Physicists have unveiled a new type of time crystal that is governed by quantum behavior that conflicts with classical models. Quantum connections between particles maintain the regular movements of the crystal, conserving its status.
Traditional crystals are defined by their order, with atoms bound in a regular repeating structure, in contrast to the liquids they cool out of, which are disordered. Most crystals contain flaws where atoms are displaced or a different element has made its way in, but the overall consistency is what defines them. If you were the size of an atom and could travel in any direction within a crystal, you would mostly experience repetition that would seem endless.
Time crystals are the name given to the once-theoretical concept of objects that show similar repetition in time, with components reverting to the same positions over and over again. If one thinks of time as the fourth dimension, these repetitions are like the placement of atoms in the three dimensions of an ordinary crystal.
On this basis a clock is a model of an extremely complex time crystal, the hands reverting to the same position every 12 hours, while the pendulum that drives a clock is a much simpler example. However, an additional feature of a time crystal is that regularity should be the result of internal processes, rather than an external driver. Researchers have also compared them to Regency or barn dances, where partners return to each other after cycling through predictable motions.
TU Wien postgraduate student Felix Russo and Professor Thomas Pohl offer an intermediary analogy, where smoke from a candle forms a series of smoke rings after the candle is put out, without external forces dictating them.
Given that one of the defining features of quantum behavior – one that physicists struggled to come to grips with in in the early 20th century – is randomness, we might expect quantum systems to lack this order. Nevertheless, recent years have seen a flood of time crystals identified, sometimes in very unexpected places.
“However, it was thought that this was only possible in very specific systems, such as quantum gases, whose physics can be well described by mean values without having to take into account the random fluctuations that are inevitable in quantum physics,” said Russo in a statement. “We have now shown that it is precisely the quantum physical correlations between the particles, which were previously thought to prevent the formation of time crystals, that can lead to the emergence of time-crystalline phases.”
“We are investigating a two-dimensional lattice of particles held in place by laser beams,” Russo said. “And here we can show that the state of the lattice begins to oscillate – due to the quantum interaction between the particles.”
Russo and Pohl report that these correlations cause particles to behave collectively in ways that can’t be explained by looking at each particle separately. It’s the subatomic equivalent of the way people in crowds will behave differently from how they might on their own, whether that reveals wisdom or madness.
As the authors put it in more technical terms, they identified “two distinct time-crystal phases that cannot be described within mean-field theory.” One of these phases only exists because of the correlations quantum particles sometimes form with others.
The authors note that the dynamics they describe could only be found in lattices with quantum spins of 1 (most bosons) not those with spins of a half (fermions such as protons and electrons). They say that exploring why this is the case could tell us a lot about continuous time crystal formation.
The study is published open access in Physical Review Letters.