We have all seen crystals, whether a simple grain of salt or sugar, or an elaborate and beautiful amethyst. These crystals are made of atoms or molecules repeating in a symmetrical three-dimensional pattern called a lattice, in which atoms occupy specific points in space.
By forming a periodic lattice, carbon atoms in a diamond, for example, break the symmetry of the space they sit in. Physicists call this “breaking symmetry.”
Scientists have recently discovered that a similar effect can be witnessed in time. Symmetry breaking, as the name suggests, can arise only where some sort of symmetry exists. In the time domain, a cyclically changing force or energy source naturally produces a temporal pattern.
Breaking of the symmetry occurs when a system driven by such a force faces a déjà vu moment, but not with the same period as that of the force. ‘Time crystals’ have in the past decade been pursued as a new phase of matter, and more recently observed under elaborate experimental conditions in isolated systems. These experiments require extremely low temperatures or other rigorous conditions to minimize undesired external influences.
To study time crystals, scientists often use Bose-Einstein condensates of magnon quasiparticles. These have to be kept at extraordinarily low temperatures, very close to absolute zero. This requires very specialized, sophisticated laboratory equipment.
In their new research, the team created a time crystal without supercooling. Their time crystals were all-optical quantum systems created at room temperature. First, they took a tiny microresonator, a disk made out of magnesium fluoride glass just one millimeter in diameter. Then, they bombarded this optical microresonator with the beams of two lasers.
“When your experimental system has energy exchange with its surroundings, dissipation and noise work hand-in-hand to destroy the temporal order,” said lead author Hossein Taheri, an assistant research professor of electrical and computer engineering in UC Riverside’s Marlan and Rosemary Bourns College of Engineering. “In our photonic platform, the system strikes a balance between gain and loss to create and preserve time crystals.”
The self-preserving subharmonic spikes (solitons) that resulted from the frequencies generated by the two laser beams indicated the creation of time crystals. The system creates a rotating lattice trap for optical solitons that then display periodicity.
The UCR-led team utilized a technique called self-injection locking of the two lasers to the resonator to achieve robustness against environmental effects. Signatures of the temporally repeating state of this system can readily be measured in the frequency domain. The proposed platform therefore simplifies the study of this new phase of matter.
Without the need for a low temperature, the system can be moved outside a complex lab for field applications. One such application could be highly accurate measurements of time. Because frequency and time are mathematical inverses of each other, accuracy in measuring frequency enables accurate time measurement.
“We hope that this photonic system can be utilized in compact and lightweight radiofrequency sources with superior stability as well as in precision timekeeping,” said Taheri.
Sources:
https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.109.160401
https://www.nature.com/articles/s412-x
https://www.nature.com/articles/nature21426
https://news.ucr.edu/articles/2022/02/14/time-crystals-leave-lab