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Europhysics Letters (EPL)

Perfect optical vortex for entangling orbital angular momentum by up-conversion in a vapour

Paul Seguigne, Laurence Pruvost

Published Jun 9, 2026• 0 citations
Publisher

Abstract

Abstract Up-conversion in an atomic vapor is known to produce photon pairs. If addressed by vortex beams, it produces pairs of orbital angular momenta (OAM), that are valuable bricks for quantum processes with OAM. The perfect vortex (PV) beam is a vortex class very pertinent in this context, because its ring radius and its Gouy phase are —in principle— independent of the OAM, giving a huge spiral bandwidth compared to the usual Laguerre-Gaussian (LG) beam. Our experimental study clarifies this point. The PV ring radius and thickness are found to vary hyperbolic-like vs . the OAM, which will limit the spiral bandwidth, but, unlike LG, the number of generated OAM pairs still remains quite comfortable for entanglement applications.

DOI
10.1209/0295-5075/ae7aba

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Abstract Up-conversion in an atomic vapor is known to produce photon pairs. If addressed by vortex beams, it produces pairs of orbital angular momenta (OAM), that are valuable bricks for quantum processes with OAM. The perfect vortex (PV) beam is a vortex class very pertinent in this context, because its ring radius and its Gouy phase are —in principle— independent of the OAM, giving a huge spiral bandwidth compared to the usual Laguerre-Gaussian (LG) beam. Our experimental study clarifies this point. The PV ring radius and thickness are found to vary hyperbolic-like vs . the OAM, which will limit the spiral bandwidth, but, unlike LG, the number of generated OAM pairs still remains quite comfortable for entanglement applications.
The orbital angular momentum (OAM) of light is a quantity explored for communication and quantum technology, its key strength being a wide set of values offering a large basis for quantum working. In this context we have studied the vortex conversion from a red optical vortex to a blue one, for OAMs ranging $\ensuremath{-}30$ to +30+30+30. The conversion is realized in a rubidium vapor, via the $5{S}_{1/2}\ensuremath{-}5{D}_{5/2}\phantom{\rule{0.16em}{0ex}}^{85}\mathrm{Rb}$ two-photon transition done with a Gaussian beam at 780nm780\phantom{\rule{4pt}{0ex}}\mathrm{nm}780nm plus a Laguerre-Gaussian beam at 776nm776\phantom{\rule{4pt}{0ex}}\mathrm{nm}776nm with the OAM $\ensuremath{\ell}$, producing a radiation at 420nm420\phantom{\rule{4pt}{0ex}}\mathrm{nm}420nm. With copropagating input beams, we demonstrate a conversion from red to blue for high-$\ensuremath{\ell}$ input vortices. We show that the output blue vortex respects the azimuthal phase matching, has a size determined by the product of the input beams, and a power decreasing with $\ensuremath{\ell}$, in agreement with their overlap. Its propagation indicates that the generated blue wave is a nearly pure Laguerre-Gaussian mode. The vortex converter thus permits a correct OAM transmission.
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.
Abstract Laguerre-Gauss beams, and more generally the orbital angular momentum of light (OAM) provide valuable research tools for optical manipulation, processing, imaging and communication. High-efficiency frequency conversion of OAM is possible via four-wave mixing in rubidium vapour. Conservation of the OAM in the two pump beams determines the total OAM shared by the generated light fields at 420 nm and 5.2 μm—but not its distribution between them. Here we experimentally investigate the spiral bandwidth of the generated light modes as a function of pump OAM. A small pump OAM is transferred almost completely to the 420 nm beam. Increasing the total pump OAM broadens the OAM spectrum of the generated light, indicating OAM entanglement between the generated light fields. This clears the path to high-efficiency OAM entanglement between widely disparate wavelengths.
The "perfect" vortex is a new class of optical vortex beam having ring radius independent of its topological charge (order). One of the simplest techniques to generate such beams is the Fourier transformation of the Bessel-Gauss beams. The variation in ring radius of such vortices require Fourier lenses of different focal lengths and or complicated imaging setup. Here we report a novel experimental scheme to generate perfect vortex of any ring radius using a convex lens and an axicon. As a proof of principle, using a lens of focal length f = 200 mm, we have varied the radius of the vortex beam across 0.3-1.18 mm simply by adjusting the separation between the lens and axicon. This is also a simple scheme to measure the apex angle of an axicon with ease. Using such vortices we have studied non-collinear interaction of photons having orbital angular momentum (OAM) in spontaneous parametric down-conversion (SPDC) process and observed that the angular spectrum of the SPDC photons are independent of OAM of the pump photons rather depends on spatial profile of the pump beam. In the presence of spatial walk-off effect in nonlinear crystals, the SPDC photons have asymmetric angular spectrum with reducing asymmetry at increasing vortex radius.
Optical vortex light engendered with integer units of orbital angular momentum (OAM) may be involved in frequency upconversion. Second harmonic generation is usually forbidden in isotropic media due to parity constraints, but it becomes allowed by six-wave mixing. Here, we present a rigorous quantum analysis for the case of a Laguerre-Gaussian input beam comprising photons endowed with a single unit of OAM. Such a process gives rise to the novel entanglement of orbital momentum in two emergent photons; it transpires that the mechanism delivers a harmonic output whose polarization is essentially parallel to the incident radiation. This investigation ascertains the character of the emission, both under forward propagation and back-reflection geometries, and identifies in detail the form of distribution in the entangled total orbital momentum. A distinctive conical spread, originating from the entangled distribution in the emission pair, affords a potential means to determine the individual angular momenta.
In this article we investigate the orbital angular momentum spectrum of photons produced in parametric down conversion process. We study how the transverse profile of the pump mode affects the spectrum as compared to a gaussian pump beam. We show that using perfect optical vortex mode as pump the spectrum can be highly narrowed down. We further study the entanglement between the two down converted photons in the orbital angular momentum basis. We observe that photons entangled in Laguerre-Gaussian modes exhibit greater higher dimensional entanglement.