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Enhancement of a nonlinear optical interaction through waveguides or resonators disclose unconventional interplay among multiple lights. Microresonator-based optical frequency comb (OFC) generation via third order nonlinearity is a typical example of such enhancements. Recently, quadratic-nonlinearity-based OFC with an external cavity configuration has been found and its on-chip implementation is highly demanded. Here we for the first time demonstrate such an on-chip OFC with a quadratic nonlinear waveguide resonator. Furthermore, we controlled the comb spectra separation by adjusting frequency difference of two pump light. This on-chip quadratic device will be useful for not only metrologies but also integrated quantum information technologies.

We demonstrate a frequency multiplexed photon pair generation based on a quadratic nonlinear optical waveguide inside a cavity which confines only signal photons without confining idler photons and the pump light. We monolithically constructed the photon pair generator by a periodicallypoled lithium niobate (PPLN) waveguide with a high reflective coating for the signal photons around 1600 nm and with anti-refrective coatings for the idler photons around 1520 nm and the pump light at 780 nm at the end faces of the PPLN waveguide. We observed a comb-like photon pair generation with a mode spacing of the free spectral range of the cavity. Unlike the conventional multiple resonant photon pair generation experiments, the photon pair generation were incessant within a range of 80 nm without missing teeth due to a mismatch of the energy conservation and the cavity resonance condition of the photons, resulting in over 1000-mode frequency multiplexed photon pairs in this range.Photon pairs produced by spontaneous parametric down conversion (SPDC) or spontaneous four wave mixing are commonly used as a resource of single photons and entangled photon pairs in photonic quantum information experiments. Unfortunately, the photon pairs include not only the genuine single photon pair but also multiple photon pair emission, which degrades the quality of the single-photon-based experiments. Typically, suppression of the multiple photon emission is achieved by setting a single photon emission probability to a value much smaller than unity, while it makes the amount of the vacuum large and the success probability of the protocol small. To boost the success probability without increasing the photon emission rate per mode, parallel processing of the protocol [1,2] or use of a larger Hilbert space [3,4] is effective. For this purpose, frequency multiplexed photon pair generation has been actively studied. Such a photon pair generator can be realized by using an optical parametric oscillator (OPO) far below threshold [5]; A nonlinear optical medium is installed in an optical cavity for confining the photons, and a sufficiently weak pump light for the photon pair generation is used. Typical nonlinear optical media have a wide bandwidth for photon pair generation, whereas the optical cavity suppresses the photon generation in nonresonant modes. As a result, photon pairs with clear frequency mode separation are produced. So far, a lot of experiments of the photon pair generation with various cavity configurations have been performed [1,6] by the quadratic nonlinerity with an external cavity [7, 8], a quadratic nonlinearity-based microring resonator [9], and Kerr nonlinearity-based microring resonators [4,[10][11][12][13][14][15]. However, to the best of our knowledge, all of the previous demonstrations for frequency multiplexed photon pair generation have used a doubly-resonant OPO which confines both the signal and the idler photons, or a triply-resonant OPO which additionally confines the pump light. While the photon pair generation by us...

No abstract

We demonstrated a frequency-multiplexed photon pair generation over 1000 modes by using a nonlinear optical waveguide inside a cavity which confines only signal photons without confining idler photons and the pump light.