For the first time, scientists have created a quantum experiment that allows them to study the dynamics, or behavior, of a specific type of theoretical wormhole. The experiment did not create an actual wormhole (a space-time rupture), but it did allow researchers to investigate links between theoretical wormholes and quantum physics, a prediction of so-called quantum gravity. Quantum gravity is a set of theories that seek to link gravity and quantum physics, two fundamental and well-studied descriptions of nature that appear inherently incompatible with one another.
“We discovered a quantum system that exhibits key properties of a gravitational wormhole while remaining small enough to be implemented on today’s quantum hardware,” says Maria Spiropulu, the Shang-Yi Ch’en Professor of Physics at Caltech and the principal investigator of the US Department of Energy Office of Science research program Quantum Communication Channels for Fundamental Physics (QCCFP). “This work is part of a larger program to test quantum gravity physics with a quantum computer. It does not replace direct probes of quantum gravity in the same way that other planned experiments that may probe quantum gravity effects in the future using quantum sensing do, but it does provide a powerful testbed for quantum gravity ideas.”
The research will be published in the journal Nature. The study’s first authors are Daniel Jafferis of Harvard University and Alexander Zlokapa (BS ’21), a former undergraduate student at Caltech who started on this project for his bachelor’s thesis with Spiropulu and has since moved on to graduate school at MIT.
The near-term significance of this type of experiment is that the gravitational perspective provides a simple way to understand an otherwise mysterious many-particle quantum phenomenon.
John Preskill
Wormholes are spacetime bridges that connect two distant regions. They have not been observed experimentally, but scientists have speculated on their existence and properties for nearly a century. In accordance with Einstein’s general theory of relativity, which describes gravity as a curvature of spacetime, Albert Einstein and Nathan Rosen described wormholes as tunnels through the fabric of spacetime in 1935. Wormholes are known as Einstein-Rosen bridges after the two physicists who proposed them, while the term “wormhole” was coined in the 1950s by physicist John Wheeler.
Juan Maldacena and Leonard Susskind proposed in 2013 that wormholes and quantum physics, specifically entanglement (a phenomenon in which two particles can remain connected across vast distances), may have a connection. The physicists hypothesized that wormholes (or “ER”) were equivalent to entanglement (also known as “EPR” after Albert Einstein, Boris Podolsky [Ph.D. ’28], and Nathan Rosen, who first proposed the concept). In essence, this work established a new type of theoretical link between the worlds of gravity and quantum physics. “It was a very daring and poetic idea,” Spiropulu says of the ER = EPR work.
Later, in 2017, Jafferis, along with his colleagues Ping Gao and Aron Wall, extended the ER = EPR idea to not just wormholes but traversable wormholes. The scientists concocted a scenario in which negative repulsive energy holds a wormhole open long enough for something to pass through from one end to the other. The researchers showed that this gravitational description of a traversable wormhole is equivalent to a process known as quantum teleportation. In quantum teleportation, a protocol that has been experimentally demonstrated over long distances via optical fiber and over the air, information is transported across space using the principles of quantum entanglement.
The present work explores the equivalence of wormholes with quantum teleportation. The Caltech-led team performed the first experiments that probe the idea that information traveling from one point in space to another can be described in either the language of gravity (the wormholes) or the language of quantum physics (quantum entanglement).
The Ronald and Maxine Linde Professor of Theoretical Physics and Mathematics at Caltech, Alexei Kitaev, demonstrated in 2015 that a simple quantum system could exhibit the same duality later described by Gao, Jafferis, and Wall, such that the model’s quantum dynamics are equivalent to quantum gravity effects. This Sachdev-Ye-Kitaev, or SYK, model (named after Kitaev, as well as Subir Sachdev and Jinwu Ye, two other researchers who worked on its development previously) prompted researchers to propose that some theoretical wormhole ideas could be studied more thoroughly by conducting experiments on quantum processors.
In 2019, Jafferis and Gao demonstrated that by entangling two SYK models, researchers should be able to perform wormhole teleportation and thus generate and measure the dynamical properties expected of traversable wormholes.
This type of experiment was carried out for the first time in the new study by a team of physicists. They used a “baby” SYK-like model that was prepared to preserve gravitational properties, and they observed the wormhole dynamics on a Google quantum device, the Sycamore quantum processor. To do so, the team had to first simplify the SYK model, which they accomplished using machine learning tools on standard computers.
“We employed learning techniques to find and prepare a simple SYK-like quantum system that could be encoded in the current quantum architectures and that would preserve the gravitational properties,” says Spiropulu. “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 curious and surprising how the optimization on one characteristic of the model preserved the other metrics! We have plans for more tests to get better insights on the model itself.”
The researchers performed the experiment by inserting a qubit (the quantum equivalent of a bit in conventional silicon-based computers) into one of their SYK-like systems and observing the information emerge from the other system. The information was transferred from one quantum system to another via quantum teleportation or, in the complementary language of gravity, through the traversable wormhole.
“In the gravity picture, we performed a type of quantum teleportation equivalent to a traversable wormhole. To do so, we had to simplify the quantum system to the simplest example that retains gravitational properties so that we could implement it on Google’s Sycamore quantum processor” Zlokapa explains.
Co-author Samantha Davis, a graduate student at Caltech, adds, “It took a really long time to arrive at the results, and we surprised ourselves with the outcome.”
“The near-term significance of this type of experiment is that the gravitational perspective provides a simple way to understand an otherwise mysterious many-particle quantum phenomenon,” says John Preskill, 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 retaining a reasonable caricature of what the gravitational picture predicts.”
In the study, the physicists report wormhole behavior expected both from the perspectives of gravity and from quantum physics. For example, while quantum information can be transmitted across the device, or teleported, in a variety of ways, the experimental process was shown to be equivalent, at least in some ways, to what might happen if information traveled through a wormhole. To do this, the team attempted to “prop open the wormhole” using pulses of either negative repulsive energy pulse or the opposite, positive energy. They observed key signatures of a traversable wormhole only when the equivalent of negative energy was applied, which is consistent with how wormholes are expected to behave.
“The high fidelity of the quantum processor we used was critical,” Spiropulu explains. “If the error rates had been increased by 50%, the signal would have been completely obscured. If they were half the size, we would have ten times the signal! “?
The researchers hope to expand their research into more complex quantum circuits in the future. Though true quantum computers are still years away, the team intends to conduct similar experiments on existing quantum computing platforms.
