When the ancient Incas wanted to archive tax and census records, they used a device made up of a number of strings called Quibo, in which the data is encoded in the form of a node. Fast forward several hundred years, physicists are on the way to it Develop a more complex modern equivalent. they “Quibo“It is a new phase of matter created inside a quantum computer, its strings are atoms, and the knots are created by patterns of laser pulses that effectively open up a second dimension of time.
This is not quite as incomprehensible as it first appeared. The new phase is one of many within the family of so-called topological phases, which were first identified in the 1980s. These materials display arrangement not based on how their components are arranged – such as the regular spacing of atoms in the crystal – but on the basis of their dynamic movements and interactions. Creating a new topological phase – a “new phase of matter” – is as simple as applying new combinations of electromagnetic fields and laser pulses to impart order, or “symmetry”, to the motions and states of atoms of matter. Such symmetries can exist in time rather than space, for example in induced repetitive motions. Time symmetries can be difficult to see directly but can be revealed mathematically by imagining real-world material as a low-dimensional projection from a higher-dimensional virtual space, similar to how a two-dimensional hologram is a low-dimensional projection of a three-dimensional object. In the case of this newly created phase, which appears in a filament of ions (electrically charged atoms), its symmetries can be distinguished by considering it as a substance that exists in a higher-dimensional reality and two time dimensions.
“It is very exciting to see this unusual phase of matter realized in an actual experiment, especially because the mathematical description relies on an ‘extra’ theoretical dimension of time,” says one of the team. Philip Domitrescu, who was working at the Flatiron Institute in New York City when the experiments were conducted. A paper describing the work has been published in temper nature On July 20.
Opening a portal to an additional time dimension – even just a theoretical one – sounds exciting, but it wasn’t the original plan for physicists. “We were very excited to see what kinds of new phases could be created,” says study co-author. Andrew Potter, a quantum physicist at the University of British Columbia. Only after visualizing the proposed new stage did team members realize that it could help protect data processed in quantum computers from errors.
Standard classical computers encode information as strings of bits – 0 or 1 – while the expected power of quantum computers stems from the ability of quantum bits, or qubits, to store values of either 0 or 1, or both at once (think Schrödinger’s cat, which can be both live and dead). Most quantum computers encode information in the state of each qubit, for example into an internal quantum property of a particle called spin, which can point up or down, corresponding to 0 or 1, or both at the same time. But any noise – say a stray magnetic field – can wreak havoc on a carefully crafted system by voluntarily spinning permutations and even completely destroy quantum effects, thus stopping the calculations.
Potter likens this loophole to transmitting a message using pieces of string, in which each string is arranged in an individual letter shape and laid out on the ground. “You can read it well until a little breeze comes in and shoots a letter away,” he says. To create a more error-resistant quantum material, Potter’s team looked at topological phases. In a quantum computer that exploits structure, information is not encoded locally in the case of each qubit but is woven across matter globally. “It’s like a knot that’s hard to undo — like a quipu,” Potter says, the Inca’s mechanism for storing enums and other data.
The study co-author adds: “Topological phases are interesting because they provide a way to guard against errors built into the material.” Justin Bonet, a quantum physicist at Quantinuum in Broomfield, Colorado, where the experiments were conducted. “This differs from traditional error-correction protocols, where they constantly take measurements on a small part of the system to check if there are errors and then step in and correct them.”
Quantinuum’s H1 quantum processor consists of 10 qubits — 10 ytterbium ions — in a vacuum chamber, where the lasers tightly control their positions and states. Such an “ion trap” is a standard technique that physicists use to manipulate ions. In their first attempt to create a topological phase that is stable against errors, Potter, Domitrescu and their colleagues sought to impart simple time symmetry to the processor by transmitting periodic kicks to ions—all lined up in one dimension—with regular repeating laser pulses. “Our calculations on the back of the envelope indicate that this would protect [the quantum processor] Potter says. This is similar to how a steady drum beat can maintain the rhythm of multiple dancers.
To see if they were right, the researchers ran the program several times on a Quantinuum processor and checked each time to see if the resulting quantum state of all the qubits matched their theoretical predictions. “It didn’t work at all,” Potter says with a laugh. “Completely incomprehensible things were coming out.” Each time, the accumulation of errors in the system degrades its performance within 1.5 seconds. The team soon realized that just adding a one-time symmetry wasn’t enough. In fact, rather than preventing the qubits from being affected by the knocks and external noise, the periodic laser pulses were amplifying the system’s small hiccups, making small perturbations even worse, Potter explains.
So he and his colleagues went back to the drawing board until they finally came up with an insight: if they could feign a pattern of pulses that were somehow arranged (rather than random) but not repeated in a regular fashion, they might create a more fluid topological phase. They calculated that such a ‘quasi-periodic’ pattern could result in multiple symmetries of the processor’s Y units while avoiding unwanted amplifications. The pattern they chose was the mathematically well-studied Fibonacci sequence, where the next number in the sequence is the sum of the two previous numbers. (So, as the regular periodic laser pulse sequence may alternate between two laser frequencies such as A, B, A, B…, the pulsed Fibonacci sequence will be triggered as A, AB, ABA, ABAAB, ABAABABA….)
Although these patterns actually originated from a rather complex arrangement of two sets of varying laser pulses, the system, according to Potter, can be thought of simply as “two lasers pulsing at two different frequencies” ensuring that the pulses do not overlap temporarily. For the purpose of his calculations, the theoretical side of the team imagined these two independent sets of pulses on two separate timelines; Each group effectively beats in its own time dimension. These two time dimensions can be traced on the surface of the torus. The quasi-periodic nature of the double timelines is illustrated by the way they each wrap around the ring over and over at “an odd angle that never repeats on itself,” says Potter.
When the team implemented the new program in a quasi-periodic sequence, the Quantinuum processor was already protected for the duration of the test: 5.5 seconds. “It doesn’t look like much in seconds, but it’s a really stark difference,” says Bonet. “It is a clear sign of the success of the demonstration.”
He agrees, “It’s so cool.” Chetan Nyak, an expert in quantum computing at Microsoft Station Q at the University of California, Santa Barbara, and was not involved in the study. He notes that two-dimensional spatial systems in general provide better protection against errors than one-dimensional systems, but are more difficult and more expensive to construct. The team-created second time effective dimension creeps around this limitation. “Their one-dimensional system works like a higher dimensional system in some ways but without the burden of creating a two-dimensional system,” he says. “It’s the best of both worlds, so you have your cake and you eat it too.”
Samuli AuttiD., a quantum physicist at Lancaster University in England, who was also not on the team, describes the tests as “elegant” and “fantastic” and was particularly impressed that they involved “dynamics” – that is, laser pulses and manipulations that stabilize the system and move its component qubits. Most previous efforts to topologically enhance quantum computers have relied on less energetic control methods, making them more stable and less flexible. Thus, says Otti, “Dynamics with topological protection is a major technical objective.”
The name researchers assigned to their new topological phase of matter recognizes its potential transformational potential, though a little sparsely: Emerging Dynamic Symmetry Protected Topological Phase, or EDSPT. “It would be nice to think of a more attractive name,” Potter admits.
There was another unexpected bonus to the project: The failed original test with periodic pulse sequencing revealed that the quantum computer was more error-prone than assumed. “This was a good way to extend and test how well the Quantinuum processor was,” says Nayak.