Strange new phase of matter created in the quantum computer seems to have two time dimensions

Strange new phase of matter created in the quantum computer seems to have two time dimensions

Strange new phase of matter created in the quantum computer seems to have two time dimensions

The Penrose tile pattern is a type of quasi-crystal, which means that it has an ordered but never repetitive structure. The pattern, composed of two shapes, is a 2D projection of a 5D square grid. Credit: None

By shining a laser pulse sequence inspired by the 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 still only a single flow of time, the physicists report on 20 July in Nature.

This thought-provoking feature offers a coveted advantage: Information stored in the phase is far more protected against errors than with alternative layouts currently used in quantum computers. As a result, the information can exist without being distorted much longer, an important milestone in making quantum calculation viable, says study leader Philipp Dumitrescu.

The approach’s use of an “extra” time dimension “is a completely different way of thinking about phases of matter,” said Dumitrescu, who worked on the project as a fellow at the Flatiron Institute’s Center for Computational Quantum Physics in New York City. “I’ve been working on these theory ideas for over five years, and it’s exciting to see them come to fruition in experiments.”

Dumitrescu led the theoretical component of the study with Andrew Potter of the University of British Columbia in Vancouver, Romain Vasseur of the University of Massachusetts, Amherst and Ajesh Kumar of the University of Texas at Austin. The experiments were performed on a quantum computer at the Quantinuum in Broomfield, Colorado, by a team led by Brian Neyenhuis.

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

Each of these atomic ions acts as what scientists call a quantum bit, or “qubit”. While traditional computers quantify information in bits (each represents a 0 or a 1), the qubits used by quantum computers utilize the strangeness of quantum mechanics to store even more information. Just as Schrödinger’s cat is both dead and alive in its box, a qubit can be a 0, a 1 or a mashup – or “superposition” – of both. The extra density of information and the way qubits interact with each other promises to allow quantum computers to deal with computational problems far beyond the reach of conventional computers.

However, there is a big problem: Just as looking in Schrödinger’s box seals the cat’s fate, so does interacting with a qubit. And that interaction does not even have to be conscious. “Even if you keep all the atoms under close control, they can lose quantity by talking to the environment, warming up or interacting with things in ways you have not planned,” says Dumitrescu. “In practice, experimental devices have many sources of error that can degrade the connection after only a few laser pulses.”

The challenge is therefore to make qubits more robust. To do this, physicists can use “symmetries”, mainly properties that can withstand change. (A snowflake, for example, has rotational symmetry because it looks the same when rotated 60 degrees.) One method is to add time symmetry by blowing up the atoms with rhythmic laser pulses. This approach helps, but Dumitrescu and his collaborators wondered if they could go further. So instead of just one time symmetry, they aimed to add two by using ordered but non-repeating laser pulses.

Strange new phase of matter created in the quantum computer seems to have two time dimensions

In this quantum computer, physicists created an unprecedented phase of matter that seems to have two dimensions to time. The phase can help protect quantum information from destruction for far longer than current methods. Credit: Quantinuum

The best way to understand their approach is by considering something else orderly but non-repetitive: “quasi-crystals.” A typical crystal has a regular, repeating structure, like the hexagons of a gingerbread. A quasi-crystal is still in order, but the patterns never repeat themselves. (Penrose tiles are one example of this.) Even more overwhelming is that quasi-crystals are crystals from higher dimensions projected, or pressed down, to lower dimensions. The higher dimensions can even be outside the three dimensions of the physical space: A 2D Penrose tile, for example, is a projected part of a 5-D grid.

For the qubits, Dumitrescu, Vasseur and Potter proposed in 2018 to make a quasi-crystal in time instead of space. While a periodic laser pulse will alternate (A, B, A, B, A, B, etc.), the researchers created a quasi-periodic laser pulse regime based on the Fibonacci sequence. In such a sequence, each part of the sequence is the sum of the two preceding parts (A, AB, ABA, ABAAB, ABAABABA, etc.). This arrangement, just like a quasi-crystal, is booked without repeating. And, like a quasi-crystal, it is a 2D pattern squeezed into a single dimension. The dimensional flattening theoretically results in two time symmetries instead of just one: The system essentially gets a bonus symmetry from a non-existent extra time dimension.

Actual quantum computers, however, are incredibly complex experimental systems, so whether the benefits promised by the theory would consist of qubits in the real world remained unproven.

Using Quantinum’s quantum computer, the experimental researchers tested the theory. They pulsed laser light on the computer’s qubits both periodically and using the sequence based on the Fibonacci numbers. The focus was on the qubits at each end of the 10-atom series; This was where the researchers expected to see the new phase of matter experience two time symmetries simultaneously. In the periodic test, the edge qubits remained in quantum for about 1.5 seconds – already an impressive length given that the qubits interacted strongly with each other. With the quasi-periodic pattern, the qubits remained quantum throughout the length of the experiment, approximately 5.5 seconds. This is because the extra time symmetry provided more protection, says Dumitrescu.

“With this quasi-periodic sequence, it’s a complicated evolution that cancels out all the flaws that live on the edge,” he says. “Because of that, the edge remains quantum mechanically coherent much, much longer than you expect.”

Although the findings show that the new phase of matter can function as long-term storage of quantum information, researchers must still functionally integrate the phase with the computational side of quantum computation. “We have this direct, tempting application, but we need to find a way to incorporate it into the calculations,” says Dumitrescu. “It’s an open problem we’re working on.”

Doubling the Cooper pairs to protect qubits in quantum computers from noise

More information:
Philipp Dumitrescu, Dynamic topological phase realized in a captured ion quantum simulator, Nature (2022). DOI: 10.1038 / s41586-022-04853-4.

Provided by the Simons Foundation

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