Diagram on top of cloud image
Source photo: Unsplash


How a tabletop experiment could test the bedrock of reality

By playing with quantum entanglement, physicists hope to probe their ideas about quantum gravity.

Diagram on top of cloud image
Source photo: Unsplash

Here’s a curious thought experiment. Imagine a cloud of quantum particles that are entangled—in other words, they share the same quantum existence. The behavior of these particles is chaotic. The goal of this experiment is to send a quantum message across this set of particles. So the message has to be sent into one side of the cloud and then extracted from the other.

The first step, then, is to divide the cloud down the middle so that the particles on the left can be controlled separately from those on the right. The next step is to inject the message into the left-hand part of the cloud, where the chaotic behavior of the particles quickly scrambles it.

Can such a message ever be unscrambled?

In a new paper, Adam Brown at Google in California and a number of colleagues, including Leonard Susskind at Stanford University, the “father of string theory,” discuss exactly how such a message can be made to surprisingly reappear.

“The surprise is what happens next,” they say. After a period in which the message seems thoroughly scrambled, it abruptly unscrambles and recoheres at a point far away from where it was originally inserted. “The signal has unexpectedly refocused, without it being at all obvious what it was that acted as the lens,” they say.

But the really extraordinary thing they point out is that such an experiment throws light on one of the deepest mysteries of the universe: the quantum nature of gravity and spacetime.

First some background explanation. The key to understanding this thought experiment lies in the nature of emergent phenomena. Brown and co say that quantum systems can display emergent phenomena in just the same way as ordinary systems do.

For example, when two people talk to each other, the phenomenon is hard to understand from the point of view of modeling each individual molecule in the air. The room in which they talk might contain a billion billion billion molecules, each one colliding with another every tenth of a nanosecond.

The conversation continues anyway. “Communication is possible despite the chaos because the system nevertheless possesses emergent collective modes—sound waves—which behave in an orderly fashion,” Brown and his colleagues write.

A similar phenomenon operates on the quantum level. And it is this emergent phenomenon, Brown and his colleagues argue, that refocuses the quantum message in the earlier example.

“When quantum effects are important, complex patterns of entanglement can give rise to qualitatively new kinds of emergent collective phenomena,” they write. “One extreme example of this kind of emergence is precisely the holographic generation of spacetime and gravity from entanglement, complexity, and chaos.” 

That’s why this thought experiment is the subject of so much interest. It allows physicists to think about a simple example of an emergent quantum phenomenon and how they might create and test one in the lab. 

So how might they go about such an experiment? Brown and co say there are several ways to approach it. The first step is to create a set of entangled quantum states that can then be separated into two sets to be handled separately.

One way to do this is to create a collection of entangled pairs known as Bell pairs. Brown and co note that these pairs have already been created using rubidium atoms and with trapped ions.

The next step is to insert quantum information into one half of these quantum states. The final step is to control the quantum evolution of the other half of the quantum states in a way that allows the message to reemerge.

This is not simple.

However, experiments have already been performed that accomplish such “quantum scrambling,” in which information is spread throughout a quantum system and subsequently recovered. Notably, a group at the University of Maryland, College Park, together with collaborators at the University of California, Berkeley, and the Perimeter Institute of Theoretical Physics in Waterloo, Ontario, published a paper in Nature in March 2019 describing their successful effort to do just that.

They used a quantum computer comprising a chain of nine ytterbium ions that are cooled by lasers while being held in a radio-frequency trap. The UMD researchers implemented a seven-qubit circuit in the middle seven of the nine ions. The first qubit was “scrambled” into three pairs of qubits, spreading the information it contained into six qubits in total (one of which was the original qubit). They then measured the seventh qubit, which had been paired with the sixth qubit. With a fidelity of about 80%, the seventh qubit was found to be in a quantum state indistinguishable from the original first qubit.

Interpreting this result is not straightforward, however the group performed several control experiments that, for technical reasons too subtle to explain here, confirmed their claim that the information initially encoded only in the first qubit was truly delocalized across the entire system.

“The scrambling-induced teleportation observed in our experiment can be reinterpreted as simulating the propagation of information through a traversable wormhole that connects a pair of black holes,” the Nature paper notes.

Such experiments suggest a number of exciting possibilities. The ability to play with analogues to an emergent form of spacetime make it possible to test certain ideas about quantum gravity.

Brown and co are clearly excited. They write: “The technology for the control of complex quantum many-body systems is advancing rapidly, and we appear to be at the dawn of a new era in physics—the study of quantum gravity in the lab.”

Ref: arxiv.org/abs/1911.06314 : Quantum Gravity in the Lab: Teleportation by Size and Traversable Wormholes.

Correction: January 14, 2020

This story originally said: “The bottom line is that this kind of experiment is beyond the state of the current quantum art. But it could be possible in the next few years, given the rate at which physicists are developing their quantum skills.” This statement was incorrect. The text has been edited to reflect a trapped ion experiment reported in the March 6, 2019, issue of Nature that accomplishes exactly the sort of scrambling, teleportation, and decoding that was being discussed.

This story has been further edited from the original version to reflect the fact that though the paper by Brown et al. published on November 14, 2019, is certainly thought-provoking, it is not the first paper to suggest that tabletop quantum computing experiments can be a useful and interesting way to gain insights into quantum gravity.

This story has also been edited throughout for clarity.

MIT Technology Review regrets the errors.