A strange new phase of matter created by a quantum computer acts as if it has two time dimensions


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A Penrose tiling is a type of quasi-crystal, meaning that it has an ordered but never repeating structure. The two-figure pattern is a two-dimensional projection of a five-dimensional square grid. Credit: No

By shooting a sequence of laser pulses inspired by Fibonacci numbers at atoms inside a quantum computer, physicists have created a remarkable, never-before-seen phase of matter. The phase has the advantages of two time dimensions, despite the fact that there is only one single flow of time, physicists report July 20 in Nature.

This mind-blowing property offers a welcome benefit: the information stored in the phase is much more error-proof than with the alternative settings currently used in quantum computers. As a result, information can exist without distortion for much longer, which is an important milestone on the path to the viability of quantum computing, said Philippe Dumitrescu, lead author of the study.

The use of an “extra” time dimension in this approach “is a completely different way of thinking about the phases of matter,” says Dumitrescu, who worked on the project as a research fellow at the Center for Computational Quantum Physics at the Flatiron Institute in New York. “I have been working on these theoretical ideas for more than five years, and it is very interesting to see how they are actually implemented in experiments.”

Dumitrescu led the theoretical part of the study, along with Andrew Potter of the University of British Columbia at Vancouver, Romain Wasser of the University of Massachusetts Amherst, and Ajesh Kumar of the University of Texas at Austin. The experiments were carried out on a quantum computer at the Quantinuum in Broomfield, Colorado, by a team led by Brian Nijenhuis.

The workhorses of the team’s quantum computer are 10 atomic ions of an element called ytterbium. Each ion is individually held and controlled by the electric fields generated by the ion trap and can be manipulated or measured using laser pulses.

Each of these atomic ions serves as what scientists call a quantum bit or “qubit”. While traditional computers measure information in bits (each of which represents 0 or 1), the qubits used by quantum computers use the oddity of quantum mechanics to store even more information. Just as Schrödinger’s cat is alive and dead in its box, a qubit can be 0, 1, or a collage—or “superposition”—of both. This extra information density and how qubits interact with each other promises to enable quantum computers to solve computational problems beyond the reach of conventional computers.

However, there is a big problem: in the same way as looking into the Schrödinger box, the fate of the cat is decided, so is the interaction with the qubit. And this interaction doesn’t even have to be intentional. “Even if you keep all the atoms under tight control, they can lose their quantumness by talking to the environment, heating up or interacting with things differently than you planned,” says Dumitrescu. “In practice, experimental devices have many sources of errors that can degrade coherence after just a few laser pulses.”

So the challenge is to make qubits more reliable. To do this, physicists can use “symmetries”, that is, properties that can be changed. (A snowflake, for example, has rotational symmetry because it looks the same when rotated 60 degrees.) One method is to add time symmetry by exploding atoms with rhythmic laser pulses. This approach helps, but Dumitrescu and his staff wondered if they could go further. Thus, instead of one temporal symmetry, they sought to add two, using ordered but non-repetitive laser pulses.

A strange new phase of matter created by a quantum computer acts as if it has two time dimensions

In this quantum computer, physicists have created a never-before-seen phase of matter that behaves as if time has two dimensions. The phase can help protect quantum information from destruction for much longer than existing methods. 1 credit

The best way to understand their approach is to consider something else ordered but non-repetitive: “quasicrystals.” A typical crystal has a regular, repeating structure, similar to hexagons in a honeycomb. There is still order in a quasicrystal, but its patterns never repeat themselves. (The Penrose tile is one example of this.) Even more stunning is that quasicrystals are crystals from higher dimensions, projected or flattened, into lower dimensions. These higher dimensions may even extend beyond the three dimensions of physical space: for example, a two-dimensional Penrose tiling is a projected fragment of a five-dimensional grid.

As for qubits, Dumitrescu, Vasser and Potter proposed in 2018 to create a quasi-crystal in time rather than space. While the periodic laser pulse alternated (A, B, A, B, A, B, etc.), the researchers created a quasi-periodic laser pulse mode based on the Fibonacci sequence. In such a sequence, each part of the sequence is the sum of the two previous parts (A, AB, ABA, ABAAB, ABAABABA, etc.). This arrangement, like the quasicrystal, is ordered without repetition. And, like a quasi-crystal, it is a two-dimensional pattern compressed into one dimension. This alignment of dimensions theoretically results in two temporal symmetries instead of one: the system essentially gains additional symmetry from a non-existent extra temporal dimension.

However, true quantum computers are incredibly complex experimental systems, so it remains unproven whether the benefits promised by theory hold up in real-world qubits.

Using the Quantinuum quantum computer, the experimenters tested the theory. They sent laser light at the computer’s qubits both periodically and using a sequence based on Fibonacci numbers. The focus was on the qubits at both ends of a 10-atom string; it was here that the researchers expected to see a new phase of matter experiencing two temporal symmetries at once. In the periodic test, the outermost qubits remained quantum for about 1.5 seconds—already an impressive time considering that the qubits strongly interacted with each other. With a quasi-periodic pattern, the qubits remained quantum throughout the entire experiment, about 5.5 seconds. That’s because the extra temporal symmetry offered more protection, Dumitrescu says.

“With this quasi-periodic sequence, a complex evolution occurs that eliminates all the errors living on the edge,” he says. “Because of this, the edge remains quantum mechanically coherent for much, much longer than you would expect.”

Although the results show that the new phase of matter can serve as a long-term storage of quantum information, researchers still need to functionally integrate this phase with the computational side of quantum computing. “We have this direct, enticing application, but we need to find a way to connect it to computing,” says Dumitrescu. “This is an open issue that we are working on.”

Cooper pair doubling to protect qubits in quantum computers from noise

Additional Information:
Philip Dumitrescu, Dynamic Topological Phase Implemented in the Trapped Ion Quantum Simulator, Nature (2022). DOI: 10.1038/s41586-022-04853-4.

Contributed by the Simons Foundation

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