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What is quantum entanglement?

Quantum entanglement is one seriously long-distance relationship.

Graphic illustrating quantum entanglement when two particles are linking together.

Quantum entanglement FAQs

How to test quantum entanglement

What is quantum entanglement used for, additional resources.

Quantum entanglement is a bizarre, counterintuitive phenomenon that explains how two subatomic particles can be intimately linked to each other even if separated by billions of light-years of space.

Despite their vast separation, a change induced in one will affect the other.

In 1964, physicist John Bell posited that such changes can be induced and occur instantaneously, even if the particles are very far apart. Bell's Theorem is regarded as an important idea in modern physics, but it conflicts with other well-established principles of physics. For example, Albert Einstein had shown years before Bell proposed his theorem that information cannot travel faster than the speed of light . Perplexed, Einstein famously described this entanglement phenomenon as "spooky action at a distance."

Related: How quantum entanglement works (infographic)

Quantum entanglement FAQs answered by an expert

We asked Alan Migdall a fellow at the Joint Quantum Institute at the University of Maryland and leader of the Quantum Optics Group at the National Institute of Standards and Technology, a few frequently asked questions about quantum entanglement.

Fellow at the Joint Quantum Institute at the University of Maryland and Leader of the Quantum Optics Group at the National Institute of Standards and Technology

Quantum entanglement is when a system is in a "superposition" of more than one state. But what do those words mean? The usual example would be a flipped coin. You flip a coin but don't look at the result. You know it is either heads or tails. You just don't know which it is. Superposition means that it is not just unknown to you, its state of heads or tails does not even exist until you look at it (make a measurement). If that bothers you, you are in good company. If it doesn't bother you, then I haven't explained it clearly enough.

You might have noticed that I explained superposition more than entanglement. The reason for that is you need superposition to understand entanglement. Entanglement is a special kind of superposition that involves two separated locations in space. The coin example is superposition of two results in one place. As a simple example of entanglement (superposition of two separate places), it could be a photon encountering a 50-50 splitter. After the splitter, the photon could be In path A, or it could be in path B. In this case, the superposition is between

a photon in path A and no photon in path B.
no photon in path A and a photon in path B.

As a normal human being, you think that it is in just one or the other, and it is just that you don't know which. But in fact, it is in both, until you actually measure it. Again, that normal human being wants to say that if I measured it and found it in path A, it was in path A even if I hadn't measured it. But making that assumption gets you into trouble. Assuming the particle has that definite characteristic before you actually measured it, leads to measured results that are just not possible.

How does quantum entanglement work?

What is an example of quantum entanglement?

An example of quantum entanglement that I work with involves a light source that emits two photons at a time. Those two photons of a pair can be entangled so that the polarizations of the individual photons can be any orientation (i.e., random), but photons of a pair always have matching polarizations.

What is polarization? The polarization of light depends on the electric field of the light wave. As the light travels from point one point to another, its electric field will oscillate transversely to that propagation direction. It might oscillate in the vertical plane, in the horizontal plane or any direction in between.

Back to those entangled pairs. So, if I measure the polarization of photon A to see if it is polarized horizontal or vertical, I get an answer and find it to be, this time, vertical. Entanglement means that when I measure whether its twin is horizontal or vertical, I find that its polarization is vertical too. If I do that experiment many times, I will always find that the two photons' polarizations match, even if I find that the result of which polarization they match to is random. (Think a pair of magical loaded dice.) So, a key point is that the measurement result will be random, but if I make the same measurement on the twin, I will get that same random result. (Again, as a normal human being, that should bother you.)

Is quantum entanglement faster than light?

Asking about speed is a very interesting question. You might as a "normal human being" think that if I measure the polarization of one photon, that sets the state of the other photon. That thinking is fine, as long as the other photon measurement happens after the first measurement. But there is already a problem. If that second photon is measured on Pluto , it might take 6 hours for light to get there, so because information cannot travel faster than the speed of light , the second photon wouldn't know what state it should be. But it turns out that that second measurement will always match the first no matter when it was measured. So, it seems like the necessary information must have traveled faster than the speed of light. Big problem, but entanglement's weirdness gets it out of an astronomical speeding ticket.

In the case of entanglement, the information that appears at your Pluto measurement station is not useful information (in the ordinary sense). It is random just like the random result that came out of that first measurement (but matching random). So, the key point is that you could not take advantage of news of a crop failure and send a buy or sell order to your stockbroker on Pluto at faster than the speed of light before the Plutonian markets had time to adjust. It is only "randomness" that appears to travel faster than light, so the galactic traffic cop just lets you off with a warning.

The inherent randomness of quantum mechanics and entanglement is a useful resource. It allows you to address two issues. It guarantees randomness and it can be arranged to guarantee it is fresh randomness. Both of those are important. For example, in a lottery, we all know that the numbers should be random, but if they existed for a long time, someone could have made a copy of them and then they would know what number is coming next.

If you have a way of guaranteeing that the randomness was generated very recently, you minimize the time someone has to hack your system and secretly copy those numbers. And then there is the use of entanglement for quantum computing, but I'll leave that for someone else.

For more than 50 years, scientists around the world experimented with Bell's Theorem but were never able to fully test the theory. In 2015, however, three different research groups were able to perform substantive tests of Bell's Theorem, and all of them found support for the basic idea.

One of those studies was led by Krister Shalm, a physicist with the National Institute of Standards and Technology (NIST) in Boulder, Colorado. Shalm and his colleagues used special metal strips cooled to cryogenic temperatures, which makes them superconducting, meaning they have no electrical resistance. A photon hits the metal and turns it back into a normal electrical conductor for a split second, and scientists can see that happen. This technique allowed the researchers to see how if at all, their measurements of one photon affected the other photon in an entangled pair.

Related: 10 mind-boggling things you should know about quantum entanglement

The results, which were published in the journal Physical Review Letters, strongly backed Bell's Theorem. "Our paper and the other two published last year show that Bell was right: any model of the world that contains hidden variables must also allow for entangled particles to influence one another at a distance," co-author Francesco Marsili, of NASA's Jet Propulsion Laboratory (JPL) in Pasadena, California, said in a statement .

In addition to proving Bell's Theorem, there are practical applications to this work as well. The "superconducting nanowire single-photon detectors'' (SNSPDs) used in that experiment, could be used in cryptography and in deep-space communications , NASA officials said.

NASA's Lunar Atmosphere Dust and Environment Explorer (LADEE) mission, which orbited the moon from October 2013 to April 2014, helped demonstrate some of this communications potential. LADEE's Lunar Laser Communication Demonstration used components on the spacecraft and a ground-based receiver similar to SNSPDs. The experiment showed that it might be possible to build sensitive laser communications arrays that would enable much more data to be up and downloaded to faraway space probes, NASA officials said.

Artistic depiction of how a quantum computer works, connecting numerous points at varying distances apart.

For a more in-depth definition and exploration of quantum entanglement, check out Jed Brody's " Quantum Entanglement (The MIT Press Essential Knowledge series) " (Knopf, 2008). Read the fascinating stories about what life was like at the time of quantum entanglement's discovery in Louisa Gilder's " The Age of Entanglement: When Quantum Physics Was Reborn " (Deckle Edge, 2008). Or, take a broader look at quantum physics as a whole in this book, " Quantum Physics for Beginners: From Wave Theory to Quantum Computing. Understanding How Everything Works by a Simplified Explanation of Quantum Physics and Mechanics Principles " by Carl J. Pratt (Independently published, 2021).

Bibliography

EmLandau, 2016. "Particles in Love: Quantum Mechanics Explored in New Study." NASA Jet Propulsion Laboratory. https://www.nasa.gov/feature/jpl/particles-in-love-quantum-mechanics-explored-in-new-study
Myrvold et al., 2019. Bell's Theorem. Stanford Encyclopedia of Philosophy. https://plato.stanford.edu/entries/bell-theorem/#EarlExpeTestBellIneq
O'Neil, 2022. "Space Station to Host 'Self-Healing' Quantum Communications Tech Demo." NASA Jet Propulsion Laboratory, California Institute of Technology. https://www.jpl.nasa.gov/news/space-station-to-host-self-healing-quantum-communications-tech-demo

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Jesse Emspak is a freelance journalist who has contributed to several publications, including Space.com, Scientific American, New Scientist, Smithsonian.com and Undark. He focuses on physics and cool technologies but has been known to write about the odder stories of human health and science as it relates to culture. Jesse has a Master of Arts from the University of California, Berkeley School of Journalism, and a Bachelor of Arts from the University of Rochester. Jesse spent years covering finance and cut his teeth at local newspapers, working local politics and police beats. Jesse likes to stay active and holds a fourth degree black belt in Karate, which just means he now knows how much he has to learn and the importance of good teaching.

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Scientists Just Teleported an Object Into Space for the First Time

S cientists have successfully teleported an object from Earth to space for the first time, paving the way for more ambitious and futuristic breakthroughs.

A team of researchers in China sent a photon from the ground to an orbiting satellite more than 300 miles above through a process known as quantum entanglement, according to MIT Technology Review . It’s the farthest distance tested so far in teleportation experiments, the researchers said. Their work was published online on the open access site arXiv .

For about a month, the scientists beamed up millions of photons from their ground station in Tibet to the low-orbiting satellite. They were successful in more than 900 cases.

“This work establishes the first ground-to-satellite up-link for faithful and ultra-long-distance quantum teleportation, an essential step toward global-scale quantum Internet,” the team said in a statement, according to MIT Technology Review .

The MIT-owned magazine described quantum entanglement as a “strange phenomenon” that occurs “when two quantum objects, such as photons, form at the same instant and point in space and so share the same existence.” “In technical terms, they are described by the same wave function,” it said.

The latest development comes almost a year after physicists successfully conducted the world’s first quantum teleportation outside of a laboratory. Scientists at that time determined quantum teleportation, which is often depicted as a futuristic tool in science-fiction films, is in fact possible.

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NASA is launching a new quantum entanglement experiment in space

The researchers will test if their tech can produce and detect quantum entanglement on the International Space Station.

By Charlotte Hu | Published Mar 10, 2022 8:00 PM EST

quantum entangled system will be hosted on ISS

NASA announced this week that it is launching a mini experiment on quantum entanglement in space later this year. The mission, called the Space Entanglement and Annealing QUantum Experiment (or SEAQUE), is set to test if two entangled photons can remain linked with one another in space. This wacky property of quantum physics could one day connect devices like telescopes and computers together at frequencies that carry information with better resolution.

The project is a collaboration between research institutions in the US, Canada, and Singapore along with a few corporate partners. Everything needed for the system would fit inside a milk-carton-sized container on the surface of the International Space Station.

It’s a complex topic, so let’s start with the basics: A photon is a basic unit of light that can behave both like a particle and a wave . Meanwhile, entangled photons would behave like they’re connected, regardless of the physical distance between them (a phenomenon also known as “spooky action at a distance”). That means that even if each photon’s properties were measured independently, their results would still be correlated because measuring one photon out of the entangled pair would affect the properties of the other photon.

So what’s the point of this kind of research? Creating and maintaining this entanglement could allow distance-separated quantum systems on the ground, like quantum computers or quantum telescopes , to communicate high-resolution data with one another. A quantum network can be used for secure communications, for programming a quantum computer remotely, and for distributed sensing.

“Our project is a stepping stone for being able to connect up quantum computers,” says Paul Kwiat, the principal investigator on the SEAQUE project from the University of Illinois Urbana-Champaign. Linking up two quantum computers can also boost their computing capacity. Rather than having, for example, two 100-qubit computers acting independently, if these computers were entangled with each other, they would behave like one 200-qubit computer.

About those qubits: Unlike classical computers, which encode information in binary bits, quantum computers can code information in qubits, which can be 0, 1, or—bizarrely—both at the same time. This property, in theory, would allow quantum computers to solve certain problems like encryption, simulating a quantum system , or searching through an unsorted database, better than classical computers.

But quantum computers are delicate. If they are a hundred miles apart, but connected with optical fiber, it’s difficult to get the quantum signals to travel from one to the other because there’s loss as it goes through the fiber. “When you go a long enough distance, basically, your quantum signals don’t make it,” Kwiat says. And because quantum states can’t be copied , engineers can’t use amplifiers for the signals. “The advantage of trying to close the link from space is that the intensity of light basically falls off, and therefore the loss is much less going through free space than it would be trying to send a signal through fiber.”

[Related: Project Icarus is creating a living map of Earth’s animals ]

The SEAQUE project has a three-part goal on the ISS: create the entanglement, distribute the entanglement, and detect the entanglement.

Previously, entangled photons were created with a crystal the size of a binder clip. Then the photons had to be collected and realigned in space. SEAQUE is going to create entanglement through a process called spontaneous parametric down-conversion, in which a single photon goes through a non-linear crystal and produces two daughter photons that are lower energy. “One of the things we’re doing differently is our source is using a little integrated optic, a waveguide chip, so it’s much smaller,” Kwiat says. “We send in our light and then our photons come out of it and we just keep the temperature stable. Per parent photon you send in, you’re more likely to produce a pair of these magic entangled daughter photons than you would be in these bulk crystals.”

“We create them in some way so that they’re correlated in some of their properties. In our case, the photons are entangled in their polarization,” Kwiat adds. “Polarization is the wiggle direction, or the oscillation direction of the light.” An everyday example of a polarized system is 3D glasses for movies, where each lens sees light going in a different direction. “No matter how you look at [these daughter photons], there’s always correlations between them,” he says. “It’s impossible to get those correlations without a quantum system.”

[Related: Spooky action at a record-breaking distance ]

In SEAQUE’s limited experiment, both photons will be created and detected in the same small package in space. For future quantum communication, they would need to add telescopes and some kind of pointing and tracking system, so that one, or both, of the photons can be transmitted, Kwiat notes.

With current technology, quantum memory cannot be stored long-term on something like a regular flash drive, so quantum information has to be sent over a link. A series of experiments out of China accomplished that through telescopes on the ground and a satellite in space .

“They have to be pointing and locked on to each other and send the quantum signals. The bigger the telescopes are, the more light you can collect with them, the higher the transmission efficiency you can get going from the ground to satellite or satellite to ground,” Kwiat explains. “The project we’re doing right now is not attempting to do that.”

After these two photons are created, the final step for SEAQUE is detection, which measures the properties of the photons. “The detector needs to be able to see single photons, and they’re extremely sensitive,” Kwiat says. There’s some loss of photons as a signal passes from Earth to space, but still, it would be a lot less loss than if it was going through fiber. “While detecting signals from Earth is beyond the scope of this technology demonstration, SEAQUE will use its detector array to count the photons generated by its entanglement source,” NASA said in its press release .

Because the photons are precious and limited, the researchers need to make sure that they can see the ones that they get, which means they have to cut out any noise that comes through the detectors.

“Typical detectors that people use are influenced by radiation damage. In outer space, you get a lot of radiation, and what that radiation does is it creates defects in the crystalline lattice of the detector material (a semiconductor or silicon),” says Kwiat. That causes noise, or dark counts, that makes the detector think it detected a photon even when no photon has passed through. These defects can accumulate over time, causing rising noise that could eventually drown out the quantum signal. If there’s too much noise quantum systems like quantum cryptography would become unsecured, and links between quantum computers would be severed.

On Earth, they seem to have found a fix for the problem. The defects from radiation are not stuck very rigidly in the lattice, and if you shake the lattice by heating it, those defects can fix themselves, Kwiat notes. But to make heating in space more cost efficient, instead of putting the entire detector in an oven-like structure, they’re going to use a bright laser to spot heal these defects. SEAQUE will test how effective this laser annealing method is in space, where there’s constant radiation damage. The laser healing will hopefully extend the mission lifetime so the whole system stays viable for longer.

It’s not yet certain how this long-distance communications will ultimately link up to an individual quantum computer. There are many different ideas about how entangled photons could be connected to quantum devices, mainly because there are lots of ideas about what a quantum computer should even look like.

[Related: IBM’s latest quantum chip breaks the elusive 100-qubit barrier ]

However, some quantum technologies in development do interact with photons. For example, trapped ions, which are used in Honeywell’s experimental systems , emit a photon when they transition from one state to another.

“You can take one of the entangled photons and try to set it into the atom, or you can interfere those two photons in a way that transfers the entanglement so you can entangle these remote systems,” Kwiat suggests. Google and IBM , on the other hand, use superconducting quantum processors with qubits (a qubit looks like an artificial atom) that talk to microwave photons. “Now the question is can you convert that to one of the photons that we’re trying to send [to space].”

Microwave photons, because they have such low energy, would be almost impossible to detect in free space. “They would be swamped by all the noise,” he adds. “So you have to do some sort of transduction where you convert from microwave to visible wavelength or the telecommunication wavelengths.”

It’s a hard physics and engineering challenge that many groups across the world are working to solve at the moment. But maybe in the next decade or so, researchers might be able to take those photons, convert them into the right frequency that can talk to the quantum bit, whether that’s a trapped ion, a neutral atom, or a superconducting qubit.

“It’s going to be a while before we have useful linked up quantum computers because we don’t yet have useful, error-corrected quantum computers, we don’t have the transductions working,” Kwiat says. “Everyone is working on their piece of the puzzle.”

Charlotte is the assistant technology editor at Popular Science. She’s interested in understanding how our relationship with technology is changing, and how we live online. Contact the author here.

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What Is Entanglement and Why Is It Important?

This article was reviewed by a member of Caltech's Faculty .

Entanglement is at the heart of quantum physics and future quantum technologies. Like other aspects of quantum science, the phenomenon of entanglement reveals itself at very tiny, subatomic scales. When two particles, such as a pair of photons or electrons, become entangled, they remain connected even when separated by vast distances. In the same way that a ballet or tango emerges from individual dancers, entanglement arises from the connection between particles. It is what scientists call an emergent property.

How do scientists explain quantum entanglement?

In the video below, Caltech faculty members take a stab at explaining entanglement. Featured: Rana Adhikari, professor of physics; Xie Chen, professor of theoretical physics; Manuel Endres, professor of physics and Rosenberg Scholar; and John Preskill, Richard P. Feynman Professor of Theoretical Physics, Allen V. C. Davis and Lenabelle Davis Leadership Chair, and director of the Institute for Quantum Information and Matter.

Unbreakable Correlation

When researchers study entanglement , they often use a special kind of crystal to generate two entangled particles from one. The entangled particles are then sent off to different locations. For this example, let's say the researchers want to measure the direction the particles are spinning, which can be either up or down along a given axis. Before the particles are measured, each will be in a state of superposition , or both "spin up" and "spin down" at the same time.

If the researcher measures the direction of one particle's spin and then repeats the measurement on its distant, entangled partner, that researcher will always find that the pair are correlated: if one particle's spin is up, the other's will be down (the spins may instead both be up or both be down, depending on how the experiment is designed, but there will always be a correlation). Returning to our dancer metaphor, this would be like observing one dancer and finding them in a pirouette, and then automatically knowing the other dancer must also be performing a pirouette. The beauty of entanglement is that just knowing the state of one particle automatically tells you something about its companion, even when they are far apart.

Are particles really connected across space?

But are the particles really somehow tethered to each other across space, or is something else going on? Some scientists, including Albert Einstein in the 1930s, pointed out that the entangled particles might have always been spin up or spin down, but that this information was hidden from us until the measurements were made. Such "local hidden variable theories" argued against the mind-boggling aspect of entanglement, instead proposing that something more mundane, yet unseen, is going on.

Thanks to theoretical work by John Stewart Bell in the 1960s, and experimental work done by Caltech alumnus John Clauser (BS '64) and others beginning in the 1970s, scientists have ruled out these local hidden-variable theories. A key to the researchers' success involved observing entangled particles from different angles. In the experiment mentioned above, this means that a researcher would measure their first particle as spin up, but then use a different viewing angle (or a different spin axis direction) to measure the second particle. Rather than the two particles matching up as before, the second particle would have gone back into a state of superposition and, once observed, could be either spin up or down. The choice of the viewing angle changed the outcome of the experiment, which means that there cannot be any hidden information buried inside a particle that determines its spin before it is observed. The dance of entanglement materializes not from any one particle but from the connections between them.

Relativity Remains Intact

A common misconception about entanglement is that the particles are communicating with each other faster than the speed of light, which would go against Einstein's special theory of relativity. Experiments have shown that this is not true, nor can quantum physics be used to send faster-than-light communications. Though scientists still debate how the seemingly bizarre phenomenon of entanglement arises, they know it is a real principle that passes test after test. In fact, while Einstein famously described entanglement as "spooky action at a distance," today's quantum scientists say there is nothing spooky about it.

"It may be tempting to think that the particles are somehow communicating with each other across these great distances, but that is not the case," says Thomas Vidick , a professor of computing and mathematical sciences at Caltech. "There can be correlation without communication," and the particles "can be thought of as one object."

Let's say you have two entangled balls, each in its own box. Each ball is in a state of superposition, or both yellow and red at the same time...

Networks of Entanglement

Entanglement can also occur among hundreds, millions, and even more particles. The phenomenon is thought to take place throughout nature, among the atoms and molecules in living species and within metals and other materials. When hundreds of particles become entangled, they still act as one unified object. Like a flock of birds, the particles become a whole entity unto itself without being in direct contact with one another. Caltech scientists focus on the study of these so-called many-body entangled systems, both to understand the fundamental physics and to create and develop new quantum technologies. As John Preskill, Caltech's Richard P. Feynman Professor of Theoretical Physics, Allen V. C. Davis and Lenabelle Davis Leadership Chair, and director of the Institute for Quantum Information and Matter, says, "We are making investments in and betting on entanglement being one of the most important themes of 21st-century science."

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Space station to host ‘self-healing’ quantum communications tech demo, jet propulsion laboratory, the power of entanglement, laser healing.

SEAQUE will be hosted on the International Space Station by the Nanoracks Bishop airlock.

The NASA-funded experiment will test two technologies that could eventually enable quantum computers to communicate with each other no matter where they are located.

A tiny experiment launching to the International Space Station later this year could set the stage for a future global quantum network. Called the Space Entanglement and Annealing QUantum Experiment (or SEAQUE), the milk-carton-size technology demonstration will test two communications technologies in the harsh environment of space.

Quantum computers hold the promise of operating millions of times faster than conventional computers, and distributed quantum sensors may lead to new understandings of Earth and our place in the universe by measuring minute changes in gravity. But for quantum computers or quantum sensors to communicate, they will require a dedicated communications network. A key component of this network will be space “nodes” that can receive and transmit quantum data to and from the ground via free-space optical communications .

SEAQUE sets out to prove the viability of technologies that could enable orbiting nodes to securely connect quantum transmitters and receivers over great distances. To do that, these nodes will need to produce and detect pairs of entangled photons . Eventually, transmitting such photons to quantum computers on the ground could provide the foundation for quantum cloud computing – the means to exchange and process quantum data regardless of where the computers are located.

Once attached to the space station’s exterior, SEAQUE will also test a technique to help space-based nodes “self-heal” from radiation damage, a continual challenge of maintaining delicate instruments in space.

“Demonstrating these two technologies builds the foundation for future global quantum networks that can connect quantum computers located hundreds or even thousands of miles apart,” said Makan Mohageg, SEAQUE co-investigator at NASA’s Jet Propulsion Laboratory in Southern California.

Like the network it’s intended to enable, the project is global. The SEAQUE collaboration includes scientists and students from the University of Illinois Urbana-Champaign, who are leading the project; the University of Waterloo in Ontario, Canada; National University of Singapore; Montana-based industrial partner AdvR, Inc.; Texas-based commercial space systems provider Nanoracks; and JPL.

Pairs of entangled photons are so intimately connected that measuring one immediately affects the results of measuring the other, even when separated by a large distance. This is a fundamental characteristic of quantum mechanical systems. SEAQUE’s entangled-photon source splits high-energy photons into pairs of entangled “daughter” photons. Those daughter photons are then counted and their quantum properties are measured by the instrument’s internal detectors.

Whereas other space-based quantum experiments have depended on bulk optics (which focus light into a special crystal) to generate entangled photons, SEAQUE relies on an integrated source of entangled photons using a waveguide – a first for spacecraft. A waveguide is a microscopic structure that acts like an expressway for photons, directing their transmission with little loss of the quantum state.

“SEAQUE will demonstrate a new and never-before-flown entanglement source based on integrated optics,” said Paul Kwiat, the project’s principal investigator at the University of Illinois Urbana-Champaign. “Such a source is inherently much smaller, more robust, and more efficient at producing photon pairs than the bulk optic entanglement sources used in previous space experiments.”

For example, where those bulk optics require delicate optical realignment by an operator on the ground after being shaken up during launch, SEAQUE’s optics will not.

“If you’re building a global quantum network, connecting hundreds of quantum ground stations on different continents, you can’t afford to have a person-in-the-loop keeping the sources at each of the nodes in optical alignment,” said Mohageg. “A monolithic waveguide-based source like the one SEAQUE is going to fly will be a huge advance toward a scalable, global quantum information network.”

The technology demonstration’s reliability could get another boost if SEAQUE proves it can also repair damage inflicted on it by radiation.

Quantum communications nodes will require highly sensitive detectors to receive the single-photon quantum signals from Earth’s surface. As high-energy particles, or radiation, from space hit the nodes’ detectors, they will create defects over time. These defects can manifest themselves as “dark counts” in a detector’s output, creating noise that will eventually overwhelm any quantum signal from the ground. Left unchecked, space radiation will ultimately degrade such detectors so much that they will need to be replaced regularly, impeding the viability of a global quantum communications network.

While detecting signals from Earth is beyond the scope of this technology demonstration, SEAQUE will use its detector array to count the photons generated by its entanglement source. And SEAQUE will use a bright laser to periodically repair radiation-induced damage that will affect the detector array’s count – another first.

“In tests on the ground, we found that this technique causes the defects in the lattice to ‘bubble away’ – a process known as annealing – thereby reducing detector noise and potentially prolonging the life of in-space quantum nodes, facilitating a robust global network,” said Kwiat.

SEAQUE will be hosted on the space station by the Bishop airlock , owned and operated by Nanoracks. Nanoracks is also providing the mission operations services and coordinating the launch services. The integrated optical entangled photon source for SEAQUE is developed by AdvR, Inc. Expected to launch no earlier than August 2022, the technology demonstration is funded by NASA’s Biological and Physical Sciences Division within the agency’s Science Mission Directorate.

Ian J. O’Neill Jet Propulsion Laboratory, Pasadena, Calif. 818-354-2649 [email protected] 2022-028

Space Station to Host ‘Self-Healing’ Quantum Communications Tech Demo

SEAQUE will be hosted on the International Space Station by the Nanoracks Bishop airlock. The blue-and-gold brackets attached to the side of the airlock are for external payloads. The technology demonstration will be installed at one of those sites.

The NASA-funded experiment will test two technologies that could eventually enable quantum computers to communicate with each other no matter where they are located.

Get the Latest JPL News

The Power of Entanglement

For example, where those bulk optics require delicate optical realignment by an operator on the ground after being shaken up during launch, SEAQUE’s optics will not.

Laser Healing

The technology demonstration’s reliability could get another boost if SEAQUE proves it can also repair damage inflicted on it by radiation.

News Media Contact

Ian J. O’Neill

Jet Propulsion Laboratory, Pasadena, Calif.

818-354-2649

[email protected]

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Published: 25 June 2019

Quantifying entanglement in a 68-billion-dimensional quantum state space

James Schneeloch ORCID: orcid.org/0000-0002-7296-9166 1 na1 ,
Christopher C. Tison 1 , 2 , 3 ,
Michael L. Fanto 1 , 4 ,
Paul M. Alsing 1 &
Gregory A. Howland ORCID: orcid.org/0000-0002-8713-5009 1 , 4 na1

Nature Communications volume 10 , Article number: 2785 ( 2019 ) Cite this article

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Quantum information
Quantum optics
Single photons and quantum effects

Entanglement is the powerful and enigmatic resource central to quantum information processing, which promises capabilities in computing, simulation, secure communication, and metrology beyond what is possible for classical devices. Exactly quantifying the entanglement of an unknown system requires completely determining its quantum state, a task which demands an intractable number of measurements even for modestly-sized systems. Here we demonstrate a method for rigorously quantifying high-dimensional entanglement from extremely limited data. We improve an entropic, quantitative entanglement witness to operate directly on compressed experimental data acquired via an adaptive, multilevel sampling procedure. Only 6,456 measurements are needed to certify an entanglement-of-formation of 7.11 ± .04 ebits shared by two spatially-entangled photons. With a Hilbert space exceeding 68 billion dimensions, we need 20-million-times fewer measurements than the uncompressed approach and 10 18 -times fewer measurements than tomography. Our technique offers a universal method for quantifying entanglement in any large quantum system shared by two parties.

High-threshold and low-overhead fault-tolerant quantum memory

Sergey Bravyi, Andrew W. Cross, … Theodore J. Yoder

Principal component analysis

Michael Greenacre, Patrick J. F. Groenen, … Elena Tuzhilina

An optical tweezer array of ultracold polyatomic molecules

Nathaniel B. Vilas, Paige Robichaud, … John M. Doyle

Introduction

Achieving a quantum advantage for information processing requires scaling quantum systems to sizes that can provide significant quantum resources, including entanglement. Large quantum systems are now realized across many platforms, including atomic simulators beyond 50 qubits 1 , 2 , 3 , nascent superconducting and trapped-ion-based quantum computers 4 , 5 , integrated-photonic circuits 6 , 7 , 8 , 9 , 10 , and photon pairs entangled in high-dimensional variables 11 , 12 , 13 , 14 , 15 , 16 .

As quantum-information-based technologies mature, it will become useful to separate the physical layer providing quantum resources (e.g., trapped ions, photons) from the logical layer that utilizes those resources. For example, many imperfect qubits may form one logical qubit 17 , 18 or thousands of atoms may coherently act as a single-photon quantum memory 19 , 20 . As with classical communication and computing, protocols and algorithms will be implemented in the logical layer with minimal concern for the underlying platform. Because real-world systems are varied and imperfect, the quantum resources they provide must be characterized before use 17 .

Certifying an amount of entanglement in a large quantum system is an essential but daunting task. While entanglement witnesses 21 , 22 and Bell tests 23 can reveal entanglement’s presence, quantification generally requires a full estimation of the quantum state 24 . Beyond moderately sized states, the number of parameters to physically measure (i.e., the number of the measurements) becomes overwhelming, making this approach unviable for current and future large-scale quantum technologies.

Any practical method for quantitative entanglement certification must require only limited data. Two ideas can dramatically reduce the needed measurement resources. First is the development of quantitative entanglement witnesses, which bound the amount of entanglement without full state estimation 25 , 26 , 27 , 28 . In a recent landmark experiment, 4.1 entangled bits (ebits) of high-dimensional biphoton entanglement was certified using partial state estimation 29 . One ebit describes the amount of entanglement in a maximally entangled, two-qubit state 24 .

Second, prior knowledge can be exploited to economize sampling. Certain features, or structure, are expected in specific systems. In highly entangled quantum systems, for example, some observables should be highly correlated, the density matrix will be low rank, or the state may be nearly pure. Such assumptions can be paired with numerical optimization to recover signals sampled below the Nyquist limit. One popular technique is compressed sensing 30 , which has massively disrupted conventional thinking about sampling. Applied to quantum systems, compressed sensing reduced measurement resources significantly for tasks, including tomography 31 , 32 , 33 , 34 , 35 , 36 , 37 and witnessing entanglement 38 , 39 .

Computational recovery techniques have substantial downsides. Because they are estimation techniques, conclusions drawn from their results are contingent on the veracity of the initial assumptions. They are therefore unsuitable for closing loopholes or verifying security. Numerical solvers are often proven correct under limited noise models and require hand-tuned parameters, potentially adding artifacts and complicating error analysis. Finally, the computational resources needed become prohibitive in very large systems. The largest quantum systems characterized using these approaches remain considerably smaller than state-of-the-art.

Here we provide an approach to entanglement quantification that overcomes these downsides. First, we improve an entropic, quantitative entanglement witness to operate on arbitrarily downsampled data. Then we develop an adaptive, multilevel sampling procedure to rapidly obtain compressed distributions suitable for the witness. Crucially, our sampling assumptions are independent of the entanglement certification, so our method can guarantee security. Because we avoid numerical optimization, error analysis is straightforward and few computational resources are needed.

Entropic witnesses of high-dimensional entanglement

Entanglement is revealed when subsystems of a quantum state are specially correlated. A common situation divides a system between two parties, Alice and Bob, who make local measurements on their portion. Given two mutually unbiased, continuous observables $\widehat {\mathbf{x}}$ and $\widehat {\mathbf{k}}$ , they can measure discrete joint probability distributions P ( X a , X b ) and P ( K a , K b ) by discretizing to pixel sizes Δ X and Δ K . Here bold notation indicates that X and K may (though need not) represent multidimensional coordinates. For example, X and K might represent cartesian position and momentum that can be decomposed into horizontal and vertical components such that X = ( X , Y ) and K = ( K ( x ) , K ( y ) ).

A recent, quantitative entanglement witness 40 uses these distributions to certify an amount of entanglement:

where, for example, H ( A | B ) is the conditional Shannon entropy for P ( A , B ). E f is the entanglement of formation, a measure describing the average number of Bell pairs required to synthesize the state. Equation ( 1 ) does not require full-state estimation but depends on an informed choice of $\widehat {\mathbf{x}}$ and $\widehat {\mathbf{k}}$ . Still, in large systems, measuring these joint distributions remains oppressive. For example, if X a has 100 possible outcomes, determining P ( X a , X b ) takes 100 2 joint measurements. Describing quantum uncertainty with information-theoretic quantities is increasingly popular 41 , 42 . Entropies naturally link physical and logical layers and have useful mathematical properties. In particular, many approximations to the joint distributions can only increase conditional entropy. Because Eq. ( 1 ) bounds E f from below, any such substitution is valid.

Improving an entropic entanglement witnesses for use with limited data

We use two entropic shortcuts to improve the entanglement witness. First, if the system is highly entangled, and $\widehat {\mathbf{x}}$ and $\widehat {\mathbf{k}}$ are well chosen, the joint distributions will be highly correlated; a measurement outcome for X a should correlate to few outcomes for X b . The distributions are therefore highly compressible. Consider replacing arbitrary groups of elements in P ( X a , X b ) with their average values to form a multilevel, compressed estimate $\tilde P({\mathbf{X}}_{\mathrm{a}},{\mathbf{X}}_{\mathrm{b}})$ . By multilevel, we mean that the new, estimated distribution will appear as if it was sampled with varying resolution—fine detail in some regions and coarse detail in others. Because coarse graining cannot decrease conditional entropy, Eq. ( 1 ) remains valid for $\tilde P({\mathbf{X}}_{\mathrm{a}},{\mathbf{X}}_{\mathrm{b}})$ and $\tilde P({\mathbf{K}}_{\mathrm{a}},{\mathbf{K}}_{\mathrm{b}})$ (see Supplemental Material : Proof arbitrary coarse-graining cannot decrease conditional entropy).

Good estimates for $\tilde P({\mathbf{X}}_{\mathrm{a}},{\mathbf{X}}_{\mathrm{b}})$ and $\tilde P({\mathbf{K}}_{\mathrm{a}},{\mathbf{K}}_{\mathrm{b}})$ can be efficiently measured by sampling at high resolution in correlated regions and low resolution elsewhere. Note that the original ( P ) and estimate ( $\tilde P$ )) are full correlation matrices with N elements, but only $M \ll N$ values measured to specify $\tilde P$ . The witness is valid for arbitrary downsampling; it works best when the approximate and actual distributions are most similar but can never overestimate E f or allow false positives.

Second, if the observables are multi-dimensional such that they can be decomposed into d marginal, component observables (e.g., horizontal and vertical components) $\widehat {\mathbf{x}} = (\hat x^{(1)},\hat x^{(2)},...,\hat x^{(d)})$ (similar for $\widehat {\mathbf{k}}$ ), the conditional entropies have the property

with equality when P ( X a , X b ) is separable. If we expect nearly separable joint-distributions, the reduced, marginal joint-distributions $P(X_{\mathrm{a}}^{(i)},X_{\mathrm{b}}^{(i)})$ can be separately measured but still capture nearly all of the correlations present. For example, in a two-dimensional cartesian scenario, we might separately measure horizontal correlations P ( X a , X b ), $P(K_{\mathrm{a}}^{(x)},K_{\mathrm{b}}^{(x)})$ and vertical correlations P ( Y a , Y b ), $P(K_{\mathrm{a}}^{(y)},K_{\mathrm{b}}^{(y)})$ . For d -component observables, this is a d th power reduction in the number of measurements. Like the first shortcut, this approximation also cannot overestimate E f .

Combining both improvements, our new quantitative entanglement witness is

Proof-of-concept experimental set-up

As a test experimental system, we use photon pairs entangled in their transverse-spatial degrees of freedom 43 , 44 , where the transverse plane is perpendicular to the optic axis. Our test bed, given in Fig. 1a , creates photon pairs via spontaneous parametric downconversion (see “Methods”). Generated photons are positively correlated in transverse-position and anti-correlated in transverse-momentum. This state closely approximates the original form of the Einstein–Podolsky–Rosen paradox. Because position $\widehat {\mathbf{x}} = (\hat x,\hat y)$ and momentum $\widehat {\mathbf{k}} = (\hat k^{(x)},\hat k^{(y)})$ (where $\widehat {\mathbf{k}} = \widehat {\mathbf{p}}/\hbar$ ) observables are continuous, this state is very high dimensional.

Experimental set-up for adaptive measurements. a An entangled photon source produces spatially entangled photon pairs, which are separated and routed through basis selection optics that switch between measuring transverse-position or transverse-momentum. Computer-controlled digital micromirror devices and photon-counting detectors perform joint spatial projections at up to 512 × 512 pixel resolution. b shows a simulated, true position joint-distribution of P ( X a , X b ) at 128 × 128 pixel resolution, while c – g show its simulated, adaptively decomposed estimate $\tilde P(X_{\mathrm{a}},X_{\mathrm{b}})$ as it is refined to higher detail via quad-tree decomposition. When the joint-intensity in a block exceeds a user-defined threshold, it is split into four sub-quadrants and the process is recursively repeated, rapidly partitioning the space to obtain a compressed distribution from very few measurements

After creation, the twin photons are separated at a beam splitter and enter identical measurement apparatuses, where a basis selection system allows for interrogating position or momentum. A digital micromirror device (DMD)—an array of individually addressable micromirrors—is placed in the output plane. By placing patterns on the signal and idler DMDs and using coincidence detection, rectangular regions of the position or momentum joint-distributions are sampled at arbitrary resolution.

Adaptive, multi-level data acquisition

We measure joint-distributions $\tilde P(X_{\mathrm{a}},X_{\mathrm{b}})$ , $\tilde P(Y_{\mathrm{a}},Y_{\mathrm{b}})$ , $\tilde P(K_{\mathrm{a}}^{(x)},K_{\mathrm{b}}^{(x)})$ , and $\tilde P(K_a^{(y)},K_{\mathrm{b}}^{(y)})$ . Finding compressed distributions requires a multilevel partitioning of the joint-space that is not known a priori. Our adaptive approach is inspired by quad-tree image compression 45 . An example is shown in Fig. 1b–g . First, all DMD mirrors are directed toward the detector to obtain a total coincidence rate R T . Then the joint-space is divided into four quadrants (c), which are independently sampled. If the count rate in the i th quadrant exceeds a threshold αR T (0 ≤ α ≤ 1), the region is recursively split and the process is repeated. The algorithm rapidly identifies important regions of the joint-space for high-resolution sampling.

We set the maximum resolution of our system to 512 × 512 pixels-per-photon for a 512 4 -dimensional joint-space. The recovered joint-distributions in position and momentum are given in Fig. 2a–d . Figure 2e, f show $\tilde P(X_{\mathrm{a}},X_{\mathrm{b}})$ with the partitioning overlaid. These display the expected strong position and momentum correlations. A histogram showing the number of partitions at various scales is given in Fig. 2g ; most partitions are either 1 × 1 or 2 × 2 pixels in size. Only 6456 partitions are needed to accurately cover the 512 4 -dimensional space—an astonishing 20-million-fold improvement versus using the unimproved witness. Over 10 21 measurements are needed to perform full, unbiased tomography.

Measured joint probability distributions at 512 × 512 pixel resolution. a – d show the four estimated joint probability distributions with their single-party marginal distributions overlaid, showing tight correlations. e shows an enlarged version of $\tilde P(X_{\mathrm{a}},X_{\mathrm{b}})$ overlaid with the adaptive partitioning, with f showing a small central region to see fine detail. The histogram g shows the number of partitions as a function of their area. Only 6456 measurements are needed instead of 2 × 512 4

The entanglement witness (Eq. ( 3 )) applied to the data in Fig. 2 is shown in Fig. 3 . For short acquisition times, there is a systematic bias toward estimating a large E f . This occurs because many of the poorly correlated regions have not yet accumulated any detection events, resulting in a systematic bias toward low conditional entropies. Statistical error is low in this region because the highly correlated regions have high count rates and rapidly reach statistical significance. With additional measurement time, the initial bias diminishes and statistical error decreases. To our knowledge, 7.11 ± .04 ebits is the largest quantity of entanglement experimentally certified in a quantum system. More than 14 maximally pairwise-entangled logical qubits are needed to describe an equal amount of entanglement. We do not require advanced post-processing such as numerical optimization, estimation, or noise reduction; however, we do post-select on coincident detection events and optionally subtract accidental coincidences (see “Methods”). Our witness does not explicitly require any post-processing and is suitable for use in adversarial scenarios given a pristine experimental system.

Entanglement quantification versus acquisition time. The entanglement of formation E f is given as a function of acquisition time-per-partition for unaltered coincidence data and accidental-subtracted data. Error bars enclosing two standard deviations are determined by propagation of error from photon-counting statistics. We confirm the validity of this error analysis strategy via Monte Carlo simulation in Supplemental Material : Monte Carlo error analysis (see Supplemental Fig. 1 )

The performance of our technique as a function of maximum discretization resolution is shown in Fig. 4 . Figure 4a shows the approximate distribution partition number as a function of discretization dimension and the improvement factor over naive sampling. Figure 4b shows the certified E f , with and without accidental subtraction, along with the ideal E f for our source under a double-Gaussian approximation 44 . Because our pump laser is not Gaussian (Fig. 1a ), the actual E f is slightly less but difficult to simulate. Error bars enclosing two standard deviations are scarcely visible. For low resolution, <1000 measurements witness entanglement. Progressively refining to higher effective resolution allows more entanglement to be certified until the maximum is reached.

Entanglement quantification versus maximum resolution. a shows the number of partitions required as a function of maximum allowed resolution and the improvement over the uncompressed approach. b shows the amount of entanglement captured as the maximum resolution increases. We see the progressive nature of the technique, which witnesses entanglement with few measurements at low resolution but more accurately quantifies it with further refinement. Our results approach the ideal maximum measurable value E f = 7.68 ebits for our source

We have shown an efficient method for performing information-based entanglement certification in a very large quantum system. An alternative, important metric for quantifying entanglement in high-dimensional systems is the entanglement dimensionality, or Schmidt rank, which describes the number of modes over which the entanglement is distributed 22 , 46 , 47 , 48 . In contrast, entanglement measures quantify entanglement as a resource of entangled bits without regard for their distribution. Efficiently certifying the entanglement dimensionality faces many of the same problems as certifying a number ebits, such as the intractability of full tomography and the desire to avoid side effects from prior assumptions. Recently, Bavaresco et al. used measurements in only two bases to efficiently certify over nine entangled dimensions between orbital-angular-momentum entangled photon pairs without special assumptions about the underlying state 49 .

The number of entangled dimensions and the number of entangled bits are complementary but distinct characterizations of entanglement 50 . If a density matrix cannot be decomposed into pure states with Schmidt rank < d , then the state is at least d -dimensionally entangled. However, a d -dimensional entangled state may possess an arbitrarily small amount of entanglement. Consider a system with a large Schmidt rank but where one coefficient of the Schmidt decomposition is much larger than the others. This system will have a large entanglement dimensionality but require few entangled bits to synthesize. In this way, a given entanglement dimensionality D provides an upper bound on the entanglement of formation E f such that $0 < E_{\mathrm{f}} \le \log _2D$ . In contrast, a given E f provides a lower bound to the entanglement dimensionality $D \ge 2^{E_{\mathrm{f}}}$ , describing the situation where all D dimensions are maximally entangled. Our quantitative witness therefore also certifies entanglement dimensionality but may dramatically underestimate when the target system is not near-maximally entangled (e.g., with additive noise or non-uniform marginals). In our case, we certify 2 7.11 ≥ 138 maximally entangled dimensions with background subtraction and 2 3.43 ≥ 10 maximally entangled dimensions without background subtraction. To our knowledge, 10 entangled dimensions is the largest certified entanglement dimensionality without assumptions about the state.

Our approach shows a path forward for certifying quantum resources in large quantum systems, where we exploit prior knowledge without conventional downsides. We show the power of an information-theoretic approach to characterizing quantum systems, and how compression can be leveraged without computational signal recovery. Though the method presented here is limited to Einstein–Podolsky–Rosen-type systems where entanglement is shared by two parties, we expect that similar techniques for many-body systems utilizing higher-order correlations will soon follow.

Experimental apparatus

The 810-nm, spatially entangled photon pairs are produced via spontaneous parametric downconversion (SPDC) 44 . The pump laser is a 405-nm diode laser (CrystaLaser DL405-025-SO) attenuated to 7.9 mW with a 356 μm ( x ) × 334 μm ( y ) beam waist. A spectral clean-up filter (Semrock Versachrome TBP01-400/16) removes unwanted the 810-nm light. The pump laser is not spatially filtered. The nonlinear crystal is a 3-mm-long BiBO crystal oriented for type-I, degenerate, collinear SPDC. The crystal is held at 32.3 °C in an oven for long-term stability. A low-pass interference filter (Semrock LP442) removes remaining pump light, followed by a telescope relay system ( f 1 = 50 mm, f 2 = 100 mm) that magnifies the SPDC field ≈2×. A half-waveplate and polarizing beamsplitter allow switching between imaging ( $\widehat {\mathbf{x}}$ ) and Fourier-transforming ( $\widehat {\mathbf{k}}$ ) beam-paths; a beam block is placed in the unused path.

The DMDs (TI Lightcrafter 4500) are computer controlled via a digital video port (HDMI). A 512 × 1024 physical-pixel area was used for data given in this manuscript. Because the DMD has twice the vertical pixel density, this corresponds to a square area. The 10-mm effective focal length, aspheric lenses (Thorlabs AC080-010) couple light into 100 micron core multi-mode fibers connected to photon-counting detector modules (Excelitas SPCM-AQ4C-10). The 810/10 nm bandpass filters (Thorlabs FBS810-10) are placed before the fiber coupling. A time-correlated single-photon counting module (PicoQuant HydraHarp400) produces histograms of photon-pair relative arrival times. We post-select on coincident detections within a 1-ns coincidence window centered on the histogram peak. With all DMD mirrors pointed toward the detectors, there are approximately 26,400 total coincidences/s.

Data collection

The apparatus must be adjusted to separately measure the four reduced, joint-probability distributions P ( X a , X b ), P ( Y a , Y b ), $P(K_{\mathrm{a}}^{(x)},K_{\mathrm{b}}^{(x)})$ , and $P(K_{\mathrm{a}}^{(y)},K_{\mathrm{b}}^{(y)})$ . For example, to access the horizontal, joint-position distribution P ( X a , X b ), we adjust the half-waveplates to direct light down the imaging beam-paths so the DMDs lie in an image plane of the nonlinear crystal. To access a particular, rectangular element of the distribution, local, one-dimensional “top-hat” patterns are placed on signal (a) and idler (b) DMDs that only vary horizontally. In the regions where light should be directed to the detectors, all vertical pixels are used. The local images’ outer-product defines the rectangular region of the joint-space P ( X a , X b ) that is being sampled.

To instead access the vertical, joint-position distribution P ( Y a , Y b ), local DMD patterns are used that only vary vertically. The joint-momentum distributions are similarly sampled, with the half-waveplates instead adjusted to send light down the Fourier-transforming optical path so that the DMDs sit in the far-field of the nonlinear crystal.

Adaptive sampling algorithm

For each configuration, experimental data are stored in nodes in a quad-tree decomposition of P whose levels describe increasingly fine detail. The i th node corresponds to a square area of $\tilde P$ at location $(x_{\mathrm{a}}^i,x_{\mathrm{b}}^i)$ with span $w_{\mathrm{a}}^i = w_{\mathrm{b}}^i = w$ . Nodes are sampled by placing the corresponding, one-dimensional local patterns on the DMDs and generating a coincidence histogram during acquisition time T a = 0.5 s. Coincidences C i are counted within a 1-ns coincidence window centered on the coincidence peak; accidental coincidences A i are counted in a 1-ns window displaced 2 ns from the coincidence window. Coincidence and accidental values are appended to a list each time the node is sampled. The estimated count rate $R_i = \left\langle {C_i} \right\rangle /\epsilon _iT_{\mathrm{a}}$ , where $\epsilon _i$ is a calibrated, relative fiber coupling efficiency. Optionally, A i can be subtracted from C i for accidental removal. Uncertainty is computed by assuming Poissonian counting statistics for C i and A i and applying standard, algebraic propagation of error through the calculation of the entanglement quantity (Eq. ( 3 )).

The data collection algorithm consists of a partitioning phase followed by an iterative phase. During partitioning, the algorithm repeatedly iterates through a scan-list of leaves of the tree. Node i is considered stable when sgn( αR T − R i ) is known to at least β standard deviations of certainty, where splitting threshold α (0 ≤ α ≤ 1) and stability criterion β are user-chosen heuristics. Stable nodes are no longer measured. If a node is stable and R i ≥ αR T , the node is split into four equal-sized sub-quadrants, which are initially unstable and added to the scan-list. Optionally, a maximum resolution (maximum tree depth) may be set.

The transition to the iterative phase occurs when the percentage of unstable leaves is < Γ , a user-chosen parameter. At this point, stability is ignored and all leaf nodes are scanned repeatedly and guaranteed to have the same total acquisition time. Various final stopping criteria can be used; we chose a fixed total run time. Note that heuristic parameters α , β , and γ may be changed during operation if desired. For the data shown in this manuscript, α = 0.002, β = 2, and Γ = 0.15 with a 30-h runtime.

The probability distribution $\tilde P$ is computed by uniformly distributing the estimated count rate (with or without accidental subtraction) from each leaf node across its constituent elements in $\tilde P$ , followed by normalization.

Data availability

The data supporting the results presented in this manuscript is available from the corresponding author G.A.H. upon request.

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Acknowledgements

We gratefully acknowledge support from the OSD ARAP QSEP program and Air Force Office of Scientific Research LRIR 14RI02COR. J.S. acknowledges support from the National Research Council Research Associate Program. Any opinions, findings, and conclusions or recommendations expressed in this article are those of the authors and do not necessarily reflect the views of AFRL.

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These authors contributed equally: James Schneeloch, Gregory A. Howland.

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Air Force Research Laboratory, Information Directorate, Rome, NY, 13441, USA

James Schneeloch, Christopher C. Tison, Michael L. Fanto, Paul M. Alsing & Gregory A. Howland

Department of Physics, Florida Atlantic University, Boca Raton, FL, 33431, USA

Christopher C. Tison

Quanterion Solutions Incorporated, Utica, NY, 13502, USA

Rochester Institute of Technology, Rochester, NY, 14623, USA

Michael L. Fanto & Gregory A. Howland

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G.A.H. and J.S. conceived of the idea and contributed equally. J.S. derived the entanglement witness and led the theoretical analysis. G.A.H. and C.C.T. developed the data collection algorithm. G.A.H. performed the experiment with help from M.L.F. and analyzed the data with help from C.C.T. and J.S. P.M.A. participated in useful scientific discussions. G.A.H. wrote the manuscript with contributions from all authors.

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Correspondence to Gregory A. Howland .

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Schneeloch, J., Tison, C.C., Fanto, M.L. et al. Quantifying entanglement in a 68-billion-dimensional quantum state space. Nat Commun 10 , 2785 (2019). https://doi.org/10.1038/s41467-019-10810-z

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The Inventory

Physicists Say Time Travel Can Be Simulated Using Quantum Entanglement

“whether closed timelike curves exist in reality, we don’t know.".

The quantum world operates by different rules than the classical one we buzz around in, allowing the fantastical to the bizarrely normal. Physicists have described using quantum entanglement to simulate a closed timelike curve—in layman’s terms, time travel.

Rewinding Reality: Cambridge Uses Time-Travel Simulations To Solve “Impossible” Problems

By University of Cambridge October 21, 2023

Researchers at the University of Cambridge have utilized quantum entanglement to simulate a scenario resembling backward time travel. This allows for past actions to be retroactively altered, potentially leading to improved present outcomes.

Physicists have shown that simulating models of hypothetical time travel can solve experimental problems that appear impossible to solve using standard physics.

If gamblers, investors, and quantum experimentalists could bend the arrow of time, their advantage would be significantly higher, leading to significantly better outcomes.

“We are not proposing a time travel machine, but rather a deep dive into the fundamentals of quantum mechanics.” — David Arvidsson-Shukur try { window._mNHandle.queue.push(function (){ window._mNDetails.loadTag("974871025", "600x250", "974871025"); }); } catch (error) {}

Researchers at the University of Cambridge have shown that by manipulating entanglement – a feature of quantum theory that causes particles to be intrinsically linked – they can simulate what could happen if one could travel backward in time. So that gamblers, investors and quantum experimentalists could, in some cases, retroactively change their past actions and improve their outcomes in the present.

Simulation and Time Loops

Whether particles can travel backward in time is a controversial topic among physicists, even though scientists have previously simulated models of how such spacetime loops could behave if they did exist. By connecting their new theory to quantum metrology, which uses quantum theory to make highly sensitive measurements, the Cambridge team has shown that entanglement can solve problems that otherwise seem impossible. The study was published on October 12 in the journal Physical Review Letters .

“Imagine that you want to send a gift to someone: you need to send it on day one to make sure it arrives on day three,” said lead author David Arvidsson-Shukur, from the Hitachi Cambridge Laboratory. “However, you only receive that person’s wish list on day two. So, in this chronology-respecting scenario, it’s impossible for you to know in advance what they will want as a gift and to make sure you send the right one.

“Now imagine you can change what you send on day one with the information from the wish list received on day two. Our simulation uses quantum entanglement manipulation to show how you could retroactively change your previous actions to ensure the final outcome is the one you want.”

Understanding Quantum Entanglement

The simulation is based on quantum entanglement, which consists of strong correlations that quantum particles can share and classical particles—those governed by everyday physics—cannot.

The particularity of quantum physics is that if two particles are close enough to each other to interact, they can stay connected even when separated. This is the basis of quantum computing – the harnessing of connected particles to perform computations too complex for classical computers.

“In our proposal, an experimentalist entangles two particles,” said co-author Nicole Yunger Halpern, researcher at the National Institute of Standards and Technology (NIST) and the University of Maryland. “The first particle is then sent to be used in an experiment. Upon gaining new information, the experimentalist manipulates the second particle to effectively alter the first particle’s past state, changing the outcome of the experiment.”

“The effect is remarkable, but it happens only one time out of four!” said Arvidsson-Shukur. “In other words, the simulation has a 75% chance of failure. But the good news is that you know if you have failed. If we stay with our gift analogy, one out of four times, the gift will be the desired one (for example a pair of trousers), another time it will be a pair of trousers but in the wrong size, or the wrong color, or it will be a jacket.”

Practical Applications and Limitations

To give their model relevance to technologies, the theorists connected it to quantum metrology. In a common quantum metrology experiment, photons—small particles of light—are shone onto a sample of interest and then registered with a special type of camera. If this experiment is to be efficient, the photons must be prepared in a certain way before they reach the sample. The researchers have shown that even if they learn how to best prepare the photons only after the photons have reached the sample, they can use simulations of time travel to retroactively change the original photons.

To counteract the high chance of failure, the theorists propose to send a huge number of entangled photons, knowing that some will eventually carry the correct, updated information. Then they would use a filter to ensure that the right photons pass to the camera, while the filter rejects the rest of the ‘bad’ photons.

“Consider our earlier analogy about gifts,” said co-author Aidan McConnell, who carried out this research during his master’s degree at the Cavendish Laboratory in Cambridge, and is now a PhD student at ETH, Zürich. “Let’s say sending gifts is inexpensive and we can send numerous parcels on day one. On day two we know which gift we should have sent. By the time the parcels arrive on day three, one out of every four gifts will be correct, and we select these by telling the recipient which deliveries to throw away.”

“That we need to use a filter to make our experiment work is actually pretty reassuring,” said Arvidsson-Shukur. “The world would be very strange if our time-travel simulation worked every time. Relativity and all the theories that we are building our understanding of our universe on would be out of the window.

“We are not proposing a time travel machine, but rather a deep dive into the fundamentals of quantum mechanics. These simulations do not allow you to go back and alter your past, but they do allow you to create a better tomorrow by fixing yesterday’s problems today.”

Reference: “Nonclassical Advantage in Metrology Established via Quantum Simulations of Hypothetical Closed Timelike Curves” by David R. M. Arvidsson-Shukur, Aidan G. McConnell and Nicole Yunger Halpern, 12 October 2023, Physical Review Letters . DOI: 10.1103/PhysRevLett.131.150202

This work was supported by the Sweden-America Foundation, the Lars Hierta Memorial Foundation, Girton College, and the Engineering and Physical Sciences Research Council (EPSRC), part of UK Research and Innovation (UKRI).

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5 comments on "rewinding reality: cambridge uses time-travel simulations to solve “impossible” problems".

So how do we fast track a path to getting the correct answer 4 out of 4 times so that we aren’t at a 25% correct answer ? That’s my question

not provable, I won’t get in to explaining other than given a persons preferences and a multiple amount of life scenarios of such person and prier choices made a list can be formulated with high to low preferences. There are something that simply cannot be changed that’s history and time is fleeting we make it what it is.

Weren’t we just here about a week ago with a similar scenario and that article mysteriously disappeared into some sort of blackhole shortly after it was published? Since the very basis of the worthiness of capitalism is to gain at the expense of others, I can definitely see its value. Of course the experiment could be run in the future with no problem whatsoever and if the answer should be wrong, it could be run again until the correct answer is achieved, then wait, for time to catch up to the tested event, that way if it were to turn out that time is by nature non-euclidean, even a filter would still produce unacceptable errors, such as what occurs in the high energy fields needed for particle accelerators where virtual particles interfer with the desired observation. Something like that, if we think of time in the same manner as we do space, because other research has already revealed that reality.

Surely this is the equivalent of trying to use entanglement to send information faster than light which is forbidden. The proposed filter must fail.

The gifts analogy is flawed. If I send four gifts one day one, on day two I find out the correct one, and day three I tell him to filter out the three incorrect gifts this is not time travel, simulated or otherwise

Taking Measure

Just a Standard Blog

Demystifying Quantum: It’s Here, There and Everywhere

A researcher wearing safety glasses reaches into a box of circuitry and other equipment, which emits a green glow.

NIST researcher Tara Fortier aligns optics to maximize the signal coming from an optical clock. The signal is measured by a frequency comb.

Recently, you have probably seen the word “quantum” used everywhere — in computing, in names for tech companies, and maybe even for explanations of love and consciousness.

So ... what is quantum? What is quantum technology? And is it worth all the hype?

First of All, What Is Quantum?

Quantum, often called quantum mechanics, deals with the granular and fuzzy nature of the universe and the physical behavior of its smallest particles.

The idea of physical granularity is like your TV image. If you zoom in on the image, you will see it is made of individual pixels. The quantum world is similar. If you zoom in on the details of matter, you will eventually see elementary units of matter and energy with their own unique characteristics.

In the physical world, matter is made up of atoms as building blocks. Atoms, in turn, are made up of electrons that surround a nucleus. Nuclear particles, such as protons and neutrons, are made up of quarks.

If you look at an individual pixel, you know nearly nothing about the larger image. However, pixels working together can create enormous complexity in color, shape and even movement from just the pixels’ colors: red, blue, green and off. The same is true of the quantum building blocks in physics.

What Does Quantum Taste Like?

In fact, the properties of all matter are defined by quantum physics. This is because the physical forces in the atom that bind it together — including the quantum properties of the elementary particles inside the atom — determine the physical and electronic structure of individual atoms.

More specifically, how the electronic charge is distributed in the atom decides the atom’s electrical properties. The atom’s electronic properties determine how atoms bond to other atoms to create molecules.

Atoms with similar electronic properties are listed in the same column of the periodic table , which describes the chemistry of those elements. The electrical structure of molecules decides how molecules work and bond together to create more complex materials, such as metals, liquids, gases and organic compounds. This even includes … you! Yes, you are quantum!

So, when we experience materials — the way they feel, smell and look — their overall “bulk” properties are determined by the physical structure of quantum’s elemental building blocks and their electrical properties. We are essentially tasting, seeing and feeling the electromagnetic fields of matter!

Where Did Quantum Come From?

While you may hear a lot about it lately, quantum physics is not new. It was deeply explored and developed around the early 1900s. The quantum nature of our world was discovered when mathematical models of the day didn’t match physical observations.

Interestingly, photons , or particles of light, were discovered by scientists trying to understand the relative intensity of different colors emitted by the newly invented lightbulb. It was a practical discovery indeed!

Ultimately, scientists learned that physics on the scale of elementary particles — and in the finest scales of energy, space and time — had different features and behaviors than objects we interact with in our ordinary lives, such as baseballs or cars. As mentioned previously, the discovery was a granular universe with unexplored behaviors, such as superposition, entanglement and quantum.

Superposition is a dynamic situation where a particle can be in different states at the same time. Superposition is a little like flipping a coin. It is neither heads nor tails, but something in between, until it stops spinning.

Entanglement , as the name implies, means two things are always connected in a way that correlates with their behavior. For the flipped coin example, correlation in five entangled coins would mean that all five coins would always land either heads up or heads down.

It is this novel nature of quantum mechanics that technologists are trying to use to advance technology in computing, communication, sensing and cryptography.

Why Is There So Much Hype?

While we’ve known about quantum mechanics for more than a century, quantum-related technology has progressed rapidly in recent years. Currently, a lot of money is being invested in quantum technologies.

In 2022, the U.S. government committed $1.8 billion in funding for quantum research and development. Private investments in quantum technology in the U.S. have been close to $5 billion in the past two years.

While that might seem like a lot of money, according to some reports, China has committed $15 billion, and the European Union has committed more than $7 billion .

A recent report by McKinsey reported a potential return on investment of $1.3 trillion by 2035!

There’s a lot at stake in developing quantum systems. In the future, we may see quantum technology:

improving computing speed and power;
creating perfectly secure communications systems through quantum cryptography; and
improving measurement capabilities by networking quantum sensors, such as atomic clocks and magnetometers.

While the above capabilities lie somewhere in the future (some more distant than others), even though you might not know it, you interact with quantum technology daily.

Quantum in the Bathroom

In modern technology, we use the bulk quantum properties of atoms, light and materials to enable technologies, such as lasers, atomic clocks and sensors.

Semiconductors

Semiconductors are materials engineered to behave somewhere between metals (conductors) and glass (insulators). Because they can be made to induce electrons to move or block electrons from moving, these powerful materials are used across multiple modern-day technologies as fast electronic switches.

For example, computers and portable electronic devices can have up to trillions of semiconductors used for computation and data storage. Motion detectors, solar panels, LEDs in lightbulbs and many lasers and sensors are based on semiconductors that convert light to electricity or vice versa. Semiconductors are so ubiquitous that the annual global market was close to $600 billion last year.

Motion Detectors

Motion detectors, mentioned above, convert light reflected from a surface, like your clothes or body, to create an electrical signal that acts like a switch. This is a quantum phenomenon called the photoelectric effect, which won Albert Einstein a Nobel Prize. Motion detectors are used to open supermarket doors, turn on a light in your house or turn on a faucet in a public restroom. Yes ... quantum is in your bathroom!

Lasers are so common in modern life that you can buy them for less than $10 to entertain your cat! They are used in construction to keep things level, in medicine for surgery and to control TVs and video boxes remotely. People also use them for data storage or for skin resurfacing and hair removal.

Lasers work on the quantum principle of stimulated emission . In stimulated emission, all the emitted light has the same color or “wavelength.” Mirrors in the laser make sure that the light comes out in the same direction.

In this case, the wavelengths of light emitted from a laser all add together, making a super-strong wave. In this wave, the peaks and valleys of the wave perfectly line up.

This is a little bit like when all instruments in an orchestra play the same note in unison, creating a powerful, resonant sound wave. When the light waves from a laser all add together, they create a directed and cooperative light source that can be used for laser machining and welding. This light power can also probe and study the electronic structure of materials, called spectroscopy , or can be used to travel massive distances in space.

Atomic Clocks and GPS

Atomic clocks have been used to help standardize time internationally since 1967. These clocks use the atom’s electronic structure to create a highly regular timing signal by cycling electrons between two quantum energy levels. Because atomic clocks are so accurate and stable, they are central to ensuring accurate navigation in GPS. Using GPS, the internet and cellular towers, the clocks on portable electronics worldwide are synchronized with atomic clocks at NIST and other national labs. Amazingly, NIST uses its clocks to help synchronize 80% of portable electronics worldwide!

How to Join the Quantum Revolution

Many of the above applications were developed during what many call the first quantum revolution. Generally speaking, these technologies take advantage of the collective electronic behavior of atoms, material and light.

Some technologists believe we are entering the second quantum revolution, which will harness the more exotic nature of quantum mechanics, namely quantum entanglement.

Entanglement, superposition and correlated behavior in quantum systems may allow a quantum computer to out-compute classical computers. Quantum systems may also create unbreakable codes for cryptography, which has concerning implications for cybersecurity. Luckily, researchers at NIST and elsewhere are working to develop post-quantum encryption that would be hard even for quantum computers to break.

On a more fun note, some of the behavior of quantum mechanics will potentially create completely new opportunities that might reveal themselves as more people get creative and involved in quantum technologies. No scientist in the 1960s thought the laser would be used for skin resurfacing or cat toys, but as participation diversifies, more novel and commercial applications begin to surface!

On this World Quantum Day , to learn more about quantum and quantum educational opportunities, check out amazing work being done around the National Q-12 Education Partnership .

Or just continue asking questions and being curious. Maybe one day, you will market quantum’s next big discovery or figure out how to bring quantum 2.0 to the masses!

About the author

Tara Fortier

Tara Fortier is a physicist and project leader in NIST’s Time and Frequency Division. She leads a research group that performs both basic and applied research in the areas of laser source development for precision optical and microwave metrology of atomic clocks and for quantum networks. Fortier is broadly involved with leadership in several scientific organizations, including NIST’s Women in STEM executive board and as a NIST representative to the White House Office of Science and Technology Policy working group on National Quantum Workforce Development, as well as being a fellow for the Optical Society of America and the American Physical Society.

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STEVENS DARK 3D FORCES MAGNOFLUX HYPOTHESIS © No dark matter/energy has been found in space as dark massless electro-magnetic forces that God created are needed to balance the WMAP 4.6% atoms in universe result 1 The dark force of mass attraction G is the weakest in deep space volume x,y,z. 2 Electromagnetic dark matter magnoflux spin x,y inertia force of about 6.28G rotates galactic stars around a magnetic black hole hub 3. Electro-static repel about 25G dark energy force in z direction is responsible for expanding the universe as stars are huge + charges. 4. Quantum photons are made of spinning magnoflux 3D momentum. If the inertia energy is not at right angles to the voltage travel direction a Cosine reduction of power is experienced and quantum VAR's are produced which can carry information ripples at very low cost in energy.

"Demystifying Quantum…" was a very interesting and well-written article. Since I retired (NIST in 1990; Cornell 2006), I have tried to understand quantum mechanics with very limited success. But apparently I am in good company. Einstein called entanglement "spukhafte Fernwirkung" or spooky action at a distance and Feynman said, “If someone says he understands quantum mechanics, it means he hasn’t thought about it deeply enough.” So my quest for understanding goes on… One minor correction to the article, it is stated that "Atoms with similar electronic properties are listed in the same row of the periodic table, …". It should have stated "same column of the periodic table".

Hi, Richard. Thanks for your comment. You are correct, and the blog post has been updated with the right information.

Keep up the good work!

Entanglement: In his book The Quantum Theory of Gravitation(2003) Vasily Yanchilin argues that a photon train consists of a whole number of waves. When at the source of entanglement a wave is split into an upper and a lower half later measurements at distances may show the same opposite situation. Atomic clocks: Yanchilin proposes to measure the ticks of clocks at low and high altitude (which I suggest to be done on the quiet beach and the 700 m high volcano of Saba) and compare after a few weeks. The result will be that near the bigger mass times passes faster and Einstein was wrong. This in agreement with the lens effect: a photon seeks a route when passing mass with as big steps (oscillations of lower frequency) as possible and a minimum of these. However a common clock is too slow and an atomic clock has to be adjusted: Where gravitation (a scalar with one dimension) is less the electrons move to a wider sphere, which has three dimensions. So more energy is involved, which translates into higher frequency. Remark: Do not talk anymore about twins or trains with almost the speed of light, for since Creation everywhere the same amount of time has passed. But the clocks differ because local gravitation determines speed of physical processes. NIST has none clock precise to a second in a billion years because the universe changes all the time. That american clock may be precise to a billionth per second. Change: Because equivalence of mass and energy a travelling photon changes the mass distribution in the universe. This requires energy. If that is not received from outside the frequency of the photon drops and redshift appears. Or the time scale may be revised.

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Quantum Time Travel: Understanding the Physics Behind Time Jumps

Quantum mechanics, the cornerstone of modern physics, has revolutionized our understanding of the fundamental nature of reality, challenging our intuitive notions of time, space, and causality. Among its many intriguing implications, the concept of quantum time travel stands as one of the most captivating and enigmatic. This theoretical possibility, entwined with the fabric of quantum theory and general relativity, explores the potential for time jumps, time dilation, and the complex interplay of quantum phenomena that could allow for journeys through time. In this article, we will delve into the fascinating world of quantum time travel, unraveling its theoretical foundations, paradoxes, and the profound insights it offers into the nature of time, reality, and the universe.

Introduction to Quantum Time Travel

Quantum time travel delves into the quantum mechanical phenomena, spacetime geometry, and the theoretical frameworks that explore the potential for time travel, time loops, and time dilation within the context of quantum mechanics, general relativity, and the mysteries of the cosmos.

Time Dilation and Relativistic Effects: Time dilation, a consequence of Einstein’s theory of special relativity, describes the slowing down of time for an observer moving relative to another inertial frame or experiencing gravitational effects, revealing the interconnectedness of time, space, and motion within the fabric of spacetime.
Quantum Mechanics and Time Evolution: Quantum mechanics, governed by the Schrödinger equation and the principles of superposition, entanglement, and quantum states, offers insights into the non-deterministic, probabilistic nature of quantum systems, time evolution, and the quantum phenomena that challenge our classical understanding of time, reality, and the quantum landscape.

Wormholes, Black Holes, and Spacetime Geometry

Theoretical constructs, such as wormholes and black holes, serve as gateways to exploring the potential for time travel, time loops, and the intricate spacetime geometry that shapes the cosmic landscape and the possibilities for traversing through time and space.

Wormholes and Einstein-Rosen Bridges: Wormholes, hypothetical passages connecting distant regions of spacetime, offer potential pathways for time travel, enabling journeys through time and space, linking distant cosmic regions, and revealing the interconnectedness of the universe through the fabric of spacetime and the curvature of reality.
Black Holes, Event Horizons, and Cosmic Singularities: Black holes, regions of spacetime exhibiting strong gravitational effects that nothing can escape from, including light, represent cosmic laboratories for exploring extreme gravitational fields, time dilation, and the potential for time loops, revealing the mysterious nature of black holes, event horizons, and the cosmic singularities that define the cosmic landscape and the boundaries of our understanding of reality.

Quantum Paradoxes and Temporal Dilemmas

Theoretical paradoxes, conundrums, and thought experiments, such as the grandfather paradox, the twin paradox, and the Möbius strip of time, challenge our intuitive notions of causality, time directionality, and the consequences of time travel within the quantum realm.

Grandfather Paradox and Causality Loops: The grandfather paradox, a classic time travel dilemma, explores the potential consequences, contradictions, and causal loops arising from changing the past, altering timelines, and the implications for causality, free will, and the interconnectedness of events within the fabric of time.
Twin Paradox and Relativistic Effects: The twin paradox, a consequence of special relativity, illustrates the differential aging, time dilation, and the relativistic effects experienced by observers traveling at different velocities or experiencing different gravitational potentials, revealing the dynamic nature of time, spacetime geometry, and the interconnectedness of time, motion, and reality within the cosmic landscape.

Quantum Mechanics, Entanglement, and Time Symmetry

Quantum mechanics introduces the concepts of entanglement, superposition, and time symmetry, offering new perspectives on the nature of time, reality, and the quantum phenomena that challenge our classical understanding of time’s arrow, quantum states, and the mysterious quantum correlations linking distant events and particles within the quantum realm.

Quantum Entanglement and Nonlocal Connections: Quantum entanglement, a phenomenon where particles become interconnected and the state of one particle instantly influences another, illustrates the nonlocality, quantum correlations, and the mysterious quantum connections that transcend classical boundaries, challenging our understanding of spacetime, locality, and the nature of reality within the quantum landscape.
Time Symmetry, Quantum States, and Timeless Quantum Mechanics: Time symmetry, quantum superposition, and the principles of quantum mechanics challenge our classical perceptions of time’s arrow, temporal directionality, and the timeless nature of quantum states, revealing the interconnectedness of time, reality, and the quantum phenomena that shape the quantum landscape, cosmic dynamics, and the mysteries of the cosmos within the fabric of quantum time travel, quantum entanglement, and the quantum realm’s profound insights into the nature of time, reality, and the universe.

Quantum time travel, with its theoretical richness, paradoxes, and profound implications, stands as a testament to the intricate interplay of quantum mechanics, general relativity, and the mysteries of the cosmos, unveiling the potential pathways, cosmic connections, and the dynamic interplay of time, spacetime geometry, and the quantum phenomena shaping the cosmic landscape.

As we explore, investigate, and unravel the mysteries of quantum time travel through scientific inquiry, theoretical physics, and the pursuit of knowledge, we embark on a journey of discovery, exploration, and enlightenment that transcends boundaries, deepens our understanding of the universe’s complexity, beauty, and the mysterious interplay of quantum forces, spacetime curvature, and the cosmic dynamics shaping our cosmic journey, destiny, and the eternal quest for truth, meaning, and the timeless wonders that inspire wonder, curiosity, and a renewed appreciation for the grandeur, diversity, and interconnectedness of the cosmos, quantum phenomena, and the boundless realms of the universe and beyond.

Read More: The Goldilocks Zone: Finding Planets Just Right for Life

Quantum Time Travel: Understanding the Physics Behind Time Jumps 4

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April 9, 2024

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Quantum entanglement in quasiparticles: A stealth mode against disorder

by Julius-Maximilians-Universität Würzburg

Physicists at Julius-Maximilians-Universität Würzburg (JMU) have made a discovery that could boost the understanding of the role of entanglement in high-temperature copper oxide superconductors. The low-energy quasiparticles of these enigmatic quantum materials, so-called Zhang-Rice singlets, were found to be remarkably resilient against extreme disorder.

This surprising resilience in an otherwise glassy electronic background is enabled by quantum entanglement—a form of quantum binding that intimately ties a hole and a spin into one effective quasiparticle and makes it harder for the particle to scatter off an impurity. The study is published in Physical Review Letters .

The robustness of quasiparticles

Imagine a couple walking hand-in-hand across the marketplace on a busy day: If it wanted to move from one side to the other, the crowd of people must step aside, locally dispersing the people in its surroundings and slowing down its own movement. When watched from above, the couple and their sidestepping surroundings would seemingly move as a unit. This unit is what condensed matter physicists call a quasiparticle, namely effective particles that determine the low energy excitation spectrum of a solid.

In a metal the quasiparticles typically consist of an electron surrounded by a polarization cloud of other electrons, with electron and polarization cloud moving coherently. In a real system, these quasiparticles scatter off impurities and disorder. Going back to our fictious marketplace, this means that our two lovebirds cannot just walk through an obstacle, such as a lamp post, standing in their way. Instead, they would have to walk around it, slowing down the couple's movement once again. In a real metal, this causes the electrons to scatter off impurities, impeding the electrons' movement and creating electrical resistance.

Dancing through possible obstacles

In the published study, the team including researchers from JMU reports that the quasiparticles in cuprate materials apparently do not abide by this scattering rule. These materials have a complex structure of copper oxide layers and are generally known for their record-breaking high-temperature superconductivity when they are doped. Their quasiparticles are Zhang-Rice singlets (ZRS), entangled composite particles where an oxygen hole teams up with a copper vacancy spin, moving through the crystal like a dancing couple.

The scientists from Würzburg tested these quasiparticles in an extremely disordered cuprate environment in which up to 40% of the copper atoms were replaced by lithium. The disorder is thus so immense—our "marketplace" is so full of obstacles—that it brings the normal electrons to a complete standstill.

Physicists call such a system a non-ergodic glass system as particles now propagate much slower compared to the typical experimental timescales. In other words, there is no more back and forth for the visitors of our marketplace, and nothing moves anymore.

The Zhang-Rice singlets' beguiling dance of hole and spin within this quantum union—despite all odds—however, is totally unaffected by the impurities standing in their way. Their quantum entanglement prevents them from scattering, and they just move through the system—as if "the marketplace" was without obstacles.

Significance of the discovery

The study has revealed the first appearance of Zhang Rice singlets in a cuprate based electron glass and shown the emerging invulnerability of ZRS quasiparticles due to quantum entanglement. Such findings could have far-reaching implications not only for our understanding of the cuprate superconductors, but also for future technologies based on quantum coherence.

In particular, the ability to stabilize quantum states with respect to external perturbations by means of quantum entanglement could play a pivotal role in the realization of quantum computing.

Journal information: Physical Review Letters

Provided by Julius-Maximilians-Universität Würzburg

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Quantum computing and AI: The future of problem-solving

Imagine a not-so-distant future where quantum computing reshapes our approach to solving some of businesses’ and society’s most pressing issues. this isn’t fantasy. just as nuclear energy, 3d printing and gene therapy transitioned from science fiction to scientific reality, quantum computing is on the brink of becoming the next transformative technology..

Quantum computers were envisioned to simulate quantum systems, which are, essentially, systems governed by quantum mechanics . Sometimes referred to as subatomic systems, quantum systems are incredibly complex and nearly impossible for even the most powerful classical supercomputers to model.

Using quantum AI for the most complex problems

At its most basic level, quantum computing is designed to use the principles of quantum mechanics to achieve computational power that rivals today’s most powerful supercomputers. This is accomplished by manipulating fundamental units of information called qubits, which expand computing beyond the traditional binary bit. Qubits can exist in a state of flux beyond 0 and 1, which brings two quantum behaviors to computing:

Superposition — A qubit can represent multiple states at once — to be a 1, 0 or both — allowing for more complex problem-solving. Extrapolating this out to hundreds of qubits means that quantum computers can explore every solution space in parallel instead of sequentially as a classical computer would.
Entanglement — Qubits can become correlated with each other, so the state of one is dependent on the state of the other; therefore, they cannot be described independently. This entanglement greatly increases computational power.

“Quantum entanglement essentially creates correlations amongst the qubits,” explains Bill Wisotsky, Lead Quantum Architect and a quantum computing researcher at SAS. “Wherever the entangled qubits are — side –by side or on opposite ends of the galaxy — their values are correlated. You know the value of one by knowing the other.”

Quantum computing could redefine data analysis and model training in AI. Quantum computers could handle the complex calculations of AI algorithms much faster than classical computers and with less data, resulting in AI that can learn and adapt in ways we can’t currently imagine.

Quantum innovation: Bridging today and tomorrow

How soon before we see these disruptive innovations? “We’re in the noisy intermediate-scale quantum era, called NISQ ,” explains Wisotsky, “Which means most quantum computers alone still can't scale to solve real-world problems because of noise and decoherence of quantum states.” This early phase of quantum computing includes limitations and constraints that future technologies could later resolve.

“What has been most promising is using quantum-classical hybrid approaches, which aim at splitting the processes and sending the pieces to quantum that quantum does best and the pieces to classical computing that classical does best,” says Wisotsky. “This could change in the future, but it's the approach that SAS is currently exploring.”

In the meantime, private and public entities are pumping billions into quantum research. While large manufacturers such as Google, IBM and Microsoft improve their computing hardware, researchers at SAS are pursuing the ideal architectural fit and perfecting quantum AI algorithms.

“Quantum computing is a very important research area for us,” says Bryan Harris, Chief Technology Officer at SAS. “While there is still considerable progress needed in the scale of quantum computing, SAS R&D teams have explored hybrid architectures combining quantum computing and classical computing that are showing some very promising results in real-world applications, and customers are taking notice.”

Diagram shows hybrid solution for solving problems with quantum and classical computing.

Practical applications taking shape

There are two main types of quantum computing:

Quantum annealing, which is very good for solving optimization problems.
The gate model, which is also called universal quantum computing.

“We have so far done a lot of work with annealers for optimization, but we are also researching the problems the gate model can solve for quantum AI , quantum machine learning, quantum chemistry and quantum simulation,” says Wisotsky.

Quantum annealing exploits the laws of quantum mechanics to find the global minimum or maximum within a large set of possible options. The process explores multiple possibilities simultaneously, allowing for more efficient solutions to complex challenges. Quantum annealers are the least flexible quantum solution for business applications but are easier to build in terms of stability of qubits and reducing errors.

The gate model Is less mature yet is making rapid improvements. Gate models express interactions between qubits as quantum logic gates, similar to a circuit. Gate models are the most powerful and flexible for many types of business applications, but it can be quite difficult to build hardware that can maintain the stability of qubits and reduce errors.

Companies have already made strides using quantum annealing. Pairing quantum computing with AI and machine learning has been used to optimize supply chains and bussing or delivery routes.

SAS has embraced the gate model for its versatility, akin to classical computing, yet exponentially more powerful. It has potential applications in molecular modeling, chemical simulations and financial modeling — areas where classical systems are reaching their limits.

“As the world becomes more digitally connected, the complexity of modeling risk is overwhelming classical methods,” explains Harris. “Quantum algorithms can improve modeling of financial markets, portfolio optimization and identifying the systemic risk associated with hyperconnectivity.”

The fusion of quantum computing and AI hints at a world where we can solve previously unsolvable problems like large-scale desalination, clean energy and fraud prevention with quantum encryption.

In the business realm, SAS researchers have been investigating four opportunities for quantum computing right now :

Drug discovery: By accurately simulating the behavior of molecules, quantum computing could drastically reduce the time and cost associated with discovering new drugs. This includes understanding the interaction between drugs and the complex biological systems they target.
Financial modeling: As the world becomes more digitally connected, the complexity of modeling risk is overwhelming classical methods. Quantum algorithms have the ability to improve the modeling of financial markets, risk assessment and portfolio optimization.
Chemical simulations: Quantum computers can simulate the behavior of atoms and molecules at a quantum level with high accuracy, … which is a challenge for classical computers because of the complex nature of quantum mechanics. This capability could revolutionize the field of chemistry, … enabling the discovery of new materials for improved sustainability … to include breakthroughs in areas such as batteries.
Optimization: As mentioned, quantum computers can tackle complex optimization problems, like the travelling salesman example, more efficiently than classical computers in most cases.

Beyond the horizon: A hybrid quantum future

Experts anticipate future processing power to be enhanced by a hybrid computational approach, where quantum processing units (QPUs) address tasks beyond the reach of classical CPUs and GPUs .

“Even though a GPU has better performance and is faster in some respects than a CPU, it’s more efficient to send highly parallelized computations to the GPU,” explains Wisotsky. “I think QPU processing will be handled the same – we’ll only send computations to the QPU that are not suited for the CPU or GPU.”

When the architectures are paired creatively, historic innovations are bound to happen more quickly than if we wait for quantum computing power and quantum technologies to be perfected. For instance, scientists believe the binary/quantum combination could end their 50-year quest to solve the protein folding problem with profound implications for diseases such as cancer, Alzheimer’s and Parkinson’s.

As Gartner Vice President Analyst Chirag Dekate suggests , we're moving toward an era where quantum computing and AI could be as ubiquitous and influential as smartphones, revolutionizing how we solve problems and innovate.

Wisotsky and Harris will be discussing quantum computing and AI at SAS Innovate in Las Vegas. Sign up now for the SAS Innovate live stream to learn more.

About author.

Waynette Tubbs is a seasoned technology journalist specializing in interviewing and writing about how leaders leverage advanced and emerging analytical technologies to transform their B2B and B2C organizations. In her current role, she works closely with global marketing organizations to generate content about artificial intelligence (AI), generative AI, intelligent automation, cybersecurity, data management, and marketing automation. Waynette has a master’s degree in journalism and mass communications from UNC Chapel Hill.

Advancing Quantum Leadership and Community

Northwestern hosted a qed-c plenary meeting for academic, corporate, and government stakeholders.

Quantum 1.0 technologies — including lasers, MRI scanners, transistors, and semiconductor devices — paved the way for ubiquitous devices and services such as smart phones, laptops, and GPS navigation. Harnessing the quantum mechanics of sub-atomic particles — the phenomena of superposition, measurement, and entanglement — quantum 2.0 technology has the potential to revolutionize artificial intelligence, communications, information technology, manufacturing, and transportation and logistics.

To advance quantum research and technology development, Northwestern University is a founding member of the Quantum Economic Development Consortium (QED-C), an alliance of academic, corporate, and government stakeholders established under the 2018 National Quantum Initiative Act with support from the National Institute of Standards and Technology to accelerate the development of quantum information science and technology applications.

QED-C aims to realize the transformative potential of quantum 2.0 technologies — from quantum computing and cryptography to quantum sensing, timing, and imaging — by supporting a robust quantum ecosystem and quantum industry supply chain and identifying strategic gaps in enabling technologies, standards and regulation, and quantum workforce development.

On March 20-21, Northwestern hosted a QED-C plenary meeting for nearly 150 members — including leaders from Northwestern Engineering and the University —to learn, network, and identify collaboration opportunities.

Northwestern President Michael H. Schill and QED-C executive director Celia Merzbacher welcomed the guests.

“One doesn't need to be an expert about quantum computing to understand that it is a critical area for Northwestern and indeed for our nation and world,” said Schill, professor of law in Northwestern’s Pritzker School of Law and professor of finance and real estate in the Kellogg School of Management. “The research emerging from this field is poised to revolutionize our lives in the coming decades, and the work that all of you are involved in certainly brings needed solutions to a whole range of societal challenges that quantum computing will have the answers to.

“Quantum is a rising priority for Northwestern and for our faculty members. In the University priorities I unveiled last summer, I highlighted the importance of research and innovation in data science, artificial intelligence, sustainability, and decarbonization. All of those things are implicated in quantum computing, so I'm really excited that we're able to host this conference and that we're a founding member of this consortium.”

One doesn't need to be an expert about quantum computing to understand that it is a critical area for Northwestern and indeed for our nation and world. The research emerging from this field is poised to revolutionize our lives in the coming decades, and the work that all of you are involved in certainly brings needed solutions to a whole range of societal challenges that quantum computing will have the answers to.

Northwestern University President Michael H. Schill

Quantum computing for transportation and logistics

Quantum computing offers significant promise for optimization, real-time decision making, monitoring, and predictive modeling in the highly complex, data-driven transportation, logistics, and supply chain sectors.

On March 19, the QED-C Use Cases Technical Advisory Committee published a study, titled “ Quantum Computing for Transportation and Logistics ,” based on a workshop NUTC facilitated last October to assess the feasibility and impact of quantum computing use cases for transportation and logistics applications.

The committee — which also included NUTC senior associate director Bret Johnson ; Kevin Glynn , adjunct lecturer in Northwestern Engineering’s Master of Science in Information Technology Program ; and postdoctoral scholar Divyakant Tahlyan (MS ’21, PhD ’23) — proposed 83 potential uses, including applications with high-impact potential in the near-term such as demand forecasting and optimization of labor, routing, and warehousing.

At the QED-C plenary meeting, Glynn moderated a panel featuring Tahlyan, chief technology officer at PCS Software Yusuf Ozturk (CS PhD) and Catherine Potts of D-Wave Quantum to discuss the findings and recommendations of the study and the computational challenges within the transportation and logistics domains.

The panelists agreed that the existing level of classical computation power is insufficient to solve the algorithmic problems across the logistics and supply chain sectors and that the development and adoption of quantum computing technologies is the next step toward increasing operational efficiency, supply chain security and resilience, and efficiencies to address sustainability.

Sustainable quantum technologies

Mahmassani explained that quantum technologies can play a crucial role in enhancing sustainability efforts within transportation and logistics by optimizing energy consumption, reducing carbon emissions through more efficient routing, and improving overall resource utilization.

“What mRNA research did for COVID is similarly what quantum computing could do for climate change,” said Nivedita Arora , the Allen K. and Johnnie Cordell Breed Junior Professor of Design and assistant professor of electrical and computer engineering at Northwestern Engineering. “The next decade is going to be very critical, and we need to accelerate radical decarbonization efforts in electricity, transport, and manufacturing.”

Prem Kumar , professor of electrical and computer engineering, opened a dialogue in a QED-C plenary breakout session on both the applications of quantum to sustainability and the sustainability of quantum technology.

Kumar and panelist Michael R. Wasielewski are executive committee members of Northwestern’s Initiative for Quantum Information Research and Engineering (INQUIRE), a transdisciplinary hub of education and research excellence in quantum sciences across areas including material informatics and data science, material synthesis, molecular quantum transduction, nanotechnology, photonics, physics, and superconducting technologies.

“If we think that we're at an incipient stage of developing quantum technology, now is the time to really think of alternatives, to be creative and see what’s possible,” said Wasielewski, who is also the director of INQUIRE and the Center for Molecular Quantum Transduction .

Student focus on next-generation quantum technologies

Students conducting research in quantum fields from Northwestern and nearby universities were invited to participate in a special track at the plenary meeting which featured speed mentoring, a science communication workshop, and a poster session.

Gamze Gül , a fifth-year PhD student in applied physics advised by Kumar, is interested in designing quantum networking protocols to manage and control quantum networks using classical bits. She presented a poster titled “ Quantum Wrapper Networking ,” which demonstrated a novel approach to operate quantum networks that is both compatible with current fiber optic infrastructure and allows for quick adjustments to address network problems — such as loss or high traffic. By creating a package of quantum bits — or qubits — wrapped with classical bits, the qubits can be transported to their destinations without measurement or disturbance to the payload.

“Some technologies that we use in our daily lives, such as lasers, GPS, and transistors, would be impossible without a deeper understanding of quantum mechanics,” Gül said. “Today, we are entering a new era of quantum technologies that could help us understand the world we live in through quantum computing or sensing. It will be crucial to connect the distributed quantum systems.”

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Moscow Metro – Part 2

Have you been to Moscow ? In all seriousness, they have the prettiest metro stations I have ever seen and I still can’t believe how immaculate and lovely every station was. There are several different stations pictured below and this is the second of several posts where I will show you the beauty of the Moscow Metro. Did you see part 1 ? There really isn’t much to say because I think the pictures speak for themselves. I have so many more pictures to share with you!

Have you ever been to Moscow? Is it someplace you have thought about visiting?

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This is the train STATION?? Oh my god… So gorgeous. Moscow has never even crossed my mind as a possible travel destination but this is gorgeous…Hmmm… LOL

I know, right? We spent several hours in the metro, just marveling at the beauty of each one. Thanks for stopping by!

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Electrostal History and Art Museum

You can spend time exploring the galleries in Electrostal History and Art Museum in Elektrostal. Take in the museums while you're in the area.

Cities near Elektrostal

Places of interest
Yuri Gagarin Cosmonaut Training Center
Peter the Great Military Academy
Central Museum of the Air Forces at Monino
History of Russian Scarfs and Shawls Museum
Balashikha Arena
Balashikha Museum of History and Local Lore
Bykovo Manor
Pekhorka Park
Malenky Puppet Theater
Drama Theatre BOOM
Ramenskii History and Art Museum
Noginsk Museum and Exhibition Center
Pavlovsky Posad Museum of Art and History
Saturn Stadium
Fairy Tale Children's Model Puppet Theater
Fifth House Gallery
Church of Vladimir
Likino Dulevo Museum of Local Lore
Malakhovka Museum of History and Culture
Orekhovo Zuevsky City Exhibition Hall

Claudia Looi

Touring the Top 10 Moscow Metro Stations

By Claudia Looi 2 Comments

Komsomolskaya metro station looks like a museum. It has vaulted ceilings and baroque decor.

Hidden underground, in the heart of Moscow, are historical and architectural treasures of Russia. These are Soviet-era creations – the metro stations of Moscow.

Our guide Maria introduced these elaborate metro stations as “the palaces for the people.” Built between 1937 and 1955, each station holds its own history and stories. Stalin had the idea of building beautiful underground spaces that the masses could enjoy. They would look like museums, art centers, concert halls, palaces and churches. Each would have a different theme. None would be alike.

The two-hour private tour was with a former Intourist tour guide named Maria. Maria lived in Moscow all her life and through the communist era of 60s to 90s. She has been a tour guide for more than 30 years. Being in her 60s, she moved rather quickly for her age. We traveled and crammed with Maria and other Muscovites on the metro to visit 10 different metro stations.

Arrow showing the direction of metro line 1 and 2

Moscow subways are very clean

To Maria, every street, metro and building told a story. I couldn’t keep up with her stories. I don’t remember most of what she said because I was just thrilled being in Moscow. Added to that, she spilled out so many Russian words and names, which to one who can’t read Cyrillic, sounded so foreign and could be easily forgotten.

The metro tour was the first part of our all day tour of Moscow with Maria. Here are the stations we visited:

1. Komsomolskaya Metro Station is the most beautiful of them all. Painted yellow and decorated with chandeliers, gold leaves and semi precious stones, the station looks like a stately museum. And possibly decorated like a palace. I saw Komsomolskaya first, before the rest of the stations upon arrival in Moscow by train from St. Petersburg.

2. Revolution Square Metro Station (Ploshchad Revolyutsii) has marble arches and 72 bronze sculptures designed by Alexey Dushkin. The marble arches are flanked by the bronze sculptures. If you look closely you will see passersby touching the bronze dog's nose. Legend has it that good luck comes to those who touch the dog's nose.

Touch the dog's nose for good luck. At the Revolution Square station

Revolution Square Metro Station

3. Arbatskaya Metro Station served as a shelter during the Soviet-era. It is one of the largest and the deepest metro stations in Moscow.

Arbatskaya Metro Station

4. Biblioteka Imeni Lenina Metro Station was built in 1935 and named after the Russian State Library. It is located near the library and has a big mosaic portrait of Lenin and yellow ceramic tiles on the track walls.

Lenin's portrait at the Biblioteka Imeni Lenina Metro Station

5. Kievskaya Metro Station was one of the first to be completed in Moscow. Named after the capital city of Ukraine by Kiev-born, Nikita Khruschev, Stalin's successor.

Kievskaya Metro Station

6. Novoslobodskaya Metro Station was built in 1952. It has 32 stained glass murals with brass borders.

Novoslobodskaya metro station

7. Kurskaya Metro Station was one of the first few to be built in Moscow in 1938. It has ceiling panels and artwork showing Soviet leadership, Soviet lifestyle and political power. It has a dome with patriotic slogans decorated with red stars representing the Soviet's World War II Hall of Fame. Kurskaya Metro Station is a must-visit station in Moscow.

Ceiling panel and artworks at Kurskaya Metro Station

8. Mayakovskaya Metro Station built in 1938. It was named after Russian poet Vladmir Mayakovsky. This is one of the most beautiful metro stations in the world with 34 mosaics painted by Alexander Deyneka.

Mayakovskaya station

One of the over 30 ceiling mosaics in Mayakovskaya metro station

9. Belorusskaya Metro Station is named after the people of Belarus. In the picture below, there are statues of 3 members of the Partisan Resistance in Belarus during World War II. The statues were sculpted by Sergei Orlov, S. Rabinovich and I. Slonim.

10. Teatralnaya Metro Station (Theatre Metro Station) is located near the Bolshoi Theatre.

Teatralnaya Metro Station decorated with porcelain figures .

Taking the metro's escalator at the end of the tour with Maria the tour guide.

Have you visited the Moscow Metro? Leave your comment below.

January 15, 2017 at 8:17 am

An excellent read! Thanks for much for sharing the Russian metro system with us. We're heading to Moscow in April and exploring the metro stations were on our list and after reading your post, I'm even more excited to go visit them. Thanks again 🙂

December 6, 2017 at 10:45 pm

Hi, do you remember which tour company you contacted for this tour?