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New phase of matter with 2D time created in quantum computer

Quantum computers hold the promise of revolutionizing information technology by harnessing the awesome physics of quantum mechanics. But playing with strange, new machinery often brings out more interesting and new physics. This is exactly what happened to quantum computing researchers in the US.

Reported on NATURE, physicists shining a pulsing laser on atoms inside a quantum computer observed a whole new phase of matter. The new state shows two dimensions of time even though there is only one flow of time.

Researchers believe that the new phase of matter can be used to create quantum computers where stored information is more protected against errors than other architectures.

See, what makes quantum computers so good is also what makes them so difficult.

Unlike classical computers, the transistor in a quantum computer exists on a quantum scale, like an atom. This allows information to be encoded not only using zeros and ones, but also a mixture, or “superposition”, of zeros and ones.

Therefore, quantum bits (or “qubits”) can store multidimensional data and quantum computers can be thousands, even millions of times faster than classical computers, and perform more efficiently.

But this same mix of 0 and 1 states in qubits is also what makes them so prone to error. That’s why a lot of quantum computing research revolves around creating machines with reduced errors in their calculations.

Read more: Australian researchers create a parallel quantum simulator

The puzzling property discovered by the authors of NATURE The paper was created by beaming a laser that shines on the atoms inside the quantum computer in a sequence inspired by Fibonacci numbers.

Using an “extra” time dimension “is a completely different way of thinking about the phases of matter”, said lead author Philipp Dumitrescu, a research associate at the Center for Computational Quantum Physics at the Flatiron Institute in New York City, US. “I’ve been working on these ideas in theory for over five years and seeing them bear out in experiments is exciting.”

The team’s quantum computer is built on ten atomic ions of ytterbium that are manipulated by laser pulses.

Quantum mechanics tells us that superpositions break down when qubits are influenced (intentionally or not), leading the quantum transistor to “choose” to be in either the 0 or 1 state. This “collapse” is a probability and cannot be determined with certainty in advance.

“Even if you keep all the atoms under tight control, they can lose their volume by interacting with their environment, heating up, or interacting with things in ways you didn’t plan for,” said Dumitrescu. “In practice, experimental devices have many sources of error that can degrade the coherence after a few laser pulses.”

Therefore, quantum computing engineers are trying to make qubits more resistant to external effects.

One way to do this is to take advantage of what physicists call “symmetries” that preserve properties despite some changes. For example, a snowflake has rotational symmetry – it looks the same when rotated through an angle.

Time symmetry can be added using rhythmic laser pulses, but Dumitrescu’s group added two-time symmetries by using ordered but non-repetitive laser pulses.

The Penrose tiling pattern is a type of quasicrystal, meaning it has an ordered non-repeating structure. The pattern, which consists of two shapes, is a 2D projection of a 5D square lattice.

Other ordered but non-repeating structures include quasicrystals. Unlike typical crystals that have a repeating structure (like honeycombs), quasicrystals have order, but no repeating pattern (like Penrose tiling). Quasicrystals are actually truncated versions, or “projections”, of higher-dimensional objects. For example, a two-dimensional Penrose tiling is a projection of a five-dimensional lattice.

Can quasicrystals be simulated in time, rather than space? That’s what Dumitrescu’s team did.

While a periodic laser pulse alternates (A, B, A, B, A, B, etc), the components of the quasi-periodic laser-pulse based on the Fibonacci sequence are the sum of the two previous components (A, AB, ABA, ABAAB, ABAABA, etc.). Like a quasicrystal, it is a two-dimensional pattern jammed into one dimension. Therefore, there is additional time symmetry as a benefit from this time-based quasicrystal.

The team fired a Fibonacci-based laser pulse sequence at the qubits at either end of the ten-atom arrangement.

Using a strictly periodic laser pulse, these edge qubits remained in their superposition for 1.5 seconds – an impressive feat in itself due to the strong interaction between the qubits. However, with quasi-periodic pulses, the qubits remain quantum for the entire length of the experiment – about 5.5 seconds.

“With this quasi-periodic sequence, there is a complex evolution that cancels all the errors that live inside,” explained Dumitrescu. “Because of that, the edge remains quantum-mechanically coherent, much longer than you would expect.” Although the findings hold great promise, the new phase of matter still needs to be integrated into a working quantum computer. “We have this direct, attractive application, but we need to find a way to hook it into calculations,” Dumitrescu said. “That’s an open problem we’re working on.”

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