Time crystals represent a fascinating departure from conventional phases of matter. While ordinary crystals—such as salt, quartz, or diamond—feature atoms arranged in a repeating spatial pattern, time crystals exhibit internal behavior that repeats in a regular rhythm over time. Crucially, this temporal repetition occurs even though the systems are not being driven by a repeating external signal. They challenge traditional thermodynamic expectations by maintaining perpetual motion or oscillation in their ground state without absorbing energy from an external periodic driver.
In the ongoing experiments at TU Dortmund, these elusive phases of matter are formed inside a specially engineered semiconductor. The material is crafted from gallium arsenide, infused with small, precise amounts of indium and silicon. These added elements play a crucial role by creating localized electrons within the semiconductor matrix. When the material is cooled to extreme temperatures close to -270 degrees Celsius—just fractions of a degree above absolute zero—a unique quantum environment takes shape. Under these frigid conditions, each localized electron interacts with roughly one million nearby nuclear spins, setting the stage for collective quantum phenomena.
To initiate the process and bring the system to life, the researchers employ a sophisticated setup utilizing a pump laser. This laser is used to meticulously align the electron spins within the semiconductor. Once aligned, the electrons transfer their polarization to the surrounding nuclear spins embedded in the crystal lattice. When a weak external magnetic field is applied to the system, the polarization of those nuclear spins begins to rotate in a coordinated fashion.
A delicate, continuous feedback loop between the electron spins and the nuclear spins is what ultimately keeps the oscillations going indefinitely, defying the natural tendency toward decoherence and decay. To observe and analyze these microscopic dynamics, a second laser is deployed, allowing the researchers to monitor precisely how the oscillations develop and evolve over time within the material.
Separate Oscillations Fall Into Sync
At the microscopic level, achieving perfection in semiconductor manufacturing is impossible. Different regions of the material naturally exhibit slight structural variations, impurities, or local fluctuations. Because of these minute local variations, time crystals that form in separate, distinct areas of the semiconductor would normally oscillate at slightly different frequencies if left entirely to themselves.
That fundamental limitation changes dramatically when the researchers illuminate many regions of the material simultaneously using a broad laser beam. Under those specific optical conditions, the previously independent oscillations can lock together and begin operating at a single, unified frequency.
This cooperative phenomenon observed in the quantum realm bears a striking conceptual resemblance to a famous historical observation made by Dutch physicist Christiaan Huygens in 1665. Huygens, the inventor of the pendulum clock, noticed that two pendulum clocks mounted on the same supporting wooden beam would gradually synchronize their swings. He realized this happened because of the weak mechanical interaction transmitted between the clocks through that shared structural support.
Inside the semiconductor, the underlying physical connection works through an entirely different mechanism, yet the macroscopic result is strikingly similar. Instead of mechanical vibrations traveling through wood or metal, the time crystals become coupled through the continuous movement and interaction of spin-polarized electrons flowing through the material.
Synchronization Across Surprising Distances
In their latest investigation, Prof. Greilich’s team sought to determine the spatial limits of this cooperative behavior. They discovered that time crystals located as far as 40 micrometers apart within the semiconductor could still successfully synchronize their rhythms. To put that scale into perspective, this distance is more than one thousand times greater than the characteristic size of a single oscillator within the system.
This long-range coordination demonstrates a remarkable degree of non-local coupling between spatially separated spin systems in a solid-state material. However, this cooperative reach has its limits. Once the physical separation between the individual time crystals becomes significantly larger than 40 micrometers, the coupling weakens beyond the threshold required for entrainment. At those greater distances, the individual time crystals no longer lock together and instead revert to oscillating independently at their own local frequencies.
The implications of these findings extend well beyond fundamental physics. By proving that multiple time crystals can form simultaneously and synchronize over extended distances within a standard semiconductor platform, the TU Dortmund research helps lay the essential groundwork for future networks of controllable spin oscillators. These insights could eventually open up entirely new possibilities for spin-based technologies, advanced quantum information processing, and novel electronic devices that harness the unique properties of time crystals.
