Diagram on top of cloud image
Source photo: Unsplash

Computing

How a tabletop experiment could test the bedrock of reality

By playing with quantum entanglement, physicists hope to probe their ideas about quantum gravity for the first time.

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?

Today, we get an answer thanks to the work of Adam Brown at Google in California and a number of colleagues, including Leonard Susskind at Stanford University, the “father of string theory.” This team shows 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 their really extraordinary claim 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, exchanging information using sound waves, the phenomenon is hard to understand from the point of view of molecular dynamics. The room in which they talk might contain 1027 molecules, each one colliding with another every 10−10 seconds or so, in a fantastically chaotic fashion.

A computer simulation of such a system would have to process 1037 bits of information every second. That makes it effectively impossible, but 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,” say Brown and co.

Exactly the same phenomenon operates on the quantum level too. And it is this emergent phenomenon that refocuses the quantum message in the earlier example.

The emergent phenomenon in question is much more significant and powerful than mere sound waves. “When quantum effects are important, complex patterns of entanglement can give rise to qualitatively new kinds of emergent collective phenomena,” say Brown and co. “One extreme example of this kind of emergence is precisely the holographic generation of spacetime and gravity from entanglement, complexity, and chaos.” In other words, one emergent quantum phenomenon forms the bedrock of reality.

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. Such an experiment would make the bedrock of reality a mere plaything for physicists to toy with.

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 large collection of entangled pairs known as Bell pairs. Brown and co say 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, which has also been achieved, albeit on a smaller scale than is necessary for Brown and co’s experiment.

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 more tricky, but physicists already know how to manipulate quantum states using electromagnetic pulses, albeit in much simpler circumstances.

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.

If these experiments can be carried out, it opens a number of exciting possibilities. The ability to play with an emergent form of spacetime makes it possible to test different ideas about quantum gravity for the first time.

Indeed, one of the breakthroughs in this paper is to show a formal mathematical link between the transmission of information across a many-body quantum system and teleportation through a wormhole in spacetime. “This suggests that we may be able to use table-top physics experiments to probe quantum gravity indirectly,” say the team.

That’s interesting work that has significant implications. Brown and co are clearly excited: “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