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Conference on Lasers and Electro-Optics

Experimental measurement of the orbital-angular-momentum spectrum in high-gain parametric down-conversion

Jeremy Rioux, Girish Ashok Kulkarni, Boris Braverman, Maria V. Chekhova, Robert W. Boyd

Published Jan 1, 2022• 0 citations
Publisher

Abstract

We measure the orbital angular momentum (OAM) spectrum of the field generated from spontaneous parametric down-conversion (SPDC) in the high-gain regime. The experimentally measured spectra and the angular Schmidt number are consistent with theoretical predictions.
DOI
10.1364/cleo_at.2022.jtu3b.48

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Abstract Light beams with orbital angular momentum (OAM) are convenient carriers of quantum information. They can also be used for imparting rotational motion to particles and providing high resolution in imaging. Due to the conservation of OAM in parametric down-conversion (PDC), signal and idler photons generated at low gain have perfectly anti-correlated OAM values. It is interesting to study the OAM properties of high-gain PDC, where the same OAM modes can be populated with large, but correlated, numbers of photons. Here we investigate the OAM spectrum of high-gain PDC and show that the OAM mode content can be controlled by varying the pump power and the configuration of the source. In our experiment, we use a source consisting of two nonlinear crystals separated by an air gap. We discuss the OAM properties of PDC radiation emitted by this source and suggest possible modifications.
We derive an exact computationally-efficient formula for the angular Schmidt spectrum of orbital angular momentum (OAM) entangled states produced by parametric down-conversion (PDC). Our formula yields the true spectrum and does not suffer from convergence issues arising due to infinite summations, as has been the case with previously derived formulas. We use this formula to experimentally characterize the angular Schmidt spectrum of entangled photons produced by PDC with noncollinear phase matching. We report measurements of very broad angular Schmidt spectra, corresponding to the angular Schmidt numbers up to 229. Our work could have important implications for OAM-based quantum information applications.
The generation of paired photons entangled in orbital angular momentum (OAM), provides a new degree of freedom that is increasingly being used as a source of quantum states. Elucidation of the OAM spectrum of OAM spectrum of the generated photons is of paramount importance, since quantum information applications require the ability to generate arbitrary entangled states with the appropriate OAM correlations. Here we discuss the OAM spectrum of the photon generated via SPDC in two complementary scenarios. On one hand, we should consider the whole geometry of the nonlinear process, taking into account azimuthal variations of the nonlinear coefficient or the phase matching conditions. On the other hand, all relevant experiments reported to date detect only a small section of the full down-conversion cone. In this scenario, the measured OAM correlations depend of the emission angle of the photons and the strength of the Poynting vector walk-off. We will present experiments in SPDC where this dependence is clearly revealed.
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