By shining a laser pulse sequence inspired by Fibonacci numbers on atoms inside a quantum computer, physicists have created a unique, unseen phase of matter. The phase has benefits in two dimensions of time even though there is only a singular course of time, physicists reported July 20 in NATURE.
This mind-blocking property offers a sought-after advantage: The information stored on stage is better protected against errors than the alternative setups currently used in quantum computers. As a result, information can exist without disruption for a long time, an important milestone for making quantum computing viable, according to the study’s lead author Philipp Dumitrescu.
Using the method of an “additional” time dimension “is a completely different way of thinking about the phases of matter,” said Dumitrescu, who works on the project as a research fellow at the Flatiron Institute’s Center. for Computational Quantum Physics in New York City. “I’ve been working on these theoretical ideas for over five years, and seeing them actually realized in experiments is exciting.”
Dumitrescu led the theoretical side 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 Quantinuum in Broomfield, Colorado, by a team led by Brian Neyenhuis.
The team’s quantum computer workhorses are 10 atomic ions in an element called ytterbium. Each ion is individually held and controlled by electric fields created by an ion trap, and can be manipulated or measured using laser pulses.
Each of these atomic ions serves what scientists call a quantum bit, or “qubit.” While traditional computers count information in pieces (each representing a 0 or a 1), the qubits used in quantum computers use the strangeness of quantum mechanics to store more information. Just as Schrödinger’s cat is dead and alive in its box, the qubit can be 0, 1 or a mashup — or “superposition” —of the two. That increased density of information and the way the qubits interact with each other promises to allow quantum computers to deal with computational problems that are not accessible to conventional computers.
There’s a big problem, though: Just as watching Schrödinger’s box covers up the cat’s fate, so does dealing with a qubit. And that interaction doesn’t have to be intentional. “Even if you keep all the atoms under tight control, they can lose their number by communicating around them, heating up or interacting with objects in ways you didn’t plan,” he said. Dumitrescu. “In practice, experimental devices have multiple sources of error that can weaken connectivity after a few laser pulses.”
So, the challenge is to make the qubits stronger. To do that, physicists can use “symmetries,” which are important properties that keep changing. (A snowflake, for example, has rotational symmetry because it looks the same when rotated 60 degrees.) One way is to increase the symmetry of time by exploding atoms using rhythmic laser pulses. This approach helped, but Dumitrescu and his colleagues wondered if they could still move on. So instead of just one -time symmetry, they aimed to add two by using ordered but not repetitive laser pulses.
The best way to understand their approach is to consider something that is ordered but not repeated: “quasicrystals.” A typical crystal has a regular, repetitive structure, like the hexagons of a ladle. The quasicrystal still has order, but its patterns can no longer be repeated. (Penrose tiling is an example of this.) Even more confusing is that quasicrystals are crystals from higher dimensions plotted, or curved, to lower dimensions. Those higher dimensions can go beyond three dimensions of physical space: A 2D Penrose tiling, for example, a planned cut of a 5-D lattice.
For qubits, Dumitrescu, Vasseur and Potter in 2018 proposed to create a quasicrystal in time rather than space. As a periodic laser pulse alternates (A, B, A, B, A, B, etc.), the researchers created a quasi-periodic laser-pulse regimen 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 a quasicrystal, is ordered without repetition. And, similar to a quasicrystal, it is a 2D pattern broken down into one dimension. That dimensional flattening theoretically results in two time symmetries instead of just one: The system in essence derives a bonus symmetry from an infinite time dimension.
Actual quantum computers are complex experimental systems, however, so whether the benefits promised in theory will last in qubits in the real world remain unproven.
Using Quantinuum’s quantum computer, the expertists tested the theory. They pulse laser light into computer qubits every now and then and use a sequence based on Fibonacci numbers. Focus rather than qubits at any top of the 10-atom lineup; There the researchers expect to see a new phase in matter that will experience two symmetries at a time. In the periodic test, the edge qubits remained quantum for about 1.5 seconds — already an impressive length because the qubits interact so strongly with each other. With the quasi-periodic pattern, the qubits remained quantum for the entire length of the experiment, about 5.5 seconds. That’s because excessive time symmetry provides additional protection, according to Dumitrescu.
“With this quasi-periodic sequence, there’s a complex evolution that cancels out all the errors that live in the interior,” he said. “As a result, the edge stays very quantum-mechanically coherent, longer than you’d expect.”
Even if the findings show that the new phase of matter can act as a long-term storage of quantum information, researchers still need to integrate the computational side of quantum computing. “We have this straightforward, exciting application, but we have to find a way to hook it into the calculations,” Dumitrescu said. “That’s an open problem we’re working on.”
Doubling Cooper pairs to protect qubits in quantum computers from noise
Philipp Dumitrescu, Dynamical topological phase accomplished in a trapped-ion quantum simulator, NATURE (2022). DOI: 10.1038 / s41586-022-04853-4. www.nature.com/articles/s41586-022-04853-4
Provided by the Simons Foundation
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