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Physical Review A

Non-Gaussian states via pump-depleted spontaneous parametric downconversion

Colin Vendromin, Samuel E. Fontaine, John E. Sipe

Published Jan 26, 2026• 3 citations
Publisher
DOI
10.1103/mg15-nzrl

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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.
Partially coherent optical beams present phenomena that may not appear in perfectly coherent beams. In this regard, spontaneous parametric downconversion is an intriguing physical process, since the optical properties of the pump laser beam are transferred to the quantum properties of the down-converted photon pairs. Here, the authors study the transfer of twist phase, a novel property of the so-called twisted Gaussian Schell model beams, from a pump beam to down-converted photons. The authors show that a non-zero twist phase can lead to highly entangled yet highly mixed states. The correlations that appear in near/far-field intensity measurements, in the two-photon covariance matrix as well as in orbital angular momentum modes, are explored. The authors derive expressions for the purity of the two-photon state and relate it to the transverse spatial coherence of the pump beam. Their main results utilize a Wigner function formalism that can be applied to a number of scenarios involving partially coherent downconversion sources and contribute to the recent interest in spatially correlated photon pairs produced in mixed states, with possible applications in quantum communication and imaging.
In this work we study, for the spontaneous parametric downconversion process, how the transverse amplitude of the pump may be transferred to one of the emitted photons in a given pair when heralded by the detection of the remaining photon. We present a theoretical description of this process along with a discussion of the short-crystal regime within which faithful transfer of the transverse amplitude occurs. We show experimental results for Gaussian and Bessel–Gauss pump beams. In both cases, we verify that the transverse amplitude mechanism occurs and for the former also shows the effects of a departure from the short crystal regime. For the latter we show that our heralded single photons exhibit a non-diffractive behavior, and for orders l = 1 and l = 2 we show that these heralded single photons are in fact vortices with orbital angular momentum transferred from the pump.