It sounds like science fiction: Researchers have created time crystals for the first time – materials that are discreetly ordered in space and time. The building blocks of these time crystals form regular lattices as in a normal crystal, but at the same time they change their state in a regular cycle. These long-standing impossible structures were theoretically postulated only a few years ago; now, two research teams in Nature are reporting the practical implementation.
The characteristic of crystals such as diamond, water ice or quartz is the regular three-dimensional lattice structure of their atoms. They form periodically repeating basic units in space. But could a crystal have such repetitive, discrete states also with respect to time? For a long time this was considered impossible.
Discrete order also in time
But a few years ago US physicists developed a theoretical model, according to which such “time crystals” could exist. According to this theory, these are crystals whose building blocks change their state periodically without external action – and therefore also have a discrete, regular structure in time.
“Our work revealed the physical basis for how such time crystals could work,” explains Shivaji Sondhi from Princeton University. “The atoms move in time, but instead of doing this fluently or in a continuous manner, they move periodically.” This is only possible if the system is not in equilibrium or resting.
But can these theoretically postulated time crystals be translated into reality? Two research teams – one at the Harvard and Princeton Universities and one under the direction of the University of Maryland – were working on this question in the last few years. Now both teams have managed to produce time crystals in different ways and with different materials.
Inverting ion spins
The first time crystal consists of ten ytterbium ions, suspended in an electric field above a surface. Christopher Monroe, of the University of Maryland and his colleagues, used these ions with regular laser pulses, which allowed the spins of the particles to be folded over, thus preventing the system from being balanced.
The strange thing, however, was that the ytterbium ions did not follow the pulse of the laser but developed their own alternating oscillations. They only changed every second pulse. According to the physicists, this suggests that this system has an independent, discrete time structure – it is a time crystal. “The oscillation and synchronization of the interacting spins shows,
Swinging gaps in the diamond lattice
The second time crystal consists of a synthetic diamond in which the carbon lattice has about one million defects, so-called nitrogen gaps. At these sites, a carbon atom is absent, but nitrogen atoms are deposited on this gap. Sonwoon Choi from Harvard University and his colleagues also solved spins in these gaps by microwaves.
This system also produced a periodic, twice as slow cycle in the alternating pattern of the spins. “This system breaks the fundamental temporal symmetry of the pulse and forms the discrete order of a time crystal,” the physicists report.
New state of matter
Thus, it seems to be proved that the time crystals that were once considered impossible can actually exist. “Both groups present evidence of a time crystal,” says Chetan Nayak of the University of California, Santa Barbara, in an accompanying commentary. “Even if it is still necessary to show that these oscillations remain in phase for long periods of time and are not blurred by fluctuations.”
The existence of the time crystals does not mean that time travel like in the series “Dr. Who” are within reach. But there could be applications for this new matter in the future. They could help, for example, to better protect data in quantum computers against interference, as the researchers explain.