Pulsed quantum frequency combs from an actively mode-locked intra-cavity generation scheme

Summary form only given. Here, we demonstrate the first scheme to generate pulsed quantum frequency combs without the need for an external laser. We constructed a laser cavity (Fig. 1a) around a 4-port integrated micro-ring resonator, featuring narrow bandpass filters to select a single resonance, an optical gain component (erbium-doped fiber amplifier), and a mode-locking element (an intensity modulator). By setting the intensity modulation frequency to the fundamental repetition rate of the cavity (determined by the external cavity length), we achieved mode-locked pulsed operation (Fig. 1b), exciting a pulsed two-photon comb. Driving the intensity modulator at multiples of the fundamental repetition rate (upwards of 4x) also resulted in stable mode-locking (Fig. 1c), significant as this enables an increase of the photon pair generation rate without sacrificing its coincidence-to-accidental ratio (CAR). We characterized the photon statistics as a function of the average ring input power (see Fig. 1d), achieving a maximum CAR of 110 that decreases with increasing power and increasing coincidence rates. Autocorrelation measurements confirmed the purity of the generated state (pure single-frequency modes, even at multiples of the repetition rate), confirming that the resonance was excited over its entire bandwidth, and obtained a heralded anti-bunching dip of 0.245<;0.5 as expected of a source operating in the single-photon regime.