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Frontiers in Optics

Quantum-enhanced Second Harmonic Generation Beyond the Spontaneous Parametric Down-conversion Regime

T. Dickinson, Ivi Afxenti, Giedre Astrauskaite, Lennart Hirsch, Samuel Nerenberg, Ottavia Jedrkiewicz, Daniele Faccio, C. Müllenbroich, A. Gatti, Matteo Clerici, Lucia Caspani

Published Jan 1, 2025• 0 citations
Publisher

Abstract

We report quantum-enhanced second harmonic generation driven by parametric down-conversion (PDC), with efficiencies exceeding that of classical light up to pumping intensities ~10 times larger than the standard classical/quantum boundary (1 PDC photon per mode).
DOI
10.1364/fio.2025.fw6c.1

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Long ago, it was predicted that stimulating two-photon processes such as two-photon absorption (TPA) and second harmonic generation (SHG) with entangled photons produced by parametric down-conversion (PDC) could result in higher efficiencies compared to classical illumination [1]. This was experimentally demonstrated for SHG [2] and TPA [3], where, however, the classical comparison was performed by adding losses to the PDC state rather than employing a true classical reference. In addition, these results considered spatially single-mode PDC radiation and were conducted at low photon fluxes, where the PDC modes are, on average, populated by less than one photon. This resulted in an extremely low SHG/TPA signal, considered of limited practical use for metrological applications (such as two-photon fluorescence microscopy or SHG imaging - see, e.g., [4]). A recent theoretical investigation [5] highlighted that a proper analysis of quantum-enhanced two-photon processes requires a fully spatiotemporal model of the nonlinear interactions. It further showed that highly multimode (in space and time) entangled PDC photons can provide a quantum advantage compared to an equivalent classical pulse, even in regimes well beyond the “one-photon-per-mode” limit, considered the boundary of quantum behavior.