How can science detect a wormhole

Cosmology: wormholes in the laboratory

How long it takes for a system to be encrypted depends on how the particles it contains interact with one another. To determine the encryption rate, scientists use what is known as a Hamilton operator, from which the most important physical properties of a system can be calculated. Black holes stand out: It follows from their Hamilton operator that they encrypt quantum information as quickly as possible (“Spectrum” June 2019, p. 24).

That led Hayden and Preskill to their conclusion. If a particle falls into a black hole, the associated information is mixed almost instantaneously with the information it contains. The Hawking radiation that then emerges is therefore almost instantaneously entangled with the new state of the black hole, which means that it also contains the new information.

If the AdS / CFT correspondence is correct, the physicists' results provide a solution to the information paradox. In order to get the information about a particle that has passed an event horizon, one would have to wait until it is half vaporized - a period of time that is far beyond the previous age of our universe.

Entangled black holes

Because of this, some scientists were not satisfied with the solution. They were looking for another way to get to the information they had swallowed - and they found it. In 2016, Ping Gao and Daniel Jafferis from Harvard University, along with Aron Wall from the Institute for Advanced Study, asked themselves what would happen if a black hole were entangled with something other than Hawking radiation, such as a second black hole. According to their calculations, this would allow the information that falls into the first collapsed star to be read from the second. According to the researchers, if the entire matter of the galactic monsters is entangled, a qubit swallowed by the first black hole would be registered practically immediately in the second.

When Gao and his colleagues examined the hypothetical process more closely, they noticed that under certain circumstances it resembles a teleportation. As early as 1997, physicists succeeded in teleporting a particle successfully in the laboratory for the first time. To do this, they used the properties of entanglement: They created two entangled particles and transferred the quantum state of the first to the second. This can then no longer be distinguished from the original first particle. Such an event is identical to the one in which the first particle is destroyed in one place and instantly appears in a different place - a teleportation.

If a particle falls into a black hole that is entangled with another, it is teleported to the second collapsed star as quickly as possible. This is because the information in the first black hole is shared by all the particles in it at maximum speed. Because the black hole is coupled to the second, the latter receives almost instantly all information about the swallowed particle.

This is how quantum information theorists interpret the process. However, if you take the AdS / CFT correspondence seriously, the channel between the entangled collapsed stars corresponds to a wormhole. In this view, the qubits travel through an abbreviation in space-time.

Quantum mechanical circuit made up of entangled, ultracold ions

While wormholes are consistent with general relativity, it was previously believed that they were impassable: if they really existed, nothing could be sent through them. In their work, Gao, Jafferis and Wall were able to show that the AdS / CFT correspondence allows wormholes to be traversed.

If the correspondence is correct, there is no need to deal directly with interlaced black holes in the AdS image. Instead, researchers can use quantum systems on the CFT side that are completely equivalent. Nezami and Brown have now worked with the renowned string theorist Leonard Susskind from Stanford University, among others, to work out a way of implementing such an experiment.

To do this, they were looking for a quantum system whose Hamilton operator corresponds to that of a black hole into which a qubit of information falls. Because collapsed stars mix up information as quickly as possible, a comparable quantum encryption system would have to be created in the laboratory.

In fact, at the beginning of 2019, physicists demonstrated such a mixing of qubits in the laboratory. At the suggestion of Yao and his colleague Beni Yoshida, Christopher Monroe and his team in Maryland created a quantum mechanical circuit from entangled, ultracold ions. The researchers used electromagnetic traps to trap the particles so that the ions were lined up in a row.

One of the greatest challenges of the experiment was to distinguish quantum encryption from other processes. Because it is impossible to completely isolate a system from the external environment, decoherence always occurs in experiments with microscopic particles, for example. Like encryption, this effect occurs when particles interact with one another. In the case of decoherence, the particles interact with particles from the environment, whereby the information from the quantum system slowly escapes and is thus irretrievably lost.

That's the big difference between decoherence and encryption: the latter can be reversed. In practice, decoherence can never be completely avoided, which makes it the ghost of quantum computers. Therefore, to ensure that the results provided are correct, each calculation must be completed before the phenomenon begins.

Ultra-cold quantum systems as wormholes

Usually a quantum system begins to interact with its environment before encryption can occur. That makes it so difficult to prove the latter within a system. But Monroe's team found that the two effects can be distinguished from each other by quantum teleportation.

To do this, the researchers used a circuit made up of seven entangled ytterbium ions arranged in a row. They split the system into two parts with three ions each; they later used the leftover particle for teleportation. By deliberately disrupting both subsystems in different ways, they mixed up the information contained therein. Now all they had to do was prove that the quantum states of the subsystems were actually encrypted - and that they had not caused decoherence.

To do this, they teleported the remaining particle from one end of the ion row to the other and back again. At the beginning of the experiment, all particle states were entangled, so teleportation was possible at that time. However, the disturbance mixed the states - if decoherence had set in, part of the information would have disappeared. In this case, the particle could not be teleported. If, on the other hand, the information is still available in the system, but only strongly mixed, the process can succeed because all states are entangled with one another. In fact, the researchers managed to successfully teleport the ytterbium ion 80 percent of the time. This enabled them to experimentally demonstrate quantum encryption for the first time.

"To simulate a realistic universe governed by Einstein's equations, you need systems that are very difficult to manufacture in the laboratory."
(Juan Maldacena)

Brown and his team are now suggesting that similar quantum mechanical circuitry could be used to recreate a passable wormhole that teleports a qubit from one black hole to another. The black holes would consist of entangled ions. Then you would have to introduce a qubit into one of the systems that encrypts it. After a certain period of time, the information would reappear unencrypted in the second quantum system. The fact that the qubit is transmitted across the systems is not really surprising - after all, the ions are coupled to one another. What is astonishing, on the other hand, is that the information in the second system does not have to be deciphered, although the first black hole had completely mixed it up.

When the theoretical physicist Brian Swingle from the University of Maryland spoke to his colleague Monroe about such an experiment in October 2019, the latter realized that the setup required was more or less the same as the one he and his team had used to prove quantum encryption . With this the experiment described by Swingle could definitely be realized.

As impressive as such an experiment would be, it cannot currently mimic the spacetime of our universe. Instead, the quantum systems would correspond to a simplified model of the cosmos, namely an anti-de-sitter space. "To simulate a realistic universe governed by Einstein's equations, you need systems that are very difficult to manufacture in the laboratory," says Maldacena.

Should the results of the proposed experiment confirm the researchers' predictions, it does not necessarily follow that the AdS / CFT correspondence is correct. Because such an experiment can just as well be viewed from a purely quantum physical point of view - you don't necessarily have to use the holographic principle to predict the outcome of the experiment. However, some of the predicted phenomena are easier to describe in terms of correspondence, for example the teleportation of a particle, than passage through a wormhole. "While one could derive all of this with the Schrödinger equation, there is a much simpler explanation that invokes black holes," says Brown.

It is hard to believe that the collapsed stars are actually related to a handful of cooled ions. However, if the AdS / CFT correspondence turns out to be correct, then the quantum systems would be more than the analogue of a black hole - they would be completely equivalent.

glossary

Microscopic particles can interact with each other crossed be. This means that the state of one is directly related to that of the other. As a result, entangled systems influence each other almost instantly.

The Event horizon of a black hole corresponds to a distance from which an object can no longer escape the collapsed star. Even massless photons are irretrievably sucked into the black hole at this point.

If information - for example in the form of a qubit - falls into a black hole, it becomes encrypted: Properties such as mass, energy or charge mix so strongly with those of the rest of the matter that it seems impossible to ever get to the information again.

Decoherence occurs when a quantum system interacts with its environment. This changes the original state of the system, and the information it initially contained is irretrievably lost.