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Research in Optical Sciences

Maximizing the Dimensionality of Orbital Angular Momentum Entanglement in Parametric Down-conversion

Jacquiline Romero, Daniel Giovannini, Filippo M. Miatto, Stephen Mark Barnett, Miles J. Padgett

Published Jan 1, 2012• 0 citations
Publisher

Abstract

The process of spontaneous parametric down-conversion is a dependable source of high-dimensional orbital angular momentum-entangled bi-photon states. We observe, analyze and maximize the number of orbital angular momentum modes produced by the down-conversion nonlinear crystal and detected by our measurement apparatus.
DOI
10.1364/qim.2012.qw3a.2

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Any practical experiment utilizing the innate DDD-dimensional entanglement of the orbital angular momentum (OAM) of photons is subject to the generation capacity of the entangled photon source and the modal capacity of the detection system. We report an experimental spontaneous parametric-down-conversion system able to generate and detect tunable high-dimensional OAM entanglement. By tuning the phase matching, we demonstrate a factor of 2 increase on the half-width of the OAM-correlation spectrum, from 10 to 20. In terms of quantum mutual information capacity, this is an increase from 3.183.183.18 to 4.954.954.95 bits/photon. Furthermore, we measure correlations in the conjugate variable, angular position, and obtain concurrence values 0.960.960.96 and 0.900.900.90. The good entanglement measures in both OAM and angular position bases indicate bipartite, DDD-dimensional entanglement where DDD is tunable.
Photon pairs generated from spontaneous parametric down-conversion are a well-established method to realize entangled bipartite photonic systems. Laguerre-Gaussian modes, which carry orbital angular momentum (OAM), are commonly exploited to engineer high-dimensional entangled quantum states. For Hilbert spaces with dimension d>2d>2d>2, maximally entangled states (MESs) help to improve the capacity and security of quantum communication protocols, among several other promising features. However, the direct generation of MESs in well-defined high-dimensional subspaces of the infinite OAM basis has remained a challenge. Here, we formalize how the spatial distribution of the pump beam and the nonlinear profile of the crystal can be simultaneously utilized to generate MESs without additional spatial filtering of OAM modes within a subspace. We illustrate our approach with maximally entangled qutrits (d=3)(d=3)(d=3) and ququints (d=5)(d=5)(d=5).
Entanglement, as a crucial feature of quantum systems, is essential for various applications of quantum technologies. High-dimensional entanglement has the potential to encode arbitrary large amount of information and enhance robustness against eavesdropping and quantum cloning. The orbital angular momentum (OAM) entanglement can achieve the high-dimensional entanglement nearly for free stems due to its discrete and theoretically infinite-dimensional Hilbert space. A stringent limitation, however, is that the phase-matching condition limits the entanglement dimension because the coincidence rate decreases significantly for high-order modes. Here we demonstrate relatively flat high-dimensional OAM entanglement based on a spontaneous parametric down conversion (SPDC) from an ultrathin nonlinear lithium niobite crystal. The difference of coincidences between the different-order OAM modes significantly decreases. To further enhance the nonlinear process, this microscale SPDC source will provide a promising and integrated method to generate optimal high-dimensional OAM entanglement.