Heralding single photons with cascaded downconversion

Patrick F. Poitras, Evan Meyer-Scott, Deny R. Hamel

is of primary importance.

Patrick F. Poitras, Deny R. Hamel, Evan Meyer-Scott

Heralded single photon sources are the subject of intense research owing to their usefulness in metrology and quantum information. These are often created using a pair of photons produced by spontaneous parametric downconversion (SPDC), from which one photon is used to certify the presence of the other. However, multiple-pair production and detector dark counts limit the quality of such sources. Here, we investigate whether cascading two downconversion events to precertify the presence of a photon pair would allow us to reduce these effects. Using analytical calculations and simulations, we find that higher purity photons, as quantified by the heralded second order correlation function, can be obtained for some detector performance ranges.

T. B. Pittman, B. C. Jacobs, J. D. Franson

Mohammed Charafi, Alexandre Z. Leger, Samridhi Gambhir +1 more

Three-photon states generated via cascaded spontaneous parametric downconversion provide a direct route to multipartite entanglement. However, current implementations require careful spectral matching between successive nonlinear stages, which constrains the choice of downconversion sources. In this work, we show that coincidence-based spectral filtering relaxes this requirement by conditionally tailoring the spectrum of the pump photon entering the second stage. By filtering the herald photon, we conditionally tailor the spectrum of its partner to match the acceptance bandwidth of the secondary nonlinear process, enabling efficient coupling between broadband and narrowband stages without altering the sources themselves. Using an electro-optically gated spectrometer, we directly measure the conditional spectra that govern the cascaded process, allowing us to quantitatively predict the enhancement in second-stage conversion probability per detected herald. We then verify this prediction through photon-triplet measurements, demonstrating improved performance at fixed heralding rates. Our results establish coincidence-based spectral engineering as a practical tool for optimizing cascaded downconversion, particularly in regimes limited by detector saturation or spectral incompatibility.

Xue-lin Liu, Tong Xiang, Xianfeng Chen

Philip F. Schewe