Cold atoms (yellow) in an optical resonator on the way to being made into a continuous time crystal. Imgae Credit: AG Hemmerich/University of Hamburg
The making of the first continuous time crystal marks a step up in the creation of these strange and important quantum substances, only six years after the first time crystal of any sort was created. Discrete and continuous time crystals are distinguished by the form of time translation symmetry that they break – the principle the laws of physics are unchanged in time.
Crystals are defined by the way they repeat a regular structure of atoms over and over again in all directions, known as broken translational symmetry because they are changed by some rotations or movements. To physicists, time is just another dimension, leading Professor Frank Wilczek to propose the idea of a set of particles in their quantum ground state whose motion repeats itself in time endlessly, since they cannot lose energy to the environment.
The atoms of a time crystal are repetitive in both time and space. It might sound like something out of a fantasy novel, but quantum physicists are used to weirder things than this – and having a Nobel Prize probably helped Wilczek get his idea taken seriously.
Wilczek proposed what is now called a continuous time crystal, but in light of challenges to their existence, others proposed a modified version known as discrete time crystals. The discrete form was observed in 2016 and has since turned up 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, a paper in the journal Science has announced the observation of the first continuous time crystal, which may prove important in its own way.
Any time crystal oscillates, but cannot shed its energy to its surroundings, leading them to be referred to as having “motion without energy”. Where discrete time crystals can sustain their status when driven by periodic external oscillations, continuous time crystals can experience ongoing drives.
The crystals described in the new paper do not exactly match Wilczek’s proposal, but the authors claim they; “Realize the spirit of Wilczek’s original.”
Dr Hans Keßler of the University of Hamburg and co-authors contained a Bose-Einstein condensate (BEC) of around 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 strength, the BEC self-organized, gaining random time phase values – like a surfer starting their board riding with no regard to where in the cycle their waves were at. Such a surfer might not do well. The BEC (a large collection of atoms that shares the wavelike behavior of subatomic particles) however, demonstrated the capacity to oscillate at its own pace unaffected by external distortions, including quantum fluctuations. Keßler described this in a statement as “a system that spontaneously breaks the continuous time translation symmetry.”
A name like “continuous time crystal” might make us think Keßler’s creation is eternal, but that’s 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 finite 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 such crystals’ existence. The authors believe their work could pave the way to improve the science of time measurement.