Quantum computers hold the promise of revolutionising information technology by utilising the whacky physics of quantum mechanics. But playing with strange, new machinery often throws up even more interesting and novel physics. This is precisely what has happened to quantum computing researchers in the US.
Reported in Nature, physicists who were shining a pulsing laser at atoms inside a quantum computer observed a completely new phase of matter. The new state exhibits two time dimensions despite there still being only a singular time flow.
The researchers believe the new phase of matter could be used to develop quantum computers in which stored information is far more protected against errors than other architectures.
See, what makes quantum computers great is also what makes them exceedingly tricky.
Unlike in classical computers, a quantum computer’s transistor is on the quantum scale, like a single atom. This allows information to be encoded not just using zeroes and ones, but also a mixture, or “superposition”, of zero and one.
Hence, quantum bits (or “qubits”) can store multidimensional data and quantum computers would be thousands, even millions of times faster than classical computers, and perform far more efficiently.
But this same mixture of 0 and 1 states in qubits is also what makes them extremely prone to error. So a lot of quantum computing research revolves around making machines with reduced flaws in their calculations.
The mind-bending property discovered by the authors of the Nature paper was produced by pulsing a laser shone on the atoms inside the quantum computer in a sequence inspired by the Fibonacci numbers.
Using an “extra” time dimension “is a completely different way of thinking about phases of matter”, says lead author Philipp Dumitrescu, a research fellow at the Flatiron Institute’s Centre for Computational Quantum Physics in New York City, US. “I’ve been working on these theory ideas for over five years and seeing them realised in experiments is exciting.”
The team’s quantum computer is built on ten atomic ions of ytterbium which are manipulated by laser pulses.
Quantum mechanics tells us that superpositions will break down when qubits are influenced (intentionally or not), leading the quantum transistor to “pick” to be either in the 0 or 1 state. This “collapse” is probabilistic and cannot be determined with certainty beforehand.
“Even if you keep all the atoms under tight control, they can lose their quantumness by talking to their environment, heating up, or interacting with things in ways you didn’t plan,” Dumitrescu says. “In practice, experimental devices have many sources of error that can degrade coherence after just a few laser pulses.”
So, quantum computing engineers try to make qubits more resistant to outside effects.
One way of doing this is to exploit what physicists call “symmetries” which preserve properties despite certain changes. For example, a snowflake has rotational symmetry – it looks the same when rotated a certain angle.
Time symmetry can be added using rhythmic laser pulses, but Dumitrescu’s team added two time symmetries by using ordered but non-repeating laser pulses.
Other ordered but non-repeating structures include quasicrystals. Unlike typical crystals which have repeating structure (like honeycombs), quasicrystals have order, but no repeating pattern (like Penrose tiling). Quasicrystals are actually the squished down versions, or “projections”, of higher-dimensional objects. For example, a two-dimensional Penrose tiling is a projection of a five-dimensional lattice.
Could quasicrystals be emulated in time, rather than space? That’s what Dumitrescu’s team was able to do.
Whereas a periodic laser pulse alternates (A, B, A, B, A, B, etc), the parts of the quasi-periodic laser-pulse based on the Fibonacci sequence are the sum of the two previous parts (A, AB, ABA, ABAAB, ABAABABA, etc.). Like a quasicrystal, this is a two-dimensional pattern jammed into a single dimension. Hence, there’s an extra time symmetry as a boon from this time-based quasicrystal.
The team fired the 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 given the strong interactions between qubits. But, with the quasi-periodic pulses, the qubits stayed quantum for the entire length of the experiment – around 5.5 seconds.
“With this quasi-periodic sequence, there’s a complicated evolution that cancels out all the errors that live on the edge,” Dumitrescu explains. “Because of that, the edge stays quantum-mechanically coherent much, much longer than you’d expect.” Though the findings bear much promise, the new phase of matter still needs to be integrated into a working quantum computer. “We have this direct, tantalising application, but we need to find a way to hook it into the calculations,” Dumitrescu says. “That’s an open problem we’re working on.”