A new phase of matter has been observed in a quantum computer after physicists pulsed light into their qubits in a pattern inspired by the Fibonacci sequence.
If you think this is mind-boggling, this strange peculiarity of quantum mechanics behaves as if it had two temporal dimensions, rather than one; a trait scientists say makes qubits more robust, able to stay stable for the duration of the experiment.
This stability is called quantum coherence and is one of the main goals of a flawless quantum computer, and one of the most difficult to achieve.
The work represents “a completely different way of thinking about the phases of matter,” according to quantum computational physicist Philipp Dumitrescu of the Flatiron Institute, lead author of a new paper describing the phenomenon.
“I’ve been working on these theoretical ideas for over five years, and seeing how they’re done in experiments is exciting.”
Quantum computing is based on qubits, the quantum equivalent of computing bits. However, when bits process information in one of two states, a 1 or a 0, the qubits can be both simultaneously, a state known as quantum overlap.
The mathematical nature of this overlay can be incredibly computationally powerful, doing a brief problem-solving job in the right circumstances.
But the blurry, unstable nature of a series of qubits also depends on how their undecided states relate to each other: a relationship called interlacing.
Frustratingly, qubits can get entangled with just about anything in their environment, making mistakes. The more delicate the blurred state of a qubit (or the more chaos there is around it), the greater the risk of losing that consistency.
Improving consistency to the point of viability is probably a multi-tactical approach to removing a major hurdle in the path of a functional quantum computer: every little bit makes a difference.
“Even if you keep all the atoms under strict control, they can lose their quantum by talking to their surroundings, warming up, or interacting with things in ways you didn’t plan to,” Dumitrescu explained.
“In practice, experimental devices have many sources of error that can degrade consistency after a few laser pulses.”
Enforcing symmetry can be a means of protecting qubits from decoherence. Turn a normal old square ninety degrees and it actually still has the same shape. This symmetry protects it from certain effects of rotation.
Touching qubits with evenly spaced laser pulses ensures that there is symmetry based not on space but on time. Dumitrescu and his colleagues wanted to know if they could compensate for this effect by adding, not symmetrical periodicity, but asymmetric quasi-periodicity.
This, they theorized, would add not one temporal symmetry, but two; one effectively buried inside the other.
The idea was based on previous work by the team that proposed the creation of something called quasicrystal in time, rather than in space. When a crystal is made up of a symmetrical network of atoms that repeats itself in space, such as a square grid jungle gym or a bee brief, the atomic pattern of a quasicrystal does not repeat itself, like a Penrose mosaic, but still tidy.
The team conducted their experiment on a state-of-the-art commercial quantum computer designed by Quantinuum, a quantum computing company. This beast uses 10 ytterbium atoms (one of the elements chosen for atomic clocks) for its qubits. These atoms are kept in an electric ion trap, from which laser pulses can be used to monitor or measure them.
Dumitrescu and his colleagues created a sequence of laser pulses based on Fibonacci numbers, where each segment is the sum of the previous two segments. This results in a sequence that is ordered but not repeated, just like a quasicrystal.
Quasicrystals can be mathematically described as smaller-sized segments of larger-sized grids. A Penrose tile can be described as a two-dimensional portion of a five-dimensional hypercube.
Similarly, the laser pulses of the equipment can be described as a one-dimensional representation of a two-dimensional pattern. Theoretically, this meant that it could impose two temporal symmetries on the qubits.
The team tested their work by flashing lasers in the array of ytterbium qubits, first in a symmetrical sequence and then almost periodically. They then measured the consistency of the two qubits at each end of the trap.
For the periodic sequence, the qubits were stable for 1.5 seconds. For the quasi-periodic sequence, they remained stable for 5.5 seconds, the duration of the experiment.
The additional temporal symmetry, the researchers said, added another layer of protection against quantum decoherence.
“With this almost periodic sequence, there is a complicated evolution that nullifies all the mistakes that live on the edge,” Dumitrescu said.
“That’s why the edge stays mechanically quantum consistent much, much longer than you’d expect.”
The work is not about to be ready for integration into functional quantum computers, but it represents an important step toward that goal, the researchers said.
The research has been published in Nature.