What are time crystals?

Time crystals are unusual physical systems whose internal states repeat in a regular rhythm over time, even when they are not driven by a repeating external signal.

In the Dortmund University of Technology experiment, a time crystal forms inside a semiconductor made from gallium arsenide containing small amounts of indium and silicon. These additional elements create localized electrons within the material. At temperatures close to -270 °C, each electron interacts with approximately 1 million nearby nuclear spins.

To initiate the process, the researchers use a pump laser to tune the electron spin. The electron then transfers its polarization to the surrounding nuclear spins. When a weak magnetic field is applied, the polarization of the nuclear spins begins to rotate.

Feedback between the electron and nuclear spins allows the oscillations to continue. A second laser enables researchers to monitor how these oscillations develop over time.

Separate time-crystal vibrations become synchronized

Different regions of a semiconductor are not completely identical at the microscopic level. Because of these local variations, time crystals that form in separate regions usually oscillate at slightly different frequencies.

That changes when researchers illuminate many areas at once with a wide laser beam. Under these conditions, separate oscillations can lock together and begin operating at the same frequency.

The effect is reminiscent of observations made by Christian Huygens in 1665 using two pendulum clocks. Huygens found that clocks mounted on the same support could gradually synchronize through weak mechanical interactions transmitted by the shared structure.

In the semiconductor, the connection works differently. Rather than being linked by mechanical vibrations, the time crystals are coupled through the movement of spin-polarized electrons.

Time crystals synchronize across distances 1,000 times their size

The researchers found that time crystals separated by as much as 40 micrometers can synchronize. This distance is more than 1,000 times the characteristic size of a single oscillator.

As the separation increases, however, the individual time crystals eventually stop locking together and continue to vibrate independently.

The finding points to nonlocal coupling between spatially separated spin systems. It could also help lay the foundation for future networks of controllable spin oscillators, opening new possibilities for spin-based technologies.