Physicists used a quantum computer to perform a new kind of quantum teleportation, the ability of quantum states to be transported between distant places, as if information could travel instantaneously. Although teleportation is an established technique in quantum technology, the purpose of the latest experiment was to simulate the behavior of a passage called a “wormhole” through a virtual universe.
The researchers behind the experiment, described in Nature November 301, say it’s a step towards using ordinary quantum physics to explore ideas about abstract universes where gravity and quantum mechanics seem to work harmoniously together. Quantum computers could help develop a quantum theory of gravity in these “toy” universes (the development of a quantum theory of gravity for our own universe is one of the biggest open questions in physics). “It’s a test of quantum gravity ideas on a real experimental test bed in the lab,” says Maria Spiropulu, a particle physicist at the California Institute of Technology who led the study.
Tunnels in space-time
Physicists Albert Einstein and Nathan Rosen proposed the idea of wormholes – passages through spacetime that could connect the centers of black holes – in 1935. They calculated that in principle, wormholes were allowed by Einstein’s general theory of relativity, which explains gravity as an effect of the curvature of spacetime. (Physicists soon realized that even if wormholes existed, they were unlikely to allow anything like the interstellar travel that characterizes science fiction.)
Because they were working with an alien toy universe, the latest research didn’t simulate anything resembling the kind of wormhole envisioned by Einstein and Rosen that could possibly exist in our universe. But their teleportation experience can be interpreted as analogous to a wormhole in their virtual system – quantum information introduced on one side of the researchers’ “wormhole” reappeared on the other side.
“The surprise is not that the message got through in any form, but that it got through in the clear,” write the authors of an accompanying News and Views article. “However, this is easily understood from the gravitational description: the message is coming in clear on the other side because it has passed through the wormhole.”
The experiment was inspired by previous research linking the physics of alien universes and their own version of gravity to a more standard, but still virtual, quantum system. The main idea is that some abstract version of spacetime emerges from the collective behavior of ordinary quantum particles living in a sort of “shadow world” – similar to how a two-dimensional hologram can create the illusion of a three-dimensional image. This “holographic” behavior dictates how emerging spacetimes bend over themselves, producing the effects of gravity.
Although physicists do not yet know how to directly write quantum theories of gravity for emerging universes, they do know that such phenomena should be fully encapsulated in shadow world physics. This means that gravitational phenomena such as black holes – which still pose puzzles to theoretical physicists – or wormholes must be compatible with quantum theory.
The latest experiment follows a scheme that co-author Daniel Jafferis, a theoretical physicist at Harvard University in Cambridge, Massachusetts, and his collaborators proposed in 2017.2. This work has focused on the simplest of these holographic correspondences, known as SYK after the initials of its creators. In this world of scale models, space has only one dimension instead of three.
In the latest study, Jafferis and his colleagues simulated an even slimmer version of such a hologram using the quantum bits, or qubits, of Google’s Sycamore processor. They expected their simulated quantum particles to replicate certain behaviors of gravity in the virtual universe, but they were limited by the capabilities of today’s quantum computers. “We needed to find a model that somehow preserves the properties of gravity and that we could code on a quantum processor that has a limited amount of qubits,” says Maria Spiropulu, a particle physicist at the California Institute of Technology who led the project. ‘study. “We reduced it to a baby model and verified that it preserved gravitational dynamics.”
“Before working on this project, it was not obvious that a system with such a small number of qubits could exhibit this phenomenon,” adds Jafferis.
Some researchers believe this line of research is a promising avenue for developing a quantum theory of gravity for our own Universe, although others see it as a dead end. The theory being tested at Google’s lab “only has a very tangential relationship to possible theories of quantum gravity in our universe,” says Peter Shor, a mathematician at the Massachusetts Institute of Technology in Cambridge.
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