Diffraction Induced Entanglement Losses
Hosted By: Quantum Optical Science and Technology Technical Group
30 September 2020, 12:00 - 13:00
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, that results in OAM entanglement decay. Another process that deteriorates entanglement of photonic spatial modes is diffraction on physical obstructions. 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 hosted by the OSA Quantum Optical Science and Technology Technical Group, Giacomo Sorelli of Laboratoire Kastler Brossel will use this convenient scenario to illustrate the main causes of OAM entanglement losses. We will first 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, studying diffraction on circular obstructions displaced with respect to the beam axis, 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, in this case, the diffraction-induced entanglement losses are a universal function of the product between the angular uncertainty and the orbital angular momentum of the encoding modes .
 G. Sorelli, V. N. Shatokhin, F. S. Roux, and A. Buchleitner, Diffraction- induced entanglement loss of orbital-angular-momentum states, Phys. Rev. A (2018).
 S. M. Barnett and D. T. Pegg, Quantum theory of rotation angles, Phys. Rev. A (1990).
 G. Sorelli, V. N. Shatokhin, and A. Buchleitner, Universal entanglement loss induced by angular uncertainty, J. Opt. (2020)
What You Will Learn:
- The webinar will include pedagogical/introductory parts on photonic orbital angular momentum and the uncertainty relation for angular position and angular momentum, as well as a more technical part on our own method to quantify how the entanglement of spatial modes of light is modified by transmission through diffracting screens.
Who Should Attend:
- Students and researchers interested in quantum optics with spatial modes of light.
About the Presenter: Giacomo Sorelli, Laboratoire Kastler Brossel
Giacomo completed his Master at the University of Florence with a thesis on how to efficiently generate entanglement useful for quantum metrology in ultra-cold atomic ensembles. Afterwards, he left his hometown to do a PhD at the University of Freiburg (Germany). In Freiburg, Giacomo got interested in the Orbital Angular Momentum (OAM) of light, and in particular to how this degree of freedom enables the generation of high-dimensional entangled states. During his PhD years, Giacomo’s research focused on how OAM-entanglement is affected by disturbances along the light propagation path. He studied how disturbances of deterministic nature, such as diffracting obstacles and apertures, as well as random perturbations, like those induced by atmospheric turbulence. In October 2019, Giacomo moved to Paris to be part of a multidisciplinary team that puts together researchers from quantum optics (Laboratoir Kastler Brossel), computer science (LIP6 - Sorbonne University) and radar science (ONERA - the French aerospace lab) to investigate new quantum technologies.