Physicists create a holographic wormhole using a quantum computer (2023)

In fact, the new experiment confirms that quantum effects, such as we can control in a quantum computer, can give rise to a phenomenon we expect in relativity - a wormhole. The evolving system of qubits on the Sycamore chip "has this really cool alternative description," he said.John Preskill, a theoretical physicist at Caltech who was not involved in the experiment. "You can think of the system as gravitational in a very different language."

Unlike an ordinary hologram, the wormhole is nothing we can see. While it could be considered "a real space-time filament," according to the co-authorDaniel Jafferisfrom Harvard University, the main developer of the wormhole teleportation protocol, is not part of the same reality in which we and the Sycamore computer live. The holographic principle holds that the two realities - that of the wormhole and that of the qubits - are alternate versions of the same physics, but how to conceptualize this type of duality remains a mystery.

Opinions will differ on the fundamental implications of the result. Crucially, the holographic wormhole in the experiment consists of a different kind of spacetime than the spacetime of our own universe. It is questionable whether the experiment supports the hypothesis that the spacetime we live in is also holographic, modeled by quantum bits.

"I think it's true that gravity in our universe emerges from a few quantum bits, just like this little one-dimensional wormhole emerges from the Sycamore chip," Jafferis said. "Of course we don't know for sure. We're trying to understand it."

in the wormhole

The history of the holographic wormhole can be traced back to two seemingly unrelated works published in 1935:1by Einstein and Rosen, known as ER,the otherby the two and Boris Podolsky, known as EPR. Both the ER and EPR roles were initially judged as ancillary works by the big E. That changed.

In the ER article, Einstein and his young assistant Rosen stumbled upon the possibility of wormholes while attempting to extend general relativity into a unified theory of everything — a description not just of spacetime but of the subatomic particles floating within it. They focused on snags in the fabric of spacetime that German soldier physicist Karl Schwarzschild found between the folds of general relativity in 1916, just months after Einstein published the theory. Schwarzschild showed that mass can attract so much that it becomes infinitely concentrated at one point, bending space-time so much that the variables become infinite and Einstein's equations break down. We now know that these "singularities" exist throughout the universe. These are points we can neither describe nor see, each tucked away at the center of a black hole that gravitationally traps all light nearby. Singularities are where a quantum theory of gravity is most needed.

Einstein and Rosen speculated that Schwarzschild's mathematics could be a way to connect elementary particles to general relativity. To make the picture work, they clipped the singularity out of their equations and swapped in new variables that replaced the sharp edge with an extradimensional tube that slides into a different part of spacetime. Einstein and Rosen wrongly but presciently argued that these "bridges" (or wormholes) could represent particles.

Ironically, in trying to connect wormholes and particles, the pair missed the strange particle phenomenon they had identified with Podolsky in the EPR study two months earlier: quantum entanglement.

Entanglement occurs when two particles interact. According to quantum rules, particles can have several possible states at the same time. This means that an interaction between particles has multiple possible outcomes, depending on what state each particle is in to begin with. However, their resulting states will always be linked - how particle A ends up depends on how particle B ends up. After such an interaction, the particles have a common formula that gives the different combined states they can be in.

The shocking consequence, which caused the EPR authors to doubt quantum theory, is what Einstein called "spooky action at a distance": the measurement of particle A (choosing one reality among its possibilities) immediately decides the corresponding state of B. No matter how far away B is.

The importance of entanglement has increased since physicists discovered in the 1990s that it enabled new types of computation. The entanglement of two qubits — particle-like quantum objects that exist in two possible states, 0 and 1 — creates four possible states with different probabilities (0 and 0, 0 and 1, 1 and 0, and 1 and 1). Three qubits make eight simultaneous possibilities, and so on; The performance of a "quantum computer" grows exponentially with each additional entangled qubit. Orchestrate the entanglement skillfully, and you can break all combinations of zeros and ones except the sequence that gives the result of a computation. Prototypes of quantum computers made up of a few dozen qubits have emerged in recent years, led by Google's 54-qubit Sycamore machine.

Meanwhile, quantum gravity researchers have fixed quantum entanglement for another reason: as possible source code for the space-time hologram.

ER = EPR

The conversation about evolving spacetime and holography began in the late 1980s after black hole theorist John Wheeler propagated the view that spacetime and everything in it can be evolving from information. Soon other researchers, including Dutch physicist Gerard 't Hooft, wondered if this formation might resemble the projection of a hologram. Examples have emerged in studies of black holes and string theory, where a description of a physical scene can be translated into an equally valid view of it with an added spatial dimension. In a 1994 article entitled “The world as a hologram“Leonard Süßkind, a quantum gravity theorist at Stanford University, developed 't Hooft's holographic principle and argued that a volume of flexible spacetime described by general relativity is equivalent or "dual" to a system of quantum particles in the lower limit dimension region.

An important example of holography came three years later.Juan Maldacena, a quantum gravity theorist now at the Institute for Advanced Study in Princeton, New Jersey,uncoveredthat a type of room called the anti-de-sitter room (AdS) is actually a hologram.

The real universe is the De Sitter space, an ever-expanding sphere propelled outward by its own positive energy. In contrast, AdS space is permeated with negative energy—resulting from a difference in the sign of a constant in the equations of general relativity—that gives space a “hyperbolic” geometry: objects shrink as they move away from the center of space remove infinitesimal at an outer limit. Maldacena showed that spacetime and gravity within an AdS universe closely match the properties of a frontier quantum system (specifically, one called conformal field theory, or CFT).

Maldacena's 1997 bombshell article describing this "AdS/CFT correspondence" has been cited 22,000 times in subsequent studies - more than twice a day on average. "Trying to explore AdS/CFT-based ideas has been the main goal of thousands of top theorists for decades," he said.Peter Woit, a mathematical physicist at Columbia University.

While Maldacena himself was exploring his AdS/CFT map between dynamic spacetime and quantum systems, he made a new discovery about wormholes. He studied a particular entanglement pattern involving two sets of particles, each particle of one set being entangled with a particle of the other. maldacenashowedthat this state is mathematically dual to a rather dramatic hologram: a pair of black holes in AdS space, their interiors connected by a wormhole.

A decade passed before Maldacena realized in 2013 (in circumstances "that I honestly don't remember," he says) that his discovery could mean a more general agreement between quantum entanglement and wormhole connectivity. He coined a cryptic little equation — ER = EPR — in an email to Susskind, who got it right away. the two quicklydeveloped the conjecturetogether and write: "We argue that the Einstein-Rosen bridge between two black holes is created by EPR-like correlations between the microstates of the two black holes" and that the duality may be more general: "It's very tempting to think about itanyThe EPR-correlated system is connected by a kind of ER bridge.”

Perhaps a wormhole connects every pair of entangled particles in the universe, forging a spatial connection that chronicles their shared history. Perhaps Einstein was right in guessing that wormholes have something to do with particles.

A strong bridge

When Jafferis heard Maldacena speak about ER=EPR at a conference in 2013, he realized that the putative duality should allow you to design tailored wormholes by adjusting the entanglement pattern.

The standard Einstein-Rosen bridges are a disappointment to sci-fi fans everywhere: if one were to form, it would quickly collapse under its own gravity and detach long before a spaceship or anything else could pass. . But Jafferis envisioned drawing a thread or some other physical connection between the two sets of confused particles that encode the two mouths of a wormhole. With this type of coupling, acting on the particles on one side would cause changes in the particles on the other side, perhaps opening the wormhole between them. "Does that make the wormhole passable?" Jafferis remembers wondering. Fascinated by wormholes since childhood—a physics prodigy, he began attending Yale University when he was 14—Jafferis pursued the question “almost for fun.”

Back at Harvard, he andPing-Gao, his then graduate student, andAaron's wall, a visiting researcher finally calculated that by coupling two sets of entangled particles one can actually perform an operation on the left set that in the dual image of higher-dimensional spacetime keeps open the wormhole leading to the right mouth and pushes a qubit through.

Jafferis, Gao e WallsDiscovery 2016This traversable holographic wormhole has given researchers a new window into the mechanics of holography. "The fact that if you do the right things on the outside you can eventually walk through it also means you can see inside the wormhole," Jafferis said. "This means that it is possible to study the fact that two entangled systems are described by a connected geometry."

Within months, Maldacena and two colleagues developed the scheme and showed that the traversable wormhole could be realized in a simple scenario - "a sufficiently simple quantum system that we can imagine," Jafferis said.

The SYK model, as it is called, is a system of matter particles that interact in groups rather than the usual pairs. First described in 1993 by Subir Sachdev and Jinwu Ye, the model suddenly gained importance as theoretical physicists from 2015 onwardsAlexei Kitaevdiscovers that it is holographic. At a talk earlier this year in Santa Barbara, California, Kitaev (who became the K in SYK) filled several panels with evidence that the specific version of the model in which matter particles interact in groups of four can be mapped mathematically to a one-dimensional black . Hole in AdS space, with identical symmetries and different properties. "Some answers are the same in both cases," he told an enthusiastic audience. Maldacena sat in the front row.

Connecting the dots, Maldacena and co-authorssuggestedthat two linked SYK models could encode the two mouths of Jafferi's, Gao's and Wall's traversable wormhole. Jafferis and Gao ran with the approach. In 2019 they found their way therea specific recipeto teleport a qubit of information from one system of four-way interacting particles to another. The rotation from all directions of particle rotation translates in the dual spacetime picture into a shockwave of negative energy that sweeps through the wormhole, kicking the qubit forward and out of the mouth at a predictable moment.

"The Jafferis wormhole is the first concrete realization of ER=EPR, where it shows that the relationship holds exactly for a given system," he said.Alex Zlokapa, a graduate student at the Massachusetts Institute of Technology and co-author of the new experiment.

Wormhole in the lab

As the theoretical work unfolded, Maria Spiropulu, an accomplished experimental particle physicist who helped discover the Higgs boson in 2012, pondered how one might use nascent quantum computers for holographic experiments on quantum gravity. In 2018, she convinced Jafferis to join her growing team, along with Google Quantum AI researchers - owners of the Sycamore device.

In order to run Jafferis and Gao's wormhole teleportation protocol on the state-of-the-art, but still small and bug-prone, quantum computer, Spiropulus' team had to greatly simplify the protocol. A complete SYK model consists practically of an infinite number of particles coupled together with random forces when four-way interactions occur. This cannot be calculated; Even using all 50 available qubits would have required hundreds of thousands of circuit operations. Researchers set out to create a holographic wormhole using just seven qubits and hundreds of operations. To do this, they had to "sparsify" the seven-particle SYK model by encoding only the strongest four-way interactions and eliminating the rest, while preserving the holographic properties of the model. "It took us a few years to find a clever way to do this," Spiropulu said.

One secret of success was Zlokapa, an orchestra boy who joined the Spiropulu research group as a graduate student at Caltech. A gifted programmer, Zlokapa mapped the particle interactions of the SYK model to the connections between neurons in a neural network, and trained the system to exclude as many network connections as possible while preserving an important wormhole signature. The process reduced the number of four-way interactions from hundreds to five.

With that, the team began programming the Sycamore qubits. Seven qubits encode 14 particles of matter - seven each in the left and right SYK system, with each particle on the left entangled with one on the right. An eighth qubit, in a probabilistic combination of states 0 and 1, is then exchanged for one of the left SYK model particles. The possible states of this qubit quickly mix with the states of the other particles on the left, spreading its information evenly between them like a drop of ink in water. This is holographically dual to the qubit entering the left mouth of a one-dimensional wormhole in AdS space.

Then comes the massive spin of all the qubits, dual to a pulse of negative energy pouring through the wormhole. The rotation causes the injected qubit to be transferred to the SYK model particles on the right. Then the information spreads out, Preskill said, "like chaos running backwards" and refocuses on the position of a single particle on the right -- the entangled partner of the left particle that was swapped. Then all states of the qubits are measured. Counting zeros and ones in many experimental runs and comparing these statistics to the primed state of the injected qubits reveals whether the qubits teleport.

The researchers are looking for a peak in the data that represents a difference between two cases: If they see the peak, it means qubit rotations, which are dual to negative energy pulses, allow qubits to teleport while rotations in the opposite direction doing this are dual to normal and positive energy pulses, don't let the qubits through. (Instead, they close the wormhole.)

One night in January, after two years of incremental improvements and noise reduction efforts, Zlokapa conducted the final protocol to Sycamore remotely from his San Francisco Bay Area nursery, where he was spending winter vacation after his first semester. 🇧🇷

The peak appeared on the computer screen.

"It was becoming clearer and clearer," he said. "I sent Maria screenshots of the summit and was very excited when I wrote, 'I think we're seeing a wormhole now'." The spike was "the first sign of seeing gravity on a quantum computer."

Spiropulu says he couldn't believe the sharp, clean peak he saw. "It was very similar to when I saw the first data from the Higgs discovery," she said. "Not because I didn't expect it, but it was right in my face."

Surprisingly, despite the skeletal simplicity of their wormhole, the researchers discovered a second signature of wormhole dynamics, a subtle pattern in the way information travels and propagates between qubits, known as "size coiling." They didn't train their neural network to receive this signal as it scatters the SYK model; Therefore, the fact that the size evolution does occur is an experimental discovery about holography.

"We didn't ask for anything about this gargantuan property, but we found it just showed up," Jafferis said. This "confirms the robustness" of holographic duality, he said. "Make one [property] appear, then you get everything else, which is sort of proof that this gravity image is the right one."

The meaning of the wormhole

Jafferis, who never expected to be part of a wormhole experiment (or any other), believes that one of the most important insights is what the experiment reveals about quantum mechanics. Quantum phenomena like entanglement are usually opaque and abstract; we do not know, for example, how a measurement of particle A remotely determines the state of particle B. But in the new experiment, an indescribable quantum phenomenon -- the teleportation of information between particles -- has a tangible interpretation as a particle that gets a boost of energy and moves from A to B at a predictable speed, point qubit vision; it moves causally,” Jafferis said. Perhaps a quantum process like teleportation "always feels gravitational to this qubit. If something like that could come out of this experiment and other related experiments, it would definitely tell us something profound about our universe.”

Susskind, who was looking at today's results, said he hopes future wormhole experiments with many more qubits could be used to probe inside the wormhole to study the quantum properties of gravity. "By measuring what went through, you interrogate and see what was in there," he said. "That seems like an interesting way forward to me."

Some physicists will say that the experiment tells us nothing about our universe since it realizes a duality between quantum mechanics and anti-de Sitter space, which our universe is not.

In the 25 years since Maldacena's discovery of the AdS/CFT correspondence, physicists have searched for a similar holographic duality for de Sitter space - a map running from a quantum system to the positively energized, expanding de Sitter universe in which we live . much slower than AdS, which makes some doubt whether the de Sitter space is holographic. "Questions like 'How about this work in the more physical case of the dS?' are not new, they are very old and the subject of tens of thousands of person-years of unsuccessful effort,” said Woit, a critic of AdS/CFT research. "What is needed are completely different ideas."

Critics argue that the two types of space are categorically different: AdS has an outer boundary and dS space does not, so there is no smooth mathematical transition that can convert one into the other. And the hard limit of AdS space is exactly what makes holography easy in this scenario, providing the quantum surface from which to project space. In comparison, in our de Sitter universe, the only limits are the farthest we can see and the infinite future. These are blurred surfaces from which one can attempt to project a space-time hologram.

Renate Loell, a prominent quantum gravity theorist at Radboud University in the Netherlands, also pointed out that the wormhole experiment involves 2D spacetime — the wormhole is a filament with one dimension of space plus the dimension of time — while gravity is more complicated in 4D spacetime is. time we really live in. "It's quite tempting to delve into the complexities of 2D toy models," she said via email, "while losing sight of the different and larger challenges that await us in 4D quantum gravity." For this theory, I fail to see how quantum computers can be of much help with their current capabilities...

Most quantum gravity researchers believe that these are all difficult but solvable problems - that the entanglement pattern weaving together 4D de Sitter space is more complicated than for 2D AdS, but we can still learn lessons from general studies that support the Study holography in simpler environments. In this field, the two types of spaces, dS and AdS, are considered similar rather than different. Both are solutions of Einstein's theory of relativity, differing only by a minus sign. The dS and AdS universes contain black holes afflicted by the same paradoxes. And when you're at the back of the AdS area, away from the outer wall, you can barely tell your surroundings from the sitter.

Still, Susskind agrees it's time to get real. "I think it's time we broke the protective layer of AdS space and opened up to a world that might be more related to cosmology," he said. "The De Sitter room is another beast."

Susskind has a new idea for this. In thea formPosted online in September, he suggested that the de Sitter space could be a hologram of a different version of the SYK model - not one with quadruple particle interactions, but one in which the number of particles involved in each interaction grows with the square root the total number of particles. This SYK model's "double scale cap" "behaves more like a de Sitter than an AdS," he said. "It's far from proof, but there is circumstantial evidence."

This quantum system is more complex than what has been programmed so far, and "whether that limit is something that can be realized in the lab, I don't know," Susskind said. What seems certain is that now that there is one holographic wormhole, more will open up.