# Integrated Frequency Comb Photon Sources

Optical frequency combs are light sources associated with a spectrum composed of many sharp individual lines. The idea to employ them as links between the radio and optical frequency domains (Nobel Prize in Physics, 2005) has changed the world in the areas of optical frequency metrology, precision measurement, and spectroscopic applications. Today these combs are commonly generated from pulsed, passively mode-locked solid-state or fiber lasers, which are however bulky and energy inefficient, limiting their application and implementation possibilities. The realization of these sources in compact devices will dramatically improve their complexity management, cost figure, and power consumption, thus allowing completely new application fields. By working in the quantum optics regime, new sources also enable advanced applications in fields like telecommunication, cryptography, and computing.

Our research group has pioneered the realization of optical frequency comb sources on a chip. By using the group’s acquired expertise in nonlinear integrated optics, we have recently proposed and demonstrated the first integrated photon pair source capable of generating multiple and simultaneous pairs of single photons – distributed on a frequency comb at telecommunication wavelengths. The source operates in a stable and robust configuration that is compatible with large-scale integration technology, as well as with state of the art quantum memories and repeaters. The device is ideal for multiplexed quantum optics applications with high pair production rates of around 300kHz per channel of the frequency comb at low continuous wave pump power below 30 mW.

Fig. 1 shows the measured photon correlations between 5 frequency channels symmetric to the pump wavelength (a) as well as the full correlation matrix between all combinations of these 5 channels, showing that photon pairs are generated only symmetric to the pump (b).

To further characterize the source, the unheralded idler-idler autocorrelation measurement (Fig 2a) can be used to measure the purity of the quantum state and amount of effective modes, where a pure state is represented by a single mode. The peak of the autocorrelation is given by 1/(1-number of modes). A measured peak of 1.537 corresponds to 1.86 effective modes, which is close to the ideal pure single mode state, confirming the high purity of the source. In order to confirm the quantum nature of the generated photons, we measure the heralded autocorrelation function (Fig. 2b). The heralded autocorrelation measurements represents a measure comparing the desired photon pair generation with respect to multiple photons generated together. A clear dip at zero time delay confirms that the source emits photon pairs, with a heralded autocorrelation dip well below the quantum limit of 0.5, confirming that the source operates in the single photon quantum regime.