## Quantum optics

This area of research is devoted to translating some of our realizations in nonlinear optics to the single photon level. Nonlinear optics processes are fundamentally interactions between photons of different frequencies and their creation/annihilation can be studied via their quantum correlations. Our works are related to photon sources and the coding of quantum information into high-dimensional Hilbert space. Applications are various and typically include quantum key distribution, optical quantum computing, quantum simulator circuits.

## Quantum frequency comb

The frequency combs demonstrated using SiN ring resonator are the result of an initial degenerate four wave mixing. The process converts two photons from an intense field into two new photons at different frequencies red and blue-detuned with respect to the initial ones. At the quantum level, that means a pair of photon is created and one can observe that event by separating the red-detuned from the blue-detuned photon and detecting them with single photon avalanche photodiodes showing they are generated at the same time. In the case of a quantum frequency comb, pairs of photons are generated on pairs of cavity resonances such that we can demonstrate the frequency-time entanglement of the generated state. [Frontiers in Optics 2011, FThE5t.]

The remarkable features of our photon source are a single photon linewidth of 1 GHz while the spectral separation between the emitted two photons range from 400 GHz to several THz. Being monolithically integrated on a chip, this source is designated for being integrated with multiple components in a low loss yet stable circuit for linear optical quantum information processing.

## Frequency coding of quantum information

As a second example, we manipulate the frequency of single photon using the process of four-wave-mixing via Bragg scattering. This work is closely related to the time-frequency entangled photon source as it allows manipulating its degree of entanglement, i.e. the frequency. Our goal is to create frequency waveplates that would act in the frequency domain as polarization waveplates in their respective degree of freedom. In a first proof of principle, we demonstrate the generation of qutrits states coded onto the frequency [CLEO 2012, QM2H.6.]

The Bragg scattering process allows the frequency conversion to longer or shorter wavelength (or both) depending on the phase matching condition. As the pump wavelength is tunable, a large set of frequency shift can be implemented. This opens the way to frequency encoding onto a high-dimensional space unlike what is allowed by the polarization.

## Frequency Multiplexed Single Photon Source:

Deterministic sources of single photons are fundamental to scalable implementations of quantum technologies in communications and information processing. These applications require a source that emits indistinguishable single photons in well-defined spatio-temporal and spectral modes. Entangled photons from parametric processes such as down-conversion and spontaneous four wave mixing have been used for proof of principle demonstrations of such sources in quantum optics. These sources however are fundamentally limited by multi-photon noise, limiting their maximum heralding efficiency to 25%. To overcome these limitations, schemes to multiplex single photons from multiple identical sources using active feed forward switching have been proposed. Using the frequency degree of freedom, we are working on a novel scheme to multiplex single photons. The scheme, illustrated in the figure , relies on low noise quantum frequency translation via Bragg scattering – four-wave mixing (BS-FWM), which has been demonstrated with close to 100% efficiency [CLEO 2015 FM3A.4]. A spectrally broad parametric single-photon source is filtered into a comb constituting the frequency channels to be multiplexed. Using BS-FWM as the “frequency switch”, multiple frequency channels are translated to a single spectral mode by tuning the wavelength of the auxiliary pumps involved in BS-FWM. This technique can be directly implemented using integrated photonic platforms, in synergy with novel ultra-bright single-photon sources such as micro-resonators, where the emission is concentrated in well-defined frequency bins [CLEO 2016 FTh1C.2]