Frequency-Domain Quantum Interference with Correlated Photons from an Integrated Microresonator

Frequency encoding of quantum information together with fiber and integrated photonic technologies can significantly reduce the complexity and resource requirements for realizing all-photonic quantum networks. The key challenge for such frequency domain processing of single photons is to realize coherent and selective interactions between quantum optical fields of different frequencies over a range of bandwidths. Here, we report frequency-domain Hong-Ou-Mandel interference with spectrally distinct photons generated from a chip-based microresonator. We use four-wave mixing to implement an active “frequency beam splitter” and achieve interference visibilities of $0.95\pm 0.02$. Our work establishes four-wave mixing as a tool for selective high-fidelity two-photon operations in the frequency domain which, combined with integrated single-photon sources, provides a building block for frequency-multiplexed photonic quantum networks.

Figure 1

Frequency beam splitter via BS-FWM: (a) Two strong classical pumps $({\omega }_{P1},{\omega }_{P2})$ mediate the interaction between two quantum fields, $({\omega }_R,{\omega }_B)$ in a medium with third-order ${\chi }^{(3)}$ nonlinearity. (b) Measured signal conversion and depletion (efficiency $\eta =97\%$). BS-FWM acts as a $50:50$ frequency beam splitter (FBS) when the pump power is such that the nonlinear interaction strength $\gamma PL=\pi /8$. (c) Two frequency-correlated photons incident on a $50:50$ FBS. Perfect HOM-type interference is observed when the two-photon amplitude for both photons being frequency translated ($RR$) and neither photon being translated ($TT$) are indistinguishable. This occurs when the BS-FWM pump separation $\mathrm{\Omega }$ matches the input photon-pair separation, resulting in $\mathrm{\Delta }\mathrm{\Omega }=0$ [see (d)].

Figure 2

Frequency domain two-photon interference via BS-FWM: Narrow band frequency-correlated photons are generated via spontaneous four-wave mixing (SFWM) in a silicon nitride microring resonator. The generated photons are coupled together with two classical pump waves through a WDM into a dispersion-shifted fiber for Bragg-scattering four-wave mixing (BS-FWM). The power of the BS-FWM pumps is set such that it acts as a $50:50$ frequency beam splitter and their frequency separation $\mathrm{\Omega }$ is set to precisely match the selected pair of correlated photons. After the nonlinear interaction, the two frequency arms are separated through a WDM followed by detection and coincidence counting. Hong-Ou-Mandel type interference is observed and both photons are bunched in the same frequency mode.

Figure 3

Experimental observation of frequency domain two-photon interference. Red curves and blue curves (dots: experiment, solid line: fit) are the normalized threefold coincidence counts when the BS-FWM pumps are on and off, respectively. The photon bandwidth is measured to be $270\pm 15\mathrm{MHz}$. (a) When $\mathrm{\Delta }\mathrm{\Omega }=0$, we observe a flat $G^{(2)}(\tau )$ with a raw visibility ${\alpha }_r=0.90\pm 0.03$ (fit visibility ${\alpha }_f=0.92\pm 0.05$. (b) For a detuning $\mathrm{\Delta }\mathrm{\Omega }/2\pi =300\mathrm{MHz}$, we observe temporal beating with a raw visibility ${\alpha }_r=0.95\pm 0.02$ (fit visibility ${\alpha }_f=0.95\pm 0.04$). (c) For $\mathrm{\Delta }\mathrm{\Omega }/2\pi =600\mathrm{MHz}$, we observe an increase in the amplitude of the side lobes of the beating signal with a measured raw visibility of ${\alpha }_r=0.86\pm 0.04$ (fit visibility ${\alpha }_f=0.90\pm 0.04$). (d) For very large detuning ($\mathrm{\Delta }\mathrm{\Omega }/2\pi =5\mathrm{GHz}$), the interference fringes are no longer resolved resulting in a double-exponential output ($\alpha =0$). Error bars are calculated assuming Poisson statistics.

Figure 4

Photon bunching via autocorrelation. Measured enhancement in two-photon bunching for small (purple, $\mathrm{\Delta }\mathrm{\Omega }/2\pi =300\mathrm{MHz}$) over large pump detunings (orange, $\mathrm{\Delta }\mathrm{\Omega }/2\pi =5\mathrm{GHz}$) with a measured peak autocorrelation of $1.93\pm 0.13$.