Integrated Optics for Single-Photon Nonlinear Interactions

An ideal platform for integrated quantum optics should combine linear optics (e.g. beam splitters)and electro-optical control (e.g. fast modulators) with nonlinear optical functions. Nonlinearprocesses, including down-conversion and quantum frequency conversion (QFC), are especiallyimportant for generating and manipulating quantum states. While many possible optical materialssuitable for integration exist, no material has been shown to be more powerful than lithium niobate(LN). This is in part due to its high nonlinear coefficient, the ability to phase-match nonlinearinteractions via quasi-phase matching (QPM), and its large transparency window. Recently, LNhas demonstrated nonlinear effects at the single-photon level, which opens up new possibilitiessuch as deterministic entanglement and nonlinear enhanced entanglement swapping.Unfortunately, the observed photon-level interaction strengths are very small, limiting the methodto extremely modest success rates. Some candidate technologies that could improve the effectivenonlinearity, like diamond-turned whispering gallery mode resonators [1], are potentially powerfulbut are not compatible with integration. While strong photon-level interactions can be achievedvia an atomic interaction in a photonic cavity, there are various practical limitations to suchsystems, such as a typical requirement for cryogenic cooling. If strong photon-photon nonlinearinteractions could instead be realized in a technology compatible with photonic integrated circuits(PICs), then photon-level nonlinearities would become practical tools enhancing other importantPIC functions, like interferometric beam combination and electro-optical switching, therebyallowing complex and efficient quantum information processing (QIP) systems on chip.In this whitepaper we propose to investigate how to realize and leverage ultra-strong nonlinearinteractions for QIP applications. We plan to exploit a recently developed processing method,where waveguides are created by rib-loading thin-film LN. The technique produces submicronmode sizes that are ~100× smaller than the incumbent technologies such as reverse protonexchange (RPE), leading to greatly enhanced effective nonlinearity. Therefore, functions likeentanglement swapping via nonlinear-assisted Bell State Measurements (BSMs) [2] will becomeorders of magnitude more efficient. Exotic state generation such as tripartite Greenberger-Horne-Zeilinger (GHZ) entangled states would also benefit from similar rate improvements [3]. Moreover,the devices are integrated on a silicon substrate and are compatible with large-scale photonicintegration. The approach has been successful in demonstrating high quality (Q-factor) integratedcavities as well as optical modulators with record high efficiencies. We will extend its use tononlinear frequency conversion including nonlinear interactions that are appreciable at the singlephotonlevel. We expect this will lead to orders of magnitude improvement in rate for functionslike heralded entangled photon generation. Furthermore, by incorporating low-loss switchingnetworks (realizable due to the extremely large electro-optical effect), it is possible to manipulatequantum signals, for instance by multiplexing indistinguishable heralded single-photon sources tocreate a near deterministic single-photon source in a specific spatio-temporal mode (thuscompatible with single photon interference schemes such as Hong-Ou-Mandel interference).Such applications take advantage of the exceptionally high performance and wide functionalcapability of LN to realize high qua