Cold atoms (yellow) in an optical resonator on the way to a continuous time crystal. Image source: AG Hemmerich/University of Hamburg
The fabrication of the first continuous time crystal marks a step forward in the creation of these strange and important quantum substances, just six years after the first time crystal of any kind was fabricated. Discrete and continuous time crystals are characterized by the form of time-translational symmetry that they break – the principle that the laws of physics are unchanging in time.
Crystals are defined by repeating a regular structure of atoms over and over in all directions, known as broken translational symmetry because they are modified by some rotations or movements. For physicists, time is just another dimension, which led Professor Frank Wilczek to propose the idea of a series of particles in their quantum ground state, whose motion repeats itself endlessly in time because they cannot release energy into the environment.
The atoms of a time crystal repeat themselves in both time and space. It might sound straight out of a fantasy novel, but quantum physicists are used to stranger things – and the Nobel Prize probably helped Wilczek take his idea seriously.
Wilczek proposed what is now referred to as a continuous-time crystal, but faced with the challenges to their existence, others proposed a modified version known as discrete-time crystals. The discrete form was observed in 2016 and has since appeared in some very unlikely places. Far from being a scientific curiosity, discrete time crystals have potential applications in gyroscopes for phones, satellites and quantum computers.
Now, an article in Science journal has heralded the observation of the first continuous time crystal, which could prove important in its own way.
Any time a crystal vibrates but cannot release its energy to its surroundings, it is referred to as “moving without energy”. Where discrete time crystals can maintain their status when driven by periodic external vibrations, continuous time crystals can experience ongoing drives.
The crystals described in the new paper don’t exactly match Wilczek’s proposal, but the authors claim they are; “Recognize the spirit of Wilczek’s original.”
dr Hans Kessler from the University of Hamburg and co-authors contained a Bose-Einstein condensate (BEC) of around 50,000 rubidium atoms in an optical resonator 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 strength, the BEC organized itself and gained random time-phase values - like a surfer launching his board, regardless of where in the cycle his waves are. Such a surfer might not do well. However, the BEC (a large assembly of atoms sharing the wave-like behavior of subatomic particles) demonstrated the ability to oscillate at its own pace, unaffected by external distortions, including quantum fluctuations. Kessler, in a statement, described this as “a system that spontaneously breaks continuous time translation symmetry”.
A name like “continuous time crystal” might lead us to think that Kessler’s creation is eternal, but that’s far from the case. The BEC loses atoms and collisions between the remaining ones “melt” the time crystal. In fact, the authors admit; “Due to the limited lifetime of the BEC, it is difficult to access the long-term behavior of the system.”
Nevertheless, it took long enough in the experiment 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.