LQuantum computers promise to revolutionize information technology using the strange physics of quantum mechanics. But playing with strange and new machines often brings up even more interesting and new physics. This is exactly what happened to quantum computer researchers in the United States.
Header image: In this quantum computer, physicists have created an unprecedented phase of matter that acts as if time had two dimensions. (quantum)
In a new study (link below), physicists shining a pulsed laser at atoms inside a quantum computer observed an entirely new phase of matter. This new state has two temporal dimensions, whereas there is only one temporal stream.
The researchers believe that this new phase of matter could be used to develop quantum computers, where stored information is much better protected against errors than in other architectures.
What makes quantum computers amazing is actually also what makes them extremely problematic.
Unlike classical computers, the transistor in a quantum computer is on a quantum scale, like a single atom. This makes it possible to encode information not only using zeros and ones, but also a mixture or “superposition” of zeros and ones.
Quantum bits (or “qubits”) can thus store multidimensional data, and quantum computers would be thousands or even millions of times faster than classical computers and would work much more efficiently.
But the same mix of 0 and 1 states in qubits is also what makes them extremely error-prone. Therefore, a large part of quantum computing research is about building machines that have reduced computational errors.
The mind-boggling property discovered by the authors of this new study was produced by pulsing a laser at atoms inside the quantum computer in a sequence inspired by the Fibonacci sequence.
According to the study’s lead author Philipp Dumitrescu, a researcher at the Center for Computational Quantum Physics at the Flatiron Institute in New York, USA:
Using an “extra” dimension of time” is a completely different way of thinking about the phases of matter. I have been working on these theoretical ideas for more than 5 years, and it is exciting to see them become a reality in experiments.
The team’s quantum computer is built on ten ytterbium atomic ions that are manipulated by laser pulses.
According to quantum mechanics, superpositions collapse when qubits are affected (intentionally or not), causing the quantum transistor to “choose” to be in state 0 or 1. This “collapse” is probabilistic and cannot be determined with certainty in a pinch.
According to Dumitrescu:
Even if you keep all atoms under tight control, they can lose their quantum character by talking to their surroundings, heating up, or interacting with things in ways you didn’t expect. In practice, experimental devices have many sources of error that can degrade coherence after only a few laser pulses.
Quantum computer engineers are therefore trying to make qubits more resistant to external effects.
One way to do this is to take advantage of what physicists call “symmetries,” which preserve properties despite some changes. For example, a snowflake has rotational symmetry: it looks the same when rotated through a certain angle.
Time symmetry can be added using rhythmic laser pulses, but Dumitrescu’s team added two time symmetries using ordered but non-repetitive laser pulses.
Quasicrystals are other ordered but non-repeating structures. Unlike typical crystals, which have a repeating structure (like gingerbread), quasicrystals have an order but no repeating pattern (like the Penrose tiling). Quasicrystals are actually smaller versions or “projections” of higher dimensional objects. For example, a two-dimensional Penrose tiling is a projection of a five-dimensional lattice.
The pattern of the Penrose tiling is a type of quasi-crystal, meaning that it has an ordered but never repeating structure. The pattern, consisting of two shapes, is a 2D projection of a 5D square mesh. (Flatiron Institute)
Could quasicrystals be mimicked in time, rather than space? That is what Dumitrescu’s team managed to do.
While 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 quasi-crystal, it is a two-dimensional pattern stuck in a single dimension. This temporal quasi-crystal thus presents an additional temporal symmetry.
The team sent the sequence of Fibonacci laser pulses to the qubits at both ends of the ten-atom structure.
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 interaction between the qubits. But with the quasi-periodic pulses, the qubit remained quantized for the duration of the experiment, or about 5.5 seconds.
Still according to Dumitrescu:
With this quasi-periodic sequence, there is a complicated evolution that cancels out any residual faults on the boundary. Thanks to this, the edge remains quantum mechanically consistent much, much longer than you might expect.
Although the results are very promising, the new phase of matter has yet to be integrated into a working quantum computer.
We have this simple and enticing application, but we need to find a way to connect it to the calculations. It is an open question that we are working on.
The study published in Nature: Dynamical topological phase realized in a trapped-ion quantum simulator and presented on the Flatiron Institute website: Strange New Phase of Matter Created in Quantum Computer Acts Like It Has Two Time Dimensions.