A new phase of matter was observed in a quantum computer after physicists pulsed light on its qubits in a pattern inspired by the Fibonacci sequence.
If you think this is mind-boggling, then this peculiar quirk of quantum mechanics behaves as if it has two time dimensions, rather than one; A trait that the scientists say makes qubits more powerful and able to remain stable for the duration of the experiment.
This stability is called quantum coherence, and it is one of the main goals of an error-free 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 computational quantum physicist Felipe Domitrescu of the Flatiron Institute, and lead author of a new research paper describing the phenomenon.
“I’ve been working on these theoretical ideas for over five years, and seeing them actually come true in experiments is exciting.”
Quantum computing is based on qubits, the quantum equivalent of computing qubits. However, when bits process information in one of two states, 1 or 0, they can be qubits at once, a condition known as quantum superposition.
The mathematical nature of this superposition can be incredibly powerful from a computational point of view, making short problem solving under the right conditions.
But the uncertain and unstable nature of a series of qubits also depends on how their oscillating states relate to each other – a relationship called entanglement.
Frustratingly, qubits can get entangled with almost anything in their environment, which leads to errors. The more sensitive a qubit’s fuzzy state is (or the more messy its environment), the greater the risk that it will lose this coherence.
Improving coherence to a point of feasibility is likely a multi-tactic approach to removing a major hurdle standing in the way of a functional quantum computer – every little bit makes a difference.
“Even if you keep all of the atoms under tight control, they can lose their quantity by talking to their environment, heating up or interacting with things in ways they didn’t plan for,” Domitrescu explained.
“In practice, experimental devices contain many error sources that can degrade coherence after a few laser pulses.”
One way to protect qubits from decoherence is to enforce symmetry. Rotate an ordinary old square ninety degrees, and it’s still effectively the same shape. This symmetry protects it from certain rotational effects.
Clicking the qubits with evenly spaced laser pulses ensures symmetry that does not depend on space, but rather in time. Domitrescu and colleagues wanted to see if they could increase this effect by adding, not symmetric periodic, but asymmetric quasi-periodic.
They assumed that this would not add a one-time symmetry, but a one-time symmetry; One is actually buried inside the other.
The idea was based on previous work by the team that proposed creating a so-called quasicrystalline in time, rather than space. When a crystal consists of a symmetrical network of atoms that repeat in space, such as a square lattice forest gym or honeycomb, the pattern of atoms on a semi-crystal is non-repetitive, like a Penrose tiling, yet still ordered.
The team conducted their experiment on a high-end commercial quantum computer designed by Quantinuum, a quantum computing company. This monster employs 10 atoms of ytterbium (one of the favorite elements of atomic clocks). These atoms are kept in an electric ion trap, through which laser pulses can be used to control or measure them.
Domitrescu and colleagues created a series of laser pulses based on Fibonacci numbers, with each part representing the sum of the previous two parts. This results in an ordered, but not repeating, sequence, just like a quasicrystal.
Semi-crystalline crystals can be described mathematically as lower dimensional sections of higher dimensional lattices. Penrose tiling can be described as a two-dimensional slice of a five-dimensional cube.
In the same way, the team’s laser pulses can be described as a one-dimensional representation of a two-dimensional pattern. In theory, this meant that it would likely impose two time symmetries on the qubits.
The team tested their work by flashing lasers into a ytterbium qubit, first in symmetrical sequences, then semi-periodically. Then they measured the coherence of two qubits on either side of the trap.
For the periodic sequence, the qubits were stable for 1.5 s. For the quasi-periodic sequences, they remained stable for 5.5 s – the duration of the experiment.
The additional time symmetry added another layer of protection against quantum decoherence, the researchers said.
“With this quasi-periodic sequence, there is a complex evolution that eliminates all the errors that live on the edge,” Domitrescu said.
“Because of that, the edge stays quantum mechanically coherent a lot, much longer than you’d expect.”
The researchers said the work is nowhere near ready to be integrated into functional quantum computers, but it does represent an important step toward that goal.
The search was published in temper nature.
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