Collaborative Research: Photonic Chip-Scale Time Crystals

Garden variety crystals like salt grains and snowflakes, or silicon crystals forming the backbone of modern-day electronics all consist of large numbers of atoms or molecules. As orderly and symmetric as water molecules sit in a snowflake, the hexagonal crystal represents not symmetry but its breaking through the reduction of the perfect symmetry of empty space into the discrete symmetry of an array of molecules.

While a small number of atoms can form molecules with new physical and chemical properties by chemical bonding, only a truly large number (~10^23, hundreds of thousands of billions of billions) of such interacting building blocks coming together can form crystals. The resultant crystals possess yet again unique properties, giving rise to their utility.

About a decade ago, scientists started theorizing time crystals – systems consisting of a large number of interacting building blocks which break symmetry, not in space but in the time dimension. It was shown that, paralleling solid-state spatial crystals, time crystals offer desirable characteristics (e.g., temporal robustness), as well as totally new physical effects (e.g., avoiding loss of crystalline order when typically heating is expected to destroy a crystal).

These inherent time crystal properties are crucial for future applications such as quantum computation where quantum bits of data are expected to preserve their information over time and after several reading operations. Experimental demonstrations of time crystals have thus far remained scarce and particularly limited to isolated systems which are not conducive to real-world applications.

They have also been largely confined to “small” crystals, typically with only 2 temporal elementary cells. The proposed research aims to surmount these limitations by realizing time crystals in non-isolated systems using photons in miniaturized nonlinear optical devices. This platform empowers investigation of unexplored aspects of time crystals and demonstrating their application in precision timekeeping.

Additionally, it accommodates “big” time crystals, hence offering the possibility of realizing temporal analogues of condensed matter physical effects and addressing open questions using the mature photonic technology. For further impact, our proposed program includes scientific professional education outreach and workshop components which will develop and deliver a curriculum to aspiring high school students and engineers.

The team will demonstrate discrete time crystals in dissipative Kerr nonlinear cavities. Leveraging the flexibilities afforded by nanofabrication of integrated photonic structures, time crystals will be realized and investigated in silicon nitride microring resonators pumped by polychromatic lasers. Dispersion engineering and judicious design and pumping of the resonator will ensure realizing big time crystals.

The state of the created time crystals will be controlled and transition between different phases will be achieved by means of frequency modulation and sweep. Stabilization of time crystals will be achieved by self-injection locking two lasers to two non-adjacent same-family optical modes of the microring resonator and tracked through monitoring the phase noise of the beatnote between the pump lasers versus that of the generated subharmonics.

The frequency division inherent to the realization of discrete time crystals in this platform results in the reduction of the phase noise. The system can be used for frequency reference transfer by locking the pump lasers to external frequency references. Successful demonstration of the proposed platform combining concepts from photonics and condensed matter physics significantly accelerates the investigation of time crystals as a new phase of matter and reveals some of their practical applications.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.