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Frontiers in Optics

Intensity correlations of twisted photons generated by spontaneous parametric down-conversion

Nijil Lal, Ali Anwar, Chithrabhanu Perumangatt, Ravindra Pratap Singh

Published Jan 1, 2017• 0 citations
Publisher

Abstract

We generate heralded single photons carrying orbital angular momentum (OAM) by spontaneous parametric down-conversion (SPDC). We study the influence of orbital angular momentum of the photon on the second-order optical coherence.
DOI
10.1364/fio.2017.jw4a.13

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Twisted light, or light with orbital angular momentum (OAM), plays an emerging role in both classical and quantum science, with important applications in areas as diverse as biophotonics, micromachines, spintronics, or quantum information. It offers fascinating opportunities for exploring new fundamental ideas in physics, as well as for being used as a tool for practical applications. One important point is to determine how to generate single photons, and two-photon states, with an appropriate OAM content. Here we describe the paraxial orbital angular momentum of entangled photon pairs generated by spontaneous parametric down-conversion (SPDC) in different non-collinear geometries. These geometries introduce a variety of new features. In particular, we find the OAM of entangled pairs generated in purely transverse-emitting configurations, where the entangled photons counter-propagate perpendicularly to the direction of propagation of the pump beam. The spatial walk-off of all interacting waves in the parametric process also determines the OAM content of the down-converted photons, and here its influence is also revealed.
Abstract In this work, we investigate the orbital angular momentum (OAM) spectrum of photons generated via spontaneous parametric down-conversion (SPDC) and study how shaping the transverse profile of the pump beam influences the resulting spectrum. Using a perfect optical vortex (POV) mode as the pump, we show that the OAM spectrum narrows significantly, leading to purer OAM states. We further examine the entanglement between the down-converted photons in the OAM basis, finding that Laguerre–Gaussian (LG) pumped photons exhibit increased higher-dimensional entanglement at the expense of mode purity.
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.
The generation of paired photons entangled in the spatial degree of freedom, i.e., in orbital angular momentum, offers a convenient physical resource to investigate the nature of entanglement in a multidimensional Hilbert space with controllable dimensionality. The two main physical processes that generate pairs of photons which show correlations in orbital angular momentum are (a) spontaneous parametric down-conversion (SPDC), and (b) Raman transitions induced in atomic ensembles. One question naturally arises: what kinds of correlations exist between the orbital angular momentum of the generated photons? The answer might be different if we consider the whole quantum state of the generated photons, i.e., all possible directions where the pairs of photons can be emitted, or if we consider only a small section of the full set of directions.