The time-programmable frequency comb and its use in quantum-limited ranging.

Two decades after its invention, the classic self-referenced frequency comb laser is an unrivalled ruler for frequency, time and distance metrology owing to the rigid spacing of its optical output 1,2 . As a consequence, it is now used in numerous sensing applications that require a combination of high bandwidth and high precision 3-5 . Many of these applications, however, are limited by the trade-offs inherent in the rigidity of the comb output and operate far from quantum-limited sensitivity. Here we demonstrate an agile programmable frequency comb where the pulse time and phase are digitally controlled with ±2-attosecond accuracy. This agility enables quantum-limited sensitivity in sensing applications as the programmable comb can be configured to coherently track weak returning pulse trains at the shot-noise limit. To highlight its capabilities, we use this programmable comb in a ranging system, reducing the required power to reach a given precision by about 5,000-fold compared with a conventional dual-comb system. This enables ranging at a mean photon per pulse number of 1/77 while retaining the full accuracy and precision of a rigid frequency comb. Beyond ranging and imaging 6-12 , applications in time and frequency metrology 1,2,5,13-23 , comb-based spectroscopy 24-32 , pump-probe experiments 33 and compressive sensing 34,35 should benefit from coherent control of the comb-pulse time and phase.