Realizing a strong photon-photon coupling is a central goal in quantum nonlinear optics, holding a promise, e.g., for single-photon nonlinear optics and deterministic optical quantum computation. While bulk nonlinear optics offers a scalable and room-temperature solution, requirements for nonlinearity and loss are extremely demanding, which have not been realized to this date. In this talk, we propose a novel route to all-optical strong coupling: ultrashort pulses on dispersion engineered nanophotonics. In addition to the transverse field confinement provided by the waveguide, ultrashort pulses form “flying cavities” that enjoy tight longitudinal confinement, significantly enhancing the nonlinear coupling. In this regime, however, strong nonlinearity inevitably induces multimode non-Gaussian quantum dynamics that naïvely require an exponentially large Hilbert space. To this, we introduce our recent developments in model reduction techniques, i.e., approaches with matrix-product states and non-Gaussians supermode model, which enables us to numerically understand the nonlinear quantum dynamics of photons beyond conventional semiclassical (i.e., linearized) treatments. We then present a novel “temporal trapping” scheme to harness the multimode nature of quantum pulses, which we can leverage to construct, e.g., a deterministic photon-photon gate. State of the art in thin-film lithium niobate waveguides suggests that we are, in principle, already in the strong coupling regime using ultrashort pulses, and we expect our work to provide a generic framework to understand and engineer the rich but complicated dynamics of broadband photons on this unique frontier.
 R. Yanagimoto, R. Hamerly et al., “Temporal trapping of ultrashort pulses enables deterministic optical quantum computation”, arXiv:2203.11909.
 R. Yanagimoto, E. Ng et al., “Onset of non-Gaussian quantum physics in pulsed squeezing with mesoscopic fields”, Optica 9, 379 (2022).
 R. Yanagimoto et al., “Efficient simulation of ultrafast quantum nonlinear optics with matrix product states”, Optica 8, 1306 (2021).
 R. Yanagimoto, T. Onodera et al., “Engineering a Kerr-Based Deterministic Cubic Phase Gate via Gaussian Operations”, Phys. Rev. Lett. 124, 240503 (2020).
Ryotatsu Yanagimoto is a senior Ph.D. student in the group of Prof. Hideo Mabuchi at Stanford University. His research interest spans AMO physics in general, while he focuses on the science of quantum devices at present. He currently works on the theoretical research of broadband non-Gaussian quantum optics, aiming at understanding and engineering coherent multimode dynamics of photons on nonlinear nanophotonics beyond the conventional framework of Gaussian quantum optics. Previously, he worked at the University of Tokyo and RIKEN on experimental research of optical lattice clocks, where he received a B.E. He is a recipient of a Masason fellowship and a Stanford Q-FARM Ph.D. fellowship.