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arXiv:2007.10514 · Physical Review A

Optimization of a lossy microring resonator system for the generation of quadrature-squeezed states

Colin Vendromin, Marc M. Dignam

Published Aug 11, 2020• 14 citations
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Abstract

The intensity buildup of light inside a lossy microring resonator can be used to enhance the generation of squeezed states via spontaneous parametric downconversion (SPDC). In this work, we model the generation of squeezed light in a microring resonator that is pumped with a Gaussian pulse via a side-coupled channel waveguide. We theoretically determine the optimum pump pulse duration and ring-to-channel coupling constant to minimize the quadrature noise (maximize the squeezing) in the ring for a fixed input pump energy. We derive approximate analytic expressions for the optimal coupling and pump pulse duration as a function of scattering loss in the ring. These results will enable researchers to easily determine the optimal design of microring resonator systems for the generation of quadrature-squeezed states.

DOI
10.1103/physreva.102.023705

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The intensity buildup of light inside a lossy microring resonator can be used to enhance the generation of squeezed states via spontaneous parametric downconversion (SPDC). In this work, we model the generation of squeezed light in a microring resonator that is pumped with a Gaussian pulse via a side-coupled channel waveguide. We theoretically determine the optimum pump pulse duration and ring-to-channel coupling constant to minimize the quadrature noise (maximize the squeezing) in the ring for a fixed input pump energy. We derive approximate analytic expressions for the optimal coupling and pump pulse duration as a function of scattering loss in the ring. These results will enable researchers to easily determine the optimal design of microring resonator systems for the generation of quadrature-squeezed states.
We present a semi-analytic formalism for calculating the squeezing and antisqueezing spectrum in a channel waveguide side-coupled to a lossy ring resonator. Our approach first uses the semi-analytic evolution of the density matrix inside the ring up to the time where the squeezing is maximized. Then noting that a conservative approximate result for the squeezing can be obtained by ignoring the effect of the pump at later times, we calculate the free evolution of the field operators in the channel waveguide at all later times. We then calculate the quadrature squeezing spectrum in the waveguide, assuming that the measurement starts at the time when the squeezing in the ring is maximized. Using these results, we determine the optimum values for the pump pulse duration and amplitude and the ring-channel coupling for the pump and signal. We find that squeezing above 10 dB can be easily achieved in the channel for antisqueezing levels of less than 22 dB.
We explore how III-V semiconductor microring resonators can efficiently generate photon pairs and squeezed vacuum states via spontaneous parametric down-conversion by utilizing their built-in quasi-phase-matching and modal dispersion. We present an analytic expression for the biphoton wave function of photon pairs generated by weak pump pulses and characterize the squeezed states that result under stronger pumping conditions. Our model includes loss and captures the statistics of the scattered photons. A detailed sample calculation shows that for low pump power conversion efficiencies of ${10}^{\ensuremath{-}5}$, corresponding to a rate of 39 MHz for a pump power of $1\phantom{\rule{4pt}{0ex}}\ensuremath{\mu}\mathrm{W}$, are attainable for rudimentary structures such as a simple microring coupled to a waveguide, in both the continuous-wave and pulsed-excitation regimes. Our results suggest that high levels of squeezing and pump depletion are attainable, possibly leading to the deterministic generation of non-Gaussian states.
Squeezed light is an essential nonclassical resource for quantum sensing, continuous-variable quantum information processing, low-noise optical communication, and new photonic computing systems. However, to make practical use of it, we need compact, low-power squeezed-light sources that can fit on a chip. In this work, we introduce a theoretical model of a dual-pump microring optical parametric amplifier for squeezed-light integrated photonics. The model outlines cavity-enhanced Kerr four-wave mixing among two pump fields, a signal mode, and the conjugate idler sideband. We use a linearized quantum Langevin approach under the undepleted-pump assumption. We derive the frequency-domain inputoutput relation and assess the output quadrature noise based on several factors: external coupling rate, intrinsic loss, pump power, signal detuning, nonlinear coefficient, and dispersion. First, we analyze a dispersion-engineered silicon nitride microring that operates in the telecom band. It shows that moderate external coupling offers the best trade-off between intracavity field enhancement and squeezed-field extraction. With the right pump detuning, the simulated silicon nitride resonator achieves over 10 dB of quadrature noise reduction at milliwatt-level input pump powers. We also compare AlGaAs and InP microring platforms, demonstrating that the squeezing performance is influenced not only by the nonlinear coefficient but also by the combined effects of material nonlinearity, cavity quality factor, loss, coupling, dispersion, and detuning. This framework gives design guidelines for low-power, chip-scale squeezed-light sources aimed at future quantum and AI-oriented photonic integrated circuits.
We develop a model for non-Gaussian state generation via spontaneous parametric down-conversion (SPDC) in InGaP microring resonators. The nonlinear Hamiltonian is written in terms of the asymptotic fields for the system, which includes a phantom channel to handle scattering loss. The full ket for the system is written as a Gaussian unitary acting on a residual non-Gaussian ket, which is vacuum initially and evolves according to a non-Gaussian Hamiltonian. We show that for realistic parameters we can access the pump depletion regime, where the Wigner function for the residual non-Gaussian ket has negativity. But we find that the non-Gaussian features for the full ket could be unobservable due to the large amount of squeezing required to lead to pump depletion. We show that a potential solution in the low-loss regime is to implement an inverse Gaussian unitary on the accessible modes to remove most of the squeezing and reveal the non-Gaussian features. This work provides a foundation for modeling pump-depleted SPDC in integrated lossy microring resonators, opening a path toward a scalable on-chip non-Gaussian source.
We develop a general Hamiltonian treatment of spontaneous four-wave mixing in a microring resonator side-coupled to a channel waveguide. The effect of scattering losses in the ring is included, as well as parasitic nonlinear effects including self- and cross-phase modulation. A procedure for computing the output of such a system for arbitrary parameters and pump states is presented. For the limit of weak pumping an expression for the joint spectral intensity of generated photon pairs, as well as the singles-to-coincidences ratio, is derived.