The next generation of space-based networks for communications, sensing, and navigation will contain optical clocks embedded within satellites. To fully realize the capabilities of such clocks, high-precision clock synchronization across the networks will be necessary. Current experiments have shown the potential for classical frequency combs to synchronize remote optical clocks over free space. However, these classical combs are restricted in precision to the standard quantum limit. Quantum frequency combs, however, which exhibit quantum properties such as squeezing and entanglement, provide pathways for going beyond the standard quantum limit. Here, we present our perspective on the prospects for practical clock synchronization in space using both classical and quantum frequency combs. We detail the current outcomes achievable with a classical frequency comb approach to synchronization, before quantifying the potential outcomes offered by quantum frequency combs. Challenges to be overcome in deploying frequency combs in space are presented, and the implications of almost-perfect synchronization for future space-based applications and experiments are discussed.
Optical frequency comb technology has been revolutionizing optical fields of ultra-precision metrology over two decades, connecting the SI unit standards of various optical clocks with utmost accuracy and stability. With an increasing demand for free-space communication and time/frequency transfer applications, the frequency comb is stretching its versatility even the outside of laboratory. Still, the environmental disturbances of free space degrade the excellent phase noise of the optical frequencies, hindering the reliable utilization of frequency comb at any desirable location. This research performs comb-to-comb synchronization over a 1.3-km Free-Space Optical link (FSO) with frequency stability of 3.80×10-15 at 0.1 s averaging time throughout the entire comb spectrum. Phase-locked loop compensates the Doppler shift of optical frequencies propagating atmospheric free-space channel, providing optical references for synchronizing distant frequency comb.