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Research is a step toward studying ‘quantum gravity’ in the lab – ScienceDaily


For the first time, scientists have developed a quantum experiment that allows them to study the dynamics, or behavior, of a special kind of theoretical wormhole. The experiment did not create an actual wormhole (a rift in space and time), but rather it allows researchers to explore the connections between theoretical wormholes and quantum physics, a prediction of so-called quantum gravity. Quantum gravity refers to a set of theories that seek to connect gravity with quantum physics, two fundamental and well-studied descriptions of nature that seem intrinsically incompatible with each other.

“We have found a quantum system that exhibits key properties of a gravitational wormhole, but is small enough to be implemented on modern quantum hardware,” says Maria Spiropoulou, principal investigator of the US Department of Energy’s Quantum Communication Channels for Fundamental Physics research program. (QCCFP) and the Shang-Yi Ch’en Professor of Physics at Caltech. “This work is a step towards a broader program of testing the physics of quantum gravity using a quantum computer. It does not replace direct studies of quantum gravity in the same way that other planned experiments may investigate the effects of quantum gravity in the future using quantum probing. , but it offers a powerful testing ground for realizing the ideas of quantum gravity.”

The study will be published on December 1 in the journal Nature. The first authors of the study are Daniel Jefferis of Harvard University and Alexander Zlokapa (BS ’21), a former Caltech student who began the project for his undergraduate thesis with Spiropoulou and has since moved on to graduate school at MIT.

Wormholes are bridges between two distant regions in space-time. They have not been observed experimentally, but scientists have theorized about their existence and properties for nearly 100 years. In 1935, Albert Einstein and Nathan Rosen described wormholes as tunnels through the fabric of spacetime, consistent with Einstein’s general theory of relativity, which describes gravity as a curvature of spacetime. Researchers call wormholes Einstein-Rosen bridges after the two physicists who called them, and the term “wormhole” itself was coined by physicist John Wheeler in the 1950s.

The idea that wormholes and quantum physics, specifically entanglement (a phenomenon where two particles can remain connected over vast distances) might be related, was first proposed in a theoretical study by Juan Maldosena and Leonardo Susskind in 2013. Physicists hypothesized that wormholes (or “ER”) were equivalent to entanglement (also known as “EPR” after Albert Einstein, Boris Podolsky [PhD ’28], and Nathan Rosen, who first proposed the concept). Essentially, this work established a new kind of theoretical connection between the worlds of gravity and quantum physics. “It was a very bold and poetic idea,” says Spiropoulou of ER = EPR.

Later, in 2017, Jefferies, along with his colleagues Ping Gao and Aaron Wall, extended the ER = EPR idea not only to wormholes, but also to traversable wormholes. The scientists came up with a scenario in which the negative repulsive energy keeps the wormhole open long enough for something to pass through from one end to the other. Researchers have shown that this gravitational description of a walkable wormhole is equivalent to a process known as quantum teleportation. In quantum teleportation, a protocol that has been experimentally demonstrated over long distances through optical fiber and through the air, information is carried across space using the principles of quantum entanglement.

This paper investigates the wormhole equivalence of quantum teleportation. A Caltech-led team has conducted the first experiments exploring the idea that information moving from one point in space to another can be described either in the language of gravity (wormholes) or in the language of quantum physics (quantum entanglement).

The key discovery that inspired the possible experiments came in 2015, when Caltech’s Alexei Kitaev, the Ronald and Maxine Linde Professor of Theoretical Physics and Mathematics, showed that a simple quantum system could exhibit the same duality later described by Gao, Jefferies and Wall, for example that the quantum dynamics of the model is equivalent to the effects of quantum gravity. This Sachdev-Ye-Kitaev model, or SYK model (named after Kitaev and two other researchers who had previously worked on the development of Subir Sachdev and Jinwu Ye), led researchers to suggest that some of the theoretical ideas of a wormhole could be explored in more depth by conducting experiments on quantum processors.

Building on these ideas, Jefferies and Gao showed in 2019 that by coupling two SYK models, researchers could perform wormhole teleportation and thus produce and measure the dynamic properties expected of a traversable wormhole.

In a new study, a team of physicists conducted such an experiment for the first time. They used a “baby” model similar to SYK, trained to preserve gravitational properties, and observed the dynamics of the wormhole on a quantum machine at Google, namely the Sycamore quantum processor. To achieve this, the team had to first reduce the SYK model to a simplified form, which they achieved using machine learning tools on ordinary computers.

“We used learning techniques to find and prepare a simple SYK-like quantum system that could be encoded in modern quantum architectures and would preserve gravitational properties,” Spiropoulou says. “In other words, we simplified the microscopic description of the SYK quantum system and studied the resulting effective model that we found on the quantum processor. It is interesting and surprising how optimizing one characteristic of the model preserved other indicators! We have plans to conduct additional tests to get a better idea of ​​the model itself.”

In the experiment, the researchers inserted a qubit — the quantum equivalent of a bit in conventional silicon computers — into one of their SYK-like systems and watched as information emerged from the other system. Information moved from one quantum system to another via quantum teleportation – or, in the additional language of gravity, quantum information passed through a wormhole.

“We have performed a kind of quantum teleportation, equivalent to a traversable wormhole in the gravitational picture. To do this, we had to simplify the quantum system to the smallest example that preserves gravitational characteristics so that we could implement it on Google’s Sycamore quantum processor,” Zlokapa says.

Co-author Samantha Davis, a graduate student at Caltech, adds, “It took a very long time to get the results, and we surprised ourselves with the results.”

“The immediate significance of this type of experiment is that the gravitational perspective provides a simple way to understand a mysterious many-particle quantum phenomenon,” says John Preskill, the Richard P. Feynman Professor of Theoretical Physics at Caltech and director of the Institute for Quantum Information and Matter (IQIM). . “What I found interesting about this new Google experiment is that, using machine learning, they were able to make the system simple enough to simulate on an existing quantum machine while maintaining a reasonable caricature of what the gravitational picture predicts.”

In the study, physicists report the behavior of a wormhole expected from both the perspective of gravity and quantum physics. For example, while quantum information can be transmitted through a device or teleported in a variety of ways, the experimental process has been shown to be equivalent, at least in some ways, to what would happen if the information traveled through a wormhole. To do this, the team tried to “open the wormhole” using pulses of either negative repulsive energy or the opposite, positive energy. They observed key signatures of a wormhole that could only be traversed when the equivalent of negative energy was applied, consistent with how a wormhole would be expected to behave.

“The high precision of the quantum processor we used was very important,” says Spiropoulou. “If the error rate was 50 percent higher, the signal would be completely obscured. If it was half, we would have 10 times the signal!”?

In the future, the researchers hope to extend this work to more complex quantum circuits. Although a true quantum computer may be years away, the team plans to continue conducting experiments of this kind on existing quantum computing platforms.

“The connection between quantum entanglement, spacetime, and quantum gravity is one of the most important questions in fundamental physics and an active area of ​​theoretical research,” says Spiropoulou. “We are very excited to take this small step toward testing these ideas on quantum hardware and will continue.”

The study, titled “Dynamics of a Traversable Wormhole on a Quantum Processor,” was funded by the US Department of Energy’s Office of Science through the QCCFP research program. Other authors include: Joseph Lykken of Fermilab; David Kolchmeier, formerly of Harvard and now a graduate student at MIT; Nikolai Lauk, a former graduate student at Caltech; and Hartmut Neven of Google.

More information can be found on the Alliance for Quantum Technologies website: https://inqnet.caltech.edu/wormhole2022.

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