Physicists observe wormhole dynamics using a quantum computer

 

Abstract concept illustration, digital worm hole

Scientists have, for the first time, developed a quantum experiment that allows them to study the dynamics, or behavior, of a special kind of theoretical wormhole. The experiment has not created an actual wormhole (a rupture in space and time), rather it allows researchers to probe 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 appear inherently incompatible with each other.

"We found a quantum system that exhibits key properties of a gravitational wormhole yet is sufficiently small to implement on today's quantum hardware," says Maria Spiropulu, the principal investigator of the U.S. Department of Energy Office of Science research program Quantum Communication Channels for Fundamental Physics (QCCFP) and the Shang-Yi Ch'en Professor of Physics at Caltech. "This work constitutes a step toward a larger program of testing quantum gravity physics using a quantum computer. It does not substitute for direct probes of quantum gravity in the same way as other planned experiments that might probe quantum gravity effects in the future using quantum sensing, but it does offer a powerful testbed to exercise ideas of quantum gravity."

The research will be published December 1 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.

Wormholes are bridges between two remote regions in spacetime. They have not been observed experimentally, but scientists have theorized about their existence and properties for close to 100 years. In 1935, Albert Einstein and Nathan Rosen described wormholes as tunnels through the fabric of spacetime in accordance 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 invoked them, while the term "wormhole" itself was coined by physicist John Wheeler in the 1950s.

The notion that wormholes and quantum physics, specifically entanglement (a phenomenon in which two particles can remain connected across vast distances), may have a connection was first proposed in theoretical research by Juan Maldacena and Leonard Susskind in 2013. The physicists speculated 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). In essence, this work established a new kind of theoretical link between the worlds of gravity and quantum physics. "It was a very daring and poetic idea," says Spiropulu 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).

A key finding that inspired possible experiments occurred 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, 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, and Subir Sachdev and Jinwu Ye, two other researchers who worked on its development previously) led researchers to suggest that some theoretical wormhole ideas could be studied more deeply by doing experiments on quantum processors.

Furthering these ideas, in 2019, Jafferis and Gao showed that by entangling two SYK models, researchers should be able to perform wormhole teleportation and thus produce and measure the dynamical properties expected of traversable wormholes.

In the new study, the team of physicists performed this type of experiment for the first time. They used a "baby" SYK-like model prepared to preserve gravitational properties, and they observed the wormhole dynamics on a quantum device at Google, namely the Sycamore quantum processor. To accomplish this, the team had to first reduce the SYK model to a simplified form, a feat they achieved using machine learning tools on conventional 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."

In the experiment, the researchers inserted a qubit -- the quantum equivalent of a bit in conventional silicon-based computers -- into one of their SYK-like systems and observed the information emerge from the other system. The information traveled from one quantum system to the other via quantum teleportation -- or, speaking in the complementary language of gravity, the quantum information passed through the traversable wormhole.

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

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, 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, via 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 gravitation 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 essential," says Spiropulu. "If the error rates were higher by 50 percent, the signal would have been entirely obscured. If they were half we would have 10 times the signal!"?

In the future, the researchers hope to extend this work to more complex quantum circuits. Though bona fide quantum computers may still be years away, the team plans to continue to perform experiments of this nature on existing quantum computing platforms.

"The relationship 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 Spiropulu. "We are excited to take this small step toward testing these ideas on quantum hardware and will keep going."

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Physicists explore the possibility of tunnels in spacetime

For decades, researchers assumed the cosmic rays that regularly bombard Earth from the far reaches of the galaxy are born when stars go supernova -- when they grow too massive to support the fusion occurring at their cores and explode.

Those gigantic explosions do indeed propel atomic particles at the speed of light great distances. However, new research suggests even supernovae -- capable of devouring entire solar systems -- are not strong enough to imbue particles with the sustained energies needed to reach petaelectronvolts (PeVs), the amount of kinetic energy attained by very high-energy cosmic rays.

And yet cosmic rays have been observed striking Earth's atmosphere at exactly those velocities, their passage marked, for example, by the detection tanks at the High-Altitude Water Cherenkov (HAWC) observatory near Puebla, Mexico. Instead of supernovae, the researchers posit that star clusters like the Cygnus Cocoon serve as PeVatrons -- PeV accelerators -- capable of moving particles across the galaxy at such high energy rates.

Their paradigm-shifting research provides compelling evidence for star forming regions to be PeVatrons and is published in two recent papers in Nature Astronomy and Astrophysical Journal Letters.

A characteristic of physics research is how collaborative it is. The research was conducted by Petra Huentemeyer, professor of physics at Michigan Technological University, along with recent graduate Binita Hona '20, doctoral student Dezhi Huang, former MTU postdoc Henrike Fleischhack (now at Catholic University/NASA GSFC/CRESST II), Sabrina Casanova at the Institute of Nuclear Physics Polish Academy of Sciences in Krakow, Ke Fang at the University of Wisconsin and Roger Blanford at Stanford, along with numerous other collaborators of the HAWC Observatory.

Huentemeyer noted that HAWC and physicists from other institutions have measured cosmic rays from all directions and across many decades of energy. It's in tracking the cosmic rays with the highest known energy, PeVs, that their origin becomes so important.

"Cosmic rays below PeV energy are believed to come from our galaxy, but the question is what are the accelerators that can produce them," Huentemeyer said.

Fleischhack said the paradigm shift the researchers have uncovered is that before, scientists thought supernova remnants were the main accelerators of cosmic rays.

"They do accelerate cosmic rays, but they are not able to get to highest energies," she said.

So, what is driving cosmic rays' acceleration to PeV energy?

"There have been several other hints that star clusters could be part of the story," Fleischhack said. "Now we are getting confirmation that they are able to go to highest energies."

Star clusters are formed from the remnants of a supernova event. Known as star cradles, they contain violent winds and clouds of swirling debris -- such as those noted by the researchers in Cygnus OB2 and cluster [BDS2003]8. Inside, several types of massive stars known as spectral type O and type B stars are gathered by the hundreds in an area about 30 parsecs (108 light-years) across.

"Spectral type O stars are the most massive," Hona said. "When their winds interact with each other, shock waves form, which is where acceleration happens."

The researchers' theoretical models suggest that the energetic gamma-ray photons seen by HAWC are more likely produced by protons than by electrons.

"We will use NASA telescopes to search for the counterpart emission by these relativistic particles at lower energies," Fang said.

The extremely high energy at which cosmic rays reach our planet is notable. Specific conditions are required to accelerate particles to such velocities.

The higher the energy, the more difficult it is to confine the particles -- knowledge gleaned from particle accelerators here on Earth in Chicago and Switzerland. To keep particles from whizzing away, magnetism is required.

Stellar clusters -- with their mixture of wind and nascent but powerful stars -- are turbulent regions with different magnetic fields that can provide the confinement necessary for particles to continue to accelerate.

"Supernova remnants have very fast shocks where the cosmic ray can be accelerated; however, they don't have the type of long confinement regions," Casanova said. "This is what star clusters are useful for. They're an association of stars that can create disturbances that confine the cosmic rays and make it possible for the shocks to accelerate them."

But how is it possible to measure atomic interactions on a galactic scale 5,000 light-years from Earth? The researchers used 1,343 days of measurements from HAWC detection tanks.

Huang explained how the physicists at HAWC trace cosmic rays by measuring the gamma rays these cosmic rays produce at galactic acceleration sites: "We didn't measure gamma rays directly; we measured the secondary rays generated. When gamma rays interact with the atmosphere, they generate secondary particles in particle showers."

"When particle showers are detected at HAWC, we can measure the shower and the charge of secondary particles," Huang said. "We use the particle charge and time information to reconstruct information from the primary gamma."

In addition to HAWC, the researchers plan to work with the Southern Wide-field Gamma-ray Observatory (SWGO), an observatory currently in the planning stages that will feature Cherenkov light detectors like HAWC but will be located in the southern hemisphere.

"It would be interesting to see what we can see in the southern hemisphere," Huentemeyer said. "We will have a good view of the galactic center that we don't have in the northern hemisphere. SWGO could give us many more candidates in terms of star clusters."

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How to spot a wormhole (if they exist)

A new study outlines a method for detecting a speculative phenomenon that has long captured the imagination of sci-fi fans: wormholes, which form a passage between two separate regions of spacetime.

Such pathways could connect one area of our universe to a different time and/or place within our universe, or to a different universe altogether.

Whether wormholes exist is up for debate. But in a paper published on Oct. 10 in Physical Review D, physicists describe a technique for detecting these bridges.

The method focuses on spotting a wormhole around Sagittarius A*, an object that's thought to be a supermassive black hole at the heart of the Milky Way galaxy. While there's no evidence of a wormhole there, it's a good place to look for one because wormholes are expected to require extreme gravitational conditions, such as those present at supermassive black holes.

In the new paper, scientists write that if a wormhole does exist at Sagittarius A*, nearby stars would be influenced by the gravity of stars at the other end of the passage. As a result, it would be possible to detect the presence of a wormhole by searching for small deviations in the expected orbit of stars near Sagittarius A*.

"If you have two stars, one on each side of the wormhole, the star on our side should feel the gravitational influence of the star that's on the other side. The gravitational flux will go through the wormhole," says Dejan Stojkovic, PhD, cosmologist and professor of physics in the University at Buffalo College of Arts and Sciences. "So if you map the expected orbit of a star around Sagittarius A*, you should see deviations from that orbit if there is a wormhole there with a star on the other side."

Stojkovic conducted the study with first author De-Chang Dai, PhD, of Yangzhou University in China and Case Western Reserve University.

A close look at S2, a star orbiting Sagittarius A*

Stojkovic notes that if wormholes are ever discovered, they're not going to be the kind that science fiction often envisions.

"Even if a wormhole is traversable, people and spaceships most likely aren't going to be passing through," he says. "Realistically, you would need a source of negative energy to keep the wormhole open, and we don't know how to do that. To create a huge wormhole that's stable, you need some magic."

Nevertheless, wormholes -- traversable or not -- are an interesting theoretical phenomenon to study. While there is no experimental evidence that these passageways exist, they are possible -- according to theory. As Stojkovic explains, wormholes are "a legitimate solution to Einstein's equations."

The research in Physical Review D focuses on how scientists could hunt for a wormhole by looking for perturbations in the path of S2, a star that astronomers have observed orbiting Sagittarius A*.

While current surveillance techniques are not yet precise enough to reveal the presence of a wormhole, Stojkovic says that collecting data on S2 over a longer period of time or developing techniques to track its movement more precisely would make such a determination possible. These advancements aren't too far off, he says, and could happen within one or two decades.

Stojkovic cautions, however, that while the new method could be used to detect a wormhole if one is there, it will not strictly prove that a wormhole is present.

"When we reach the precision needed in our observations, we may be able to say that a wormhole is the most likely explanation if we detect perturbations in the orbit of S2," he says. "But we cannot say that, 'Yes, this is definitely a wormhole.' There could be some other explanation, something else on our side perturbing the motion of this star."

Though the paper focuses on traversable wormholes, the technique it outlines could indicate the presence of either a traversable or non-traversable wormhole, Stojkovic says. He explains that because gravity is the curvature of spacetime, the effects of gravity are felt on both sides of a wormhole, whether objects can pass through or not.

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