Simulations of ‘Backwards Time Travel’ Can Improve Scientific Experiments

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

Adjunct Assistant Professor and JQI affiliate Nicole Yunger Halpern and her colleagues 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 backwards in time. If such an experiment can be performed, it will be as if the quantum experimentalists are gamblers that can retroactively change their past actions to improve their outcomes in the present.(Credit: Time is Slipping Away (cropped) from Bennilover on Flick under CC BY-ND 2.0 DEED)(Credit: Time is Slipping Away (cropped) from Bennilover on Flick under CC BY-ND 2.0 DEED)

Whether particles can travel backwards in time is a controversial topic among physicists, but 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 appears 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,” says lead author David Arvidsson-Shukur, from the Cambridge Hitachi Laboratory. However, you only receive that persons wish list on day two. So, in this chronology-respecting scenario, its 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.”

The simulation is based on quantum entanglement, where the fates of quantum particles are intrinsically linked in a way that never occurs in the physics of relatively large items like people or even grains of sand. Entanglement plays an essential role in quantum computing—the harnessing of connected particles to perform computations too complex for classical computers.

“In our proposal, an experimentalist entangles two particles,” says co-author Yunger Halpern, who is also a Fellow of the Joint Center for Quantum Information and Computer Science and a physicist at the National Institute of Standards and Technology. “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 colour, or it will be a jacket.”

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,” says 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,” says 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 yesterdays problems today.”

Story by Vanessa Bismuth

This story was prepared by the University of Cambridge and adapted with permission.

Reference Publications
Nonclassical Advantage in Metrology Established via Quantum Simulations of Hypothetical Closed Timelike CurvesD. Arvidsson-Shukur, A. McConnell, and N. Halpern, Phys. Rev. Lett., 131, 150202, (2023)

Pablo Jarillo-Herrero to Give Prange Prize Lecture on Oct. 24

Pablo Jarillo-Herrero, the Cecil and Ida Green Professor of Physics at the Massachusetts Institute of Technology, has been named the recipient of the Richard E. Prange Prize and Lectureship in Condensed Matter Theory and Related Areas for 2023. He will give his lecture, "The Magic of Moiré Quantum Matter," on Tues., Oct. 24 at 4 p.m. in room 1410 of the John S. Toll Physics Building. Refreshments will be served at 3:30 p.m. 

The Prange Prize, established by the UMD Department of Physics and Condensed Matter Theory Center (CMTC), honors the late Professor Richard E. Prange, whose distinguished professorial career at Maryland spanned four decades (1961-2000). The Prange Prize is made possible by a gift from Dr. Prange's wife, Dr. Madeleine Joullié, a professor of chemistry at the University of Pennsylvania.Pablo Jarillo-Herrero (courtesy of MIT)Pablo Jarillo-Herrero (courtesy of MIT)

Richard E. PrangeRichard E. PrangeDr. Prange was a member of the Maryland condensed matter theory group for more than 40 years and was an affiliate of CMTC with its inception in 2002. He edited a highly-respected book on the quantum Hall effect and made important theoretical contributions to the subject. His interests extended into all aspects of theoretical physics, and continued after his retirement, recalled Sankar Das Sarma, who holds the Richard E. Prange Chair in Physics at UMD and is also a Distinguished University Professor and director of the CMTC.

While earning his Ph.D. at the University of Chicago under Nobelist Yoichiro Nambu, Prange also worked with Murray Gell-Mann and Marvin Goldberger. 

Jarillo-Herrero joined MIT in 2008, and has received an Alfred P. Sloan Fellowship, a David and Lucile Packard Fellowship, a DOE Early Career Award, a Presidential Early Career Award for Scientists and Engineers, an ONR Young Investigator Award, a Moore Foundation Experimental Physics in Quantum Systems Investigator Award, the Oliver E. Buckley Condensed Matter Physics Prize and the 2020 Wolf Prize in Physics. His work showing that slight rotations of adjacent layers of graphene allowed control of its electronic properties was named the Physics World 2018 Breakthrough of the Year. 

Jarillo-Herrero joins a prestigious list of Prange Prize recipients: Philip W. Anderson, Walter Kohn, Daniel Tsui, Andre Geim, David Gross, Klaus von Klitzing, Frank Wilczek, Juan Maldacena and Charles Kane.

In addition to the Tuesday lecture, Jarillo-Herrero will deliver the CMTC JLDS Seminar on Wednesday, October 25 at 10 a.m. in room 4402 of the Atlantic Building. 

 

 

Embracing Uncertainty Helps Bring Order to Quantum Chaos

In physics, chaos is something unpredictable. A butterfly flapping its wings somewhere in Guatemala might seem insignificant, but those flits and flutters might be the ultimate cause of a hurricane over the Indian Ocean. The butterfly effect captures what it means for something to behave chaotically: Two very similar starting points—a butterfly that either flaps its wings or doesn’t—could lead to two drastically different results, like a hurricane or calm winds.

But there's also a tamer, more subtle form of chaos in which similar starting points don’t cause drastically different results—at least not right away. This tamer chaos, known as ergodicity, is what allows a coffee cup to slowly cool down to room temperature or a piece of steak to heat up on a frying pan. It forms the basis of the field of statistical mechanics, which describes large collections of particles and how they exchange energy to arrive at a shared temperature. Chaos almost always grows out of ergodicity, forming its most eccentric variant.A system is ergodic if a particle traveling through it will eventually visit every possible point. In quantum mechanics, you never know exactly what point a particle is at, making ergodicity hard to track. In this schematic, the available space is divided into quantum-friendly cells, and an ergodic particle (left) winds through each of the cells, while a non-ergodic one (right) only visits a few. (Credit: Amit Vikram/JQI)A system is ergodic if a particle traveling through it will eventually visit every possible point. In quantum mechanics, you never know exactly what point a particle is at, making ergodicity hard to track. In this schematic, the available space is divided into quantum-friendly cells, and an ergodic particle (left) winds through each of the cells, while a non-ergodic one (right) only visits a few. (Credit: Amit Vikram/JQI)

Where classical, 19th-century physics is concerned, ergodicity is pretty well understood. But we know that the world is fundamentally quantum at the smallest scales, and the quantum origins of ergodicity have remained murky to this day—the uncertainty inherent in the quantum world makes classical notions of ergodicity fail. Now, Victor Galitski and colleagues in the Joint Quantum Institute (JQI) have found a way to translate the concept of ergodicity into the quantum realm. They recently published their results in the journal Physical Review Research. This work was supported by the DOE Office of Science (Office of Basic Energy Sciences).

“Statistical mechanics is based on the assumption that systems are ergodic,” Galitski says. “It’s an assumption, a conjecture, and nobody knows why. And our work sheds light on this conjecture.”

In the classical world, ergodicity is all about trajectories. Imagine an air hockey puck bouncing around a table. If you set it in motion, it will start bouncing off the walls, changing direction with each collision. If you wait long enough, that puck will eventually visit every point on the table's surface. This is what it means to be ergodic—to visit every nook and cranny available, given enough time. If you paint the puck’s path as you go, you will eventually color in the whole table. If lots of pucks are unleashed onto the table, they will bump into each other and eventually spread out evenly over the table.

To translate this idea of ergodicity into the quantum world of individual particles is tough. For one, the very notion of a trajectory doesn't quite make sense. The uncertainty principle dictates that you cannot know the precise position and momentum of a particle at the same time, so the exact path it follows ends up being a little bit fuzzy, making the normal definitions of chaos and ergodicity challenging to apply. 

Physicists have thought up several alternate ways to look for ergodicity or chaos in quantum mechanics. One is to study the particle’s quantum energy levels, especially how they space out and bunch up. If the way they bunch up has a particular kind of randomness, the theory goes, this is a type of quantum chaos. This might be a nice theoretical tool, but it’s difficult to connect to the actual motion of a quantum particle. Without such a connection to dynamics, the authors say there’s no fundamental reason to use this energy level signature as the ultimate definition of quantum chaos. “We don't really know what quantum chaos [or ergodicity] is in the first place,” says Amit Vikram, a graduate student in physics at JQI and lead author of the paper. “Chaos is a classical notion. And so what people really have are different diagnostics, essentially different things that they intuitively associate with chaos.”

Galitski and Vikram have found a way to define quantum ergodicity that closely mimics the classical definition. Just as an air hockey puck traverses the surface of the table, quantum particles traverse a space of quantum states—a surface like the air hockey table that lives in a more abstract world. But to capture the uncertainty inherent to the quantum world, the researchers break the space up into small cells rather than treating it as individual points. It's as if they divided the abstract air hockey table into cleverly chosen chunks and then checked to see if the uncertainty-widened particle has a decent probability of visiting each of the chunks.

“Quantum mechanically you have this uncertainty principle that says that your resolution in trajectories is a little bit fuzzy. These cells kind of capture that fuzziness,” Vikram says. “It's not the most intuitive thing to expect that some classical notion would just carry over to quantum mechanics. But here it does, which is rather strange, actually.”

Picking the correct cells to partition the space into is no easy task—a random guess will almost always fail. Even if there is only one special choice of cells where the particle visits each one, the system is quantum ergodic according to the new definition. The team found that the key to finding that magic cell choice, or ruling that no such choice exists, lies in the particle’s quantum energy levels, the basis of previous definitions of quantum chaos. This connection enabled them to calculate that special cell choice for particular cases, as well as connect to and expand the previous definition.

One advantage of this approach is that it's closer to something an experimentalist can see in the dynamics—it connects to the actual motion of the particle. This not only sheds light on quantum ergodicity, quantum chaos and the possible origins of thermalization, but it may also prove important for understanding why some quantum computing algorithms work while others do not.

As Galitski puts it, every quantum algorithm is just a quantum system trying to fight thermalization. The algorithm will only work if the thermalization is avoided, which would only happen if the particles are not ergodic. “This work not only relates to many body systems, such as materials and quantum devices, but that also relates to this effort on quantum algorithms and quantum computing,” Galitski says.

Original story by Dina Genkina: https://jqi.umd.edu/news/embracing-uncertainty-helps-bring-order-quantum-chaos

Reference Publications Dynamical quantum ergodicity from energy level statistics, A. Vikram Anand, and V. Galitski, Physical Review Research, 5, (2023)

Thomas Antonsen Honored by the American Physical Society

Distinguished University Professor Thomas M. Antonsen will receive the American Physical Society’s (APS) 2023 James Clerk Maxwell Prize for Plasma Physics for “pioneering contributions in the theory of magnetized plasma stability, RF, current drive, laser-plasma interactions, and charged particle beam dynamics”.  He will be honored at the 65th Annual Meeting of the APS Division of Plasma Physics in October.Thomas AntonsenThomas Antonsen

The James Clerk Maxwell Prize annually recognizes outstanding contributions to the field of plasma physics.  The prize is named after a nineteenth century Scottish physicist known for his work with electricity, magnetism and light.

Antonsen joined the department, then known as the Department of Electrical Engineering and Physics, in 1984.  He is highly recognized in his esearch fields of plasma theory, nonlinear dynamics and chaos, and currently holds appointments in Electrical and Computer Engineering (ECE), the Institute for Research in Electronics and Applied Physics (IREAP), Physics, and the Maryland Energy Innovation Institute.

In 2017, he was appointed University of Maryland Distinguished University Professor, the highest recognition for faculty members.  Other awards include the Clark School of Engineering Outstanding Research Award, the IEEE Plasma Science and Applications Award, the John R. Pierce Award for Excellence in Vacuum Electronics, and the IEEE Marie Sklodowska-Curie Award for contributions to plasma science. He is a fellow of IEEE and APS.

Antonsen will receive $10,000 and recognition at the 65th Annual Meeting of the APS Division of Plasma Physics this fall in Denver, Colorado. 

Previous UMD physicists who have won the Maxwell Prize include Hans R. Griem, Roald Sagdeev, James Drake, Phillip A. Sprangle and Ronald C. Davidson.