Strange new phase of matter created in quantum computer acts like it has two time dimensions

Scientists have generated a spectacular, never-before-seen phase of matter by beaming a sequence of laser pulses at atoms within a quantum computer that is inspired by the Fibonacci numbers. The physicists disclose their findings in Nature on July 20, stating that although there is still only one unique flow of time, the phase has the advantages of two time dimensions.

This perplexing feature provides a desired advantage: compared to other configurations now employed in quantum computers, information stored in the phase is significantly more safeguarded against mistakes. Because of this, information may remain undistorted for a far longer period of time, which is a critical step toward the viability of quantum computing, according to research lead author Philipp Dumitrescu.

An "extra" time dimension is used in the method, which "is a completely different way of thinking about phases of matter," according to Dumitrescu, who worked on the project as a research fellow at the Center for Computational Quantum Physics at the Flatiron Institute in New York City. It's thrilling to see these theoretical concepts come to fruition in trials after more than five years of labor.

Together with Andrew Potter from the University of British Columbia in Vancouver, Romain Vasseur from the University of Massachusetts, Amherst, and Ajesh Kumar from the University of Texas in Austin, Dumitrescu led the study's theoretical component. A group under the direction of Brian Neyenhuis conducted the tests on a quantum computer at Quantinuum in Broomfield, Colorado.

Ten atomic ions of the element ytterbium serve as the main components of the team's quantum computer. An ion trap generates electric fields that hold and regulate each ion separately. Laser pulses may be used to manipulate or measure each ion.

These individual atomic ions each function as what is known as a quantum bit, or "qubit." The qubits used in quantum computers take use of the peculiarities of quantum physics to store much more information than ordinary computers, which quantify information in bits, each of which either a 0 or a 1. A qubit can be a0, a1, or a mashup—or "superposition"—of both, much as Schrödinger's cat can be both dead and alive in its cage. Quantum computers are expected to be able to solve computational problems that are much above the capabilities of traditional computers because of this increased information density and the way qubits interact with one another.

However, there's a major issue: engaging with a qubit also seals the cat's doom, much like peering into Schrödinger's box did. It's not even necessary for such exchange to be intentional. "It is possible for atoms to lose their quantumness even with strict control—they can interact with their surroundings, heat up, or do other unexpected things," notes Dumitrescu. "In actual use, a few laser pulses can cause coherence to be lost due to numerous sources of error in experimental devices."

Therefore, strengthening qubits is the problem. Physicists can employ "symmetries," or qualities that are resistant to change, to achieve that. (A snowflake, for example, exhibits rotational symmetry as its appearance remains unchanged upon rotation of 60 degrees.) An approach is to introduce temporal symmetry by subjecting the atoms to periodic laser pulses. While this method is helpful, Dumitrescu and his associates questioned if they could take it a step further. Thus, by employing ordered but non-repeating laser pulses, they hoped to add two time symmetry instead of simply one.

The easiest way to comprehend their methodology is to think of something else that is non-repeating yet ordered: "quasicrystals." Similar to the hexagons in a honeycomb, the structure of a typical crystal is regular and repetitive. Order exists in a quasicrystal, but its patterns are non-repeating. (One illustration of this is Penrose tiling.) The fact that quasicrystals are crystals from higher dimensions that have been projected—or squashed down—into lower dimensions is even more astounding. The three dimensions of physical space may not even be reached by those higher dimensions: An example of a projected slice of a 5-D lattice is a 2D Penrose tiling.

In 2018, Dumitrescu, Vasseur, and Potter suggested that a quasicrystal be formed for qubits in time as opposed to space. The researchers developed a quasi-periodic laser-pulse regimen based on the Fibonacci sequence, in contrast to a periodic laser pulse, which would alternate (A, B, A, B, A, B, etc.). Each segment in such a series (A, AB, ABA, ABAAB, ABAABABA, etc.) equals the total of the two preceding segments. Similar to a quasicrystal, this arrangement is repeat-free and organized. It's also a two-dimensional pattern compressed into one dimension, similar to a quasicrystal. Theoretically, this dimensional flattening produces two temporal symmetries rather than simply one: The existence of an extra time dimension gives the system effectively an extra symmetry.

However, the practicality of the theoretical advantages in real-world qubits has not been demonstrated, as actual quantum computers are exceedingly intricate experimental devices.

To test the theory, the experimenters used Quantinuum's quantum computer. They used a sequence based on the Fibonacci numbers and fired laser light at the computer's qubits on a regular basis. The qubits at each end of the 10-atom array were the focus as it is where the researchers anticipated to find the new phase of matter simultaneously experiencing two time symmetries. The edge qubits in the periodic test maintained their quantum state for almost 1.5 seconds, which is remarkable considering the intense interaction between the qubits. For 5.5 seconds, or the duration of the experiment, the qubits maintained their quantum state with the quasi-periodic pattern. According to Dumitrescu, this is because the additional temporal symmetry offered more safety.

According to him, "there's a complicated evolution that cancels out all the errors that live on the edge with this quasi-periodic sequence." "That's why the edge remains quantum-mechanically coherent for a much longer period of time than one might anticipate."

Researchers still need to effectively integrate the new phase of matter with the computational side of quantum computing, even if the findings show that it can work as long-term quantum information storage. Dumitrescu states, "We have this direct, tantalizing application, but we need to figure out how to hook it into the calculations." "That is an open issue that we are addressing."