Exotic ‘time crystals’ could be used as memory in quantum computers, promising research finds
Time crystals could help create quantum computing data storage that lasts minutes, according to new research – a vast improvement over the millisecond duration of existing quantum data storage.
As part of this new research, scientists conducted experiments on how time crystals interact with mechanical waves. Although time crystals are widely considered extremely fragile, researchers have shown that they can couple the time crystal to a mechanical surface wave without it being destroyed.
“This is the most interesting part for me”, co-author of the study Jere Makinenresearcher at Aalto University in Finland, told Live Science. “It’s that you can actually couple time crystals in a meaningful way to another system and exploit the inherent robustness of time crystals.”
The researchers described their results in a study published October 16 in the journal Natural communications.
Making waves in time crystal research
Traditional crystal structures have a regular arrangement of atoms or molecules in space, but time crystals return to a certain state after regular periods of time. This is not the same as a pendulum, for example, where the swing frequency simply reflects the frequency of the downward swing force, while the gravitational pull competes with the changing orientation of the tension. In the case of a time crystal, although in practice an initial prompt to action is necessary, the periodicity is acquired spontaneously, without anything driving it to that frequency.
Since they were the first proposed in 2012various configurations that act as time crystals have been reported. Mäkinen and his collaborators based theirs on quasiparticles called magnons – collective waves with the value of a quantum property known as spin. They created magnons in “superfluid helium-3,” a helium whose nuclei have two protons and a single neutron so that the spins of the particles in the nucleus cannot cancel out.
They cooled helium-3 to cryogenic temperatures so that the dynamics of the atoms caused them to attract each other efficiently, albeit weakly, and rearrange into quasiparticles called Cooper pairs. As Cooper pairs, these quasiparticles are limited to a single available quantum state, thereby eliminating fluid viscosity.
It turns out that the back-and-forth motion of the superfluid helium-3 with a mechanical surface wave has an interesting effect that boils down to the influence of the surface on the spin and orbital angular momentum of the Cooper pairs, which are the properties used to characterize the superfluid. To illustrate this, let’s think about the influence of a wall on the possible orbits of a ball spun at the end of a string: in free space, the orbitals of the ball can take any orientation in three dimensions, but if we bring it closer to a wall, some of these orbitals are no longer possible.
Mäkinen and his collaborators recognized that this would influence the Magnon time crystal period. In their experiments, they found that the time crystal could survive the interaction for up to a few minutes. This suggests that it might be possible to couple data from quantum computers to the time crystal via a similar interaction for storage.
In quantum computers, each qubit can be in a superposition of two binary states at once, which provides the basis for theoretically higher processing power. The memory of quantum computers must therefore store data that preserves this indefinite quality of the qubit state.
Memory technologies in current quantum computers typically use spin orientation to store data, but these spin states are easily disrupted by environmental disturbances such as thermal noise. These disturbances push them into one or another possible state, meaning that the quantum nature of the stored data is lost. As such, spin quantum memory lasts only a few milliseconds.
In contrast, the magnons created by Mäkinen and colleagues lasted a few minutes, even with the disruption of the surface mechanical wave. Since the surface wave leaves an imprint on the frequency of the magnon time crystal, it can be used to “write” the quantum data to be stored. With longer quantum memory, more quantum processing operations can be performed on the data before it deteriorates, enabling more complex tasks.
Textbook Analogies
After examining the experimental data, the team also discovered several similarities with optomechanics, where light and mechanical resonators interact. An example is the barely perceptible impact of a photon hitting a mirror attached to a spring, where the spring gains or loses energy as the photon bounces off the mirror.
Drawing parallels between time crystals and optomechanics could reveal a theory from the well-established field of optomechanics that can be applied to time crystals subjected to a mechanical wave, providing a head start in understanding these interactions.
“Optomechanics is a very general topic in many areas of physics, so you can use it in a wide variety of different systems,” Mäkinen said.
Nikolai Jeludev, a professor of physics and astronomy at the University of Southampton who also studies time crystals and optomechanics but was not involved in the study, described the study as “interesting”. “It opens a research direction in the physics of non-equilibrium systems with potential implications for advancing quantum sensing and quantum control,” he told Live Science in an email.
Mäkinen said he wanted to explore different types of configurations to mechanically couple to the time crystal, such as with a nanofabricated electromechanical resonator, which would have a much lower mass than the superfluid surface wave. “The obvious idea is to really go to the quantum limit and see how far we can push it,” he said.
