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Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC)

Conservation of Orbital Angular Momentum on a Single-Photon Level

Lea Kopf, Rafael F. Barros, Shashi Prabhakar, Enno Giese, Robert Fickler

Published Jun 23, 2025• 0 citations
Publisher

Abstract

Conservation laws are at the heart of our natural scientific understanding, as they dictate why certain phenomena do or do not exist in nature. In the realm of nonlinear optics — where light-matter interactions enable the generation of photon pairs — these principles are also essential. In the common interpretation of spontaneous parametric down-conversion (SPDC), the conservation of momenta is attributed to the transfer of the well-defined linear or orbital angular momenta (OAM) of the single pump photon to the generated photon pair [1], [2]. However, due to the low conversion efficiencies, this conservation rule has only been tested with a strong coherent laser pumping the process. Here, we show for the first time that the OAM of a single pump photon is indeed conserved and that an OAM quanta of up to 2 is transferred to the generated photon pair.

DOI
10.1109/cleo/europe-eqec65582.2025.11109832

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Conservation laws are at the heart of our natural scientific understanding, as they dictate why certain phenomena do or do not exist in nature. In the realm of nonlinear optics — where light-matter interactions enable the generation of photon pairs — these principles are also essential. In the common interpretation of spontaneous parametric down-conversion (SPDC), the conservation of momenta is attributed to the transfer of the well-defined linear or orbital angular momenta (OAM) of the single pump photon to the generated photon pair [1], [2]. However, due to the low conversion efficiencies, this conservation rule has only been tested with a strong coherent laser pumping the process. Here, we show for the first time that the OAM of a single pump photon is indeed conserved and that an OAM quanta of up to 2 is transferred to the generated photon pair.
Identifying conservation laws is central to every subfield of physics, as they illuminate the underlying symmetries and fundamental principles. A prime example can be found in quantum optics: the conservation of orbital angular momentum (OAM) during spontaneous parametric down-conversion (SPDC) enables the generation of a photon pair with entangled OAM. In this Letter, we report on the observation of OAM conservation in SPDC pumped on the single-photon level by a preceding SPDC process. We implement this cascaded down-conversion scheme in free space, without waveguide confinement, and thereby set the stage for experiments on the direct generation of multiphoton high-dimensional entanglement using all degrees of freedom of light.
States of light that are simultaneously eigenstates of orbital, intrinsic, and total angular momentum are found and eigenstates describing pairs of photons from spontaneous parametric down-conversion reveal classes of photon conversion with angular momentum conservation and cases where the initial angular momentum from the pump laser beam is shared between the converted photons and the generating medium. These angular momentum conservation laws open up a wide range of applications to be explored.
Orbital angular momentum (OAM)-entangled states produced by spontaneous parametric down-conversion (SPDC) are considered ideal for realizing high-dimensional entangled states, which have several advantages for quantum technologies. However, the limited sensitivity of current two-photon OAM detectors is a major roadblock not only for realizing such technologies but also for resolving foundational questions, such as OAM conservation in SPDC. The current theoretical understanding is that OAM is not conserved in Type-II SPDC but is conserved in Type-I. Experimentally, although non-conservation in TypeII has not been demonstrated, conservation in Type-I has been reported frequently and has become an underlying assumption for techniques generating high-dimensional OAM entangled states. In this work, we experimentally demonstrate a high-sensitivity two-photon OAM detector, using which, contrary to the current understanding, we report non-conservation of OAM in Type-I SPDC. We attribute this to a spatial walk-off effect and prove it using a framework free of standard phase-matching approximations.
We show that the transfer of the plane-wave spectrum of the pump beam to the fourth-order transverse spatial correlation function of the two-photon field generated by spontaneous parametric down-conversion leads to the conservation and entanglement of orbital angular momentum of light. By means of a simple experimental setup based on fourth-order (or two-photon) interferometry, we show that our theoretical model provides a good description for down-converted fields carrying orbital angular momentum.
We show that the orbital angular momentum (OAM) correlations, and the amount of spatial entanglement, of paired photons generated in spontaneous parametric downconver- sion are strongly affected by the use of noncollinear configurations and by the presence of Poynting vector walk-off. We discuss how our results are related to the question of the conservation of OAM in a nonlinear optics process, and we derive selection rules that allow to determine the sought-after OAM correlations, especially for those configurations used in current quantum information experiments.