Cold (yellow) atoms in an optical resonator being transformed into a continuous-time crystal. Imgae credit: AG Hemmerich/University of Hamburg
The making of the first continuous time crystal marks a milestone in the creation of these strange and important quantum substances, only six years after the first crystal of any kind was created. Discrete and continuous time crystals are distinguished by the form of time translation symmetry they break – the principle that the laws of physics are unchanged over time.
Crystals are defined by the way they repeat a regular structure of atoms over and over in all directions, known as broken translational symmetry as they are changed by certain rotations or movements. For physicists, time is just another dimension, which led Professor Frank Wilczek to propose the idea of a set of particles in their quantum ground state whose movement repeats itself indefinitely in time, because they cannot lose energy to the environment.
Atoms in a time crystal are repeating in both time and space. It may sound like something out of a fantasy novel, but quantum physicists are used to stranger things than that – and winning a Nobel probably helped Wilczek take his idea seriously.
Wilczek proposed what is now called a continuous-time crystal, but in light of the challenges to their existence, others proposed a modified version known as discrete-time crystals. The inconspicuous form was observed in 2016 and has since appeared in very unlikely places. Far from being a scientific curiosity, discrete-time crystals have potential applications in gyroscopes for phones, satellites, and quantum computers.
Now, a paper in the journal Science has announced the sighting of the first continuous time crystal, which could prove significant in its own way.
Each time the crystal oscillates, but cannot transmit its energy to its surroundings, causing them to be referred to as “energyless motion”. Where discrete-time crystals can maintain their status when driven by periodic external oscillations, continuous-time crystals can undergo continuous drives.
The crystals described in the new paper do not exactly match Wilczek’s proposal, but the authors claim so; “Realize the spirit of Wilczek’s original.”
Dr. Hans Keßler of the University of Hamburg and his co-authors contained a Bose-Einstein condensate (BEC) of about 50,000 rubidium atoms in an optical cavity and pumped it with a laser. The wavelength chosen for the laser was a fraction of a percent shorter than that of the relevant rubidium transition.
Above a certain pump force, the BEC self-organizes, gaining random time phase values - like a surfer starting to ride his board without caring where his waves were in the cycle. Such a surfer might not do well. The BEC (a large collection of atoms that share the wave-like behavior of subatomic particles) has, however, demonstrated the ability to oscillate at its own pace unaffected by external distortions, including quantum fluctuations. Keßler described this in a statement as “a system which spontaneously breaks the continuous translational symmetry of time”.
A name like “continuous time crystal” might make us think that Keßler’s creation is eternal, but that is far from the case. The BEC loses atoms and collisions between those that remain “melt” the time crystal. Indeed, the authors admit; “Due to the limited lifetime of the BEC, it is difficult to access the long-term behavior of the system.”
Nevertheless, in the experiment, it lasted long enough to prove the possibility of the existence of such crystals. The authors believe their work could pave the way to improving the science of timekeeping.