High-dimensional (entangled) quantum states are an important resource to increase the channel capacity and to enhance the security of quantum communication protocols. The orbital angular momentum (OAM) of photons spans an infinite-dimensional Hilbert space, and it is therefore suitable to encode such high-dimensional quantum states. However, disturbances along the light propagation path can deteriorate the information encoded in OAM photonic states. For example, the turbulence-induced phase distortions lead to crosstalk among different OAM modes, which results in OAM entanglement decay. One of the main differences between diffraction and propagation across turbulence is that, in the former case, the scattering of the wave is deterministic. Thus the quantum state corresponding to a diffracted wave is a pure, infinite-dimensional state of a twisted biphoton, whose entanglement can be quantified by a bona fide entanglement measure such as concurrence.
In this webinar, we use this scenario to illustrate the main causes of OAM entanglement losses. Giacomo Sorelli will present an analytical expression that quantifies diffraction-induced entanglement losses in terms of the mutual overlap between the diffracted images of the optical modes used to encode the entanglement. We then use this formula to investigate the role of the radial structure of the encoding OAM modes in mitigating the phase distortions and consequently the entanglement decay. In particular, we prove that entanglement encoded into Bessel-Gaussian modes experiences reduced losses when compared to that encoded into Laguerre-Gaussian modes. In the second part of the webinar, we will study the entanglement losses induced by angular apertures. Using the uncertainty relation for angular position and angular momentum, we demonstrate that the diffraction-induced entanglement losses are a universal function of the product between the angular uncertainty and the OAM of the encoding modes.