Time crystals are quantum systems whose lowest energy-state is where its particles are in constant motion. First proposed in 2012 by Frank Wilczek, time crystals are analogous to common crystals, except, whereas common crystals have atoms arranged periodically in space, time crystals atoms’ are arranged periodically in time and space. Time crystals don’t bleed energy to the environment and come to a state of rest because they are in a quantum ground state. Time crystals have motion without energy. Time crystals properties have led scientists to believe that they could some day help to achieve quantum computing. Recently, scientists led by Prof. Samuel Autti linked two time crystals in a seemingly impossible experiment that has brought the world closer to a new form of quantum computing. 

A Rupture With Symmetry

Common crystals represent a rupture in the symmetry of nature. Nature’s symmetry is reflected in the law of physics. In other words, the laws of physics affect everything equally. A planet is not, for example, less governed by gravitational forces than other planets. The laws of physics not only apply equally, they work in all directions, so, ceteris paribus, no matter how you tilt a physical experimental setting, you will get the same outcomes. Crystals break this symmetry by having atoms arranged periodically, breaking spontaneous symmetry so that while the laws of physics are still symmetric, the arrangement of atoms is periodic.

The Impossibility of Observation or Interaction Under Quantum Mechanics

When Wilczek proposed time crystals, it was with the understanding that the laws of physics were symmetry not just across space, but across time as well. So not only should changes in the physical setting of an experiment not affect outcomes, but changes in time should not either. It struck him that common crystals must have a time analogue. It was from that insight that scientists were able to develop time crystals.

Time crystals should be impossible: nothing should be able to exist in a state of constant motion. Yet they exist. We know from the laws of thermodynamics that systems in a state of equilibrium tend toward entropy, so the perpetual motion of a time crystal should tend toward disorder, that is, toward rest. This is because time crystals are not subject to the laws of thermodynamics, they are instead subject to quantum mechanics.

The thing with quantum mechanics is that perpetual motion is that it should come to a rest once we observe it. The authors of the study begin their article by noting that, “Perpetual ground state motion in equilibrium defines a time crystal, but observing such motion is famously unfeasible”. Interaction with the environment should break down this remarkable state of affairs. In fact, if interaction with the environment is strong enough, not only will perpetual motion stop, time crystals will stop being time crystals.

By this time, you can understand why this experiment seemed impossible. At the subatomic scale in which quantum mechanics reigns, the researchers had to find a way to interact with these time crystals without bringing their perpetual motion to a halt or breaking down the time crystals. What makes time crystals interesting is that they seem to exist along the continuum between the world of quantum mechanics and classical physics. That is very suggestive for anyone interested in that great mystery of how we move across the continuum.

The Experiment

Autti and his team built a time crystal using magnons, or spin-wave quasiparticles, a collective excitation of an electrons' spin structure in a crystal lattice. They cooled helium-3 until it was within the zero temperature limit, turning it into a Bose-Einstein condensate. The electron spin waves work in concert in a Bose-Einstein condensate, creating magnons. The waves that make up time crystals sway perpetually, backwards and forwards, forming time crystals.

Two sets of magnons existing as time crystals were brought near enough to each other to interact, creating a single time crystal system with two different states. 

By successfully completing this experiment, the team hopes that it can help physics take one step closer to understanding how quantum mechanics interacts with classical physics. Ultimately, they want to be able to have time crystals interact with their environment without a resulting breakdown of those time crystals. If this can be done, then those perpetual motion machines could be used for some purpose. If you’re thinking of alternative energy, that isn’t a possibility given that, as we said at the top of the article, time crystals have “motion without energy”. However, time crystals could be used for quantum computing. 

Quantum computing is made possible by the ability to have two different states at the same time within a single computing system. In a traditional computer, computers can only be in on ebit state at a time, “1”, or “0”. A quantum computer transcends this with its use of qubits, which can be in different places at the same time, resulting in greater computing power.