Did quantum computers create wormholes False!
Last fall, a group of physicists announced that they had transmitted a quantum bit through a holographic wormhole in a quantum computer. Now, another group believes this is not the case.
In January 2022, a small team of physicists watched breathlessly as data flowed from Google's quantum computer, Sycamore.
(Sycamore)" from Google's quantum computer: a sharp spike indicated that their experiment had worked. They mixed a unit of quantum information into a cloud of wispy particles and watched it emerge from a connected cloud: it was like watching an egg scrambling itself in one bowl and unscrambling itself in another.
Simulation of a traversable wormhole Hamiltonian from the SYK model in the 2022 experiment.
In several key respects, the event closely resembles a familiar movie scene: a spacecraft enters a black hole only to jump out of a completely different one. Wormholes, as these theoretical paths suggest, are a classic gravitational phenomenon. With the experimental results, scientists theoretically have reason to believe that the quantum bit has passed through a quantum system that behaves exactly like a wormhole (a so-called "holographic wormhole") - and that is exactly what the researchers have concluded. When it was published in November, the experiment was featured on the cover of the journal Nature and was widely reported in the media.
Now, another group of physicists have analyzed the results and say with certainty that while the experiment may have produced something vaguely resembling a wormhole, it was not a holographic wormhole in any sense of the word. Based on the new analysis, the researchers began to wonder what exactly the teleportation experiment had to do with gravity after all - experiments in which the evidence for a gravitational explanation was waning.
However, the earlier group did teleport something on the Sycamore chip, and they did so in a way that (at least on the surface) looked more like a wormhole than anything produced by the earlier experiments. The controversy over how to interpret the experiment involves the rapid development of holography: holography functions as a kind of mathematical 3D glasses that allow physicists to see a quantum system as a gravitational system. The study of wormholes through gravitational lensing and the discovery of new ways to transmit quantum information have raised hopes that such quantum experiments could one day go in another direction: detecting quantum gravity in the laboratory.
Because of quantum entanglement, quantum systems can show non-classical patterns of association, even when the parts are far apart. The holographic principle tells us that some non-gravitational quantum states have an alternative description in terms of higher-dimensional gravitational states - an alternative description known as a holographic dyad. in 2022, Jafferis et al. produced some kind of highly entangled quantum state between the two halves of a quantum computer, choosing its holographic dyad to be one called "emergent wormhole" (emergent wormhole), an entity that extends between two external regions. They then simulated a message that travels through this wormhole.
But the wormhole debate highlights the fact that determining when holography works, and whether quantum gravity is possible to obtain on a quantum computer, may require greater subtlety than physicists imagine, and greater subtlety. So, does quantum gravity exist in the laboratory?
Wormholes have long existed in the works of science fiction writers who needed a mechanism to move characters quickly and traverse the vastness of space. The wormholes that emerged from Einstein's theory of gravity initially seemed highly improbable, requiring tricky manipulations of spacetime that inevitably led to the "grandmother paradox.
That changed in 2016, when Harvard's Ping Gao, Daniel Jafferis and Aron Wall, then at the Institute for Advanced Study, discovered an unexpectedly simple and paradox-free way to prop open a wormhole with a negative energy shock wave.
"It was quite beautiful." Hrant Gharibyan, a quantum physicist at Caltech, said, "This development started the whole thinking in this direction."
This work is based on one of the more popular trends in modern physics - holography.
Holography involves a profound study of duality relations (dualities). On the surface, dual systems look completely different: they have different parts and operate according to different rules; however, if two systems are dual, every aspect of one can be precisely related to some element of the other (e.g., electric and magnetic fields are dual). One of the major discoveries of modern physics is that duality also seems to link certain gravitational systems to quantum systems.
For example, we can consider a collection of interacting particles entirely within the framework of quantum theory. Or, like putting on a pair of 3D glasses, we can think of the collection of particles as a black hole governed by the rules of gravity.
Physicists have spent decades developing mathematical "dictionaries" that allow them to translate quantum elements into gravitational elements and vice versa. They observe how particles, black holes and wormholes shift between the two perspectives. Calculations that are difficult to perform from one perspective are often easier from the other. A major vision of the field is to develop mysterious rules of quantum gravity by studying better-understood quantum theories.
However, problems abound. Does every conceivable quantum theory suddenly become a theory of gravity when viewed in a holographic fashion? Will physicists be able to understand gravity in our universe by finding its better quantum twin? No one knows.
In 2016, Gao, Jafferis, and Wall had already suggested that wormholes might have a quantum explanation: invisible transmission of quantum information. A few years later, another team fleshed out their speculation.
Daniel Jafferis, a theoretical physicist at Harvard University, helped develop the wormhole teleportation protocol. He was also one of the leaders of the wormhole team last year.
In 2019, Gharibyan and his collaborators translated traversable wormholes into quantum language, unveiled a curious quantum experimental step, and demonstrated the nature of holography. Wearing 3D glasses, the experimenter is presented with a wormhole: objects enter one black hole, cross a kind of space-time bridge, and leave the other; however, removing the glasses, the experimenter is presented with a double quantum system.
The two black holes become two huge clouds of particles, the space-time bridge becomes quantum entanglement, and the act of crossing the wormhole becomes an event that seems quite amazing from a quantum perspective: a particle carrying a quantum bit enters an entanglement cloud, and subsequently, that quantum bit leaves the entanglement cloud as another particle. This is like seeing a butterfly torn apart by a hurricane in Houston, while seeing an identical butterfly jump out of a typhoon in Tokyo.
This process makes a lot of sense. But entangled clouds of particles are not the wormholes of our universe - although they are equivalent to wormholes. That means they behave in a way that matches each other for anything a traversable wormhole can do: including transmitting a quantum bit.
That's what the team announced in their November Nature paper. They simulated the behavior of two clouds of entangled particles in a quantum computer and performed a stealthy transfer of states that captured the basic information about traversing a wormhole from a holographic perspective.
But this is not the only way to explain their experiment - gravity is not always essential.
Gravity scrambles information in a very specific way. In fact, theorists believe that black holes must be the most effective scramblers in nature. But when Gharibyan and his colleagues used particle clouds perturbed by quantum rules different from those of gravity, they realized that these particle clouds could still be transported by perturbation, albeit less efficiently. And when they looked at these "alternative clouds" through holography, they saw nothing - no wormholes.
A team led by Harvard physicist Norman Yao may have falsified last year's wormhole paper.
Gharibyan's group and another group led by Norman Yao at Berkeley (who has since moved to Harvard) published two simultaneous papers in 2021 that put it all together.
These papers listed a number of features that distinguished gravitational teleportation from teleportation via more ordinary kinds of perturbations. In particular, they identify a feature of all quantum systems - size winding - that can be holographically linked to the speed at which a particle falls into a wormhole.
Last spring, the scientists conducted an experiment on two quantum computers, one operated by IBM and the other by Quantinuum, to achieve invisible state transfer through perturbation. They called their teleportation demonstration "wormhole-inspired" because they knew their quantum model used a non-gravitational perturbation method. At the time, they suspected that a true experimental demonstration of gravitational teleportation would take a decade or more.
It is important to note that these quantum computers do not really contain clouds of particles; instead, they contain quantum bits (quantum bits can be made from actual or artificial atoms). When scientists program a computer, they need to change the quantum bits according to the Hamiltonian function; the Hamiltonian function describes how the quantum bits change from one moment to the next. In effect, this equation allows them to customize the laws of quantum physics for quantum bits: when the computer runs, it performs a kind of simulation - a simulation of how a real cloud of particles governed by these laws would act.
Here's the problem: in order to explicitly demonstrate gravitational transport, a large particle cloud is needed. How big? The bigger, the better. Theorists have done all the math with essentially infinitely large particle clouds; meanwhile, researchers generally agree that 100 particles per cloud is enough to make wormholes appear.
However, as the number of particles increases, the Hamiltonian equations become too large to be realized. If particles are modeled using a more tractable model of gravity (the SYK model), the Hamiltonian equations must reflect the fact that each member of a set of particles can directly affect every other member.
The Hamiltonian equation for 100 densely connected particles is an equation with a staggering 3,921,225 terms: this is far beyond what today's quantum computers can model with a few dozen quantum bits. Even if one were willing to settle for a fuzzy wormhole pairing with only 20 particle clouds, the Hamiltonian equation would have 4,845 terms - prohibitive. This obstacle is a key reason why the team believes that real wormhole simulations are still a decade away.
Then last November, a team of researchers led by Jafferis, Joseph Lykken of Fermi National Accelerator Laboratory and Maria Spiropulu of Caltech announced that they had run a perfect quantum experiment with just seven particles, a key sign that is thought to establish the existence of a gravitational dyad, and therefore a wormhole; even more surprising is that they were able to cram the behavior of this seven-particle system into a Hamiltonian equation with only five terms.
Last year's experiments were conducted on seven quantum bits of Google's Sycamore quantum computing chip.
The core of the group's work was to sort out in a new way those many particle-to-particle connections described by the SYK Hamiltonian. Many physicists have "diluted" the given quantum SYK model by removing random terms, finding that simpler versions can maintain the holographic properties of the original Hamiltonian.
Instead of removing random connections, Jafferis and his co-workers thought it would be better to use machine learning to intelligently prune only those connections that do not affect the ability to pass invisible states.
The researchers targeted the 10-particle SYK model: it has a 210-item Hamiltonian. They simulated the invisible transmission state between the 10-particle cloud on a standard computer and designed a machine learning algorithm that simplified the Hamiltonian model as much as possible without destroying its transmission capability. The algorithm returned an extremely sparse Hamiltonian equation that measured only five terms, capturing the invisible transfer state between two seven-particle clouds. The equation is simple enough to run on Google's Sycamore quantum processor, which is a noteworthy achievement.
The Sycamore experiment confirmed that the Hamiltonian can perform invisible state transfer, just as it was trained to do. But what really excited the researchers was that the gang of quantum bits also showed perfect dimensional entanglement - which should be the hallmark of a gravitational dyad. The researchers appear to have done the equivalent of boiling down a tornado into a handful of molecules that, despite being largely unable to interact, still manage to maintain the characteristic funnel shape.
Many in the field have been struck by the simplicity of this model. In particular, Norman Yao and his Berkeley colleagues Bryce Kobrin and Thomas Schuster set out to investigate how such a simple model could capture the ineffable and chaotic gravity.
On Feb. 15, the three released their findings, which involve analyzing the mathematical properties and behavior of the simple Hamiltonian of Team Nature; the paper is not yet peer-reviewed. Their main finding is that this simple model deviates from its parent gravity model in key ways. The team believes that these discrepancies mean that the signal that the researchers thought was a sign of gravity no longer applies, and because of this, the best description of what the Jafferis group saw last year was not gravitational transport.
A comparison of the size winding behavior of established models and other random small-size full-flux Hamiltonian quantities.
The researchers also proposed a non-gravitational explanation for the supposed features of holography. The five-term Hamiltonian does have this feature, but so do the other random five-term permutations of the Hamiltonian they tested. Moreover, when they tried to increase the number of particles while maintaining the commutation feature, the size winding signal should have strengthened. Instead, it disappeared. The physicists came to a conclusion that the researchers had not previously grasped because no one had studied this simple model in a holographic way: many small Hamiltonians that are perfectly commutative seem to have perfect size winding, even though these models have no gravitational duality. This finding implies that in small systems, perfect size winding is not a sign of gravity - it is just a side effect of the system.
For now, both groups have declined to comment while they resolve their differences through peer-reviewed publications. the Yao group has submitted their analysis to Nature, and the Jafferis, Lykken, and Spiropulu groups will likely respond in the near future.
Reference links:
[1] https://arxiv.org/abs/2205.14081
[2]https://arxiv.org/pdf/2302.07897.pdf
[3] https://www.quantamagazine.org/wormhole-experiment-called-into-question-20230323/#comments
