Direct Detection of the Angular Momentum of Light

Mohammad Mirhosseini, Mehul Malik, Zhimin Shi +1 more

Liang Fang, Lan Luo, Rujin Zhao +1 more

The utilization of orbital angular momentum (OAM) in light for information encoding is a promising avenue, as it exhibits resilience against environmental disturbances. However, the direct detection of OAM poses a significant challenge due to beam distortion during transmission, which ultimately limits the practical application of OAM-based optical communication. We propose an environmentally friendly and intuitive detection method based on optical differentiation using a weak-measurement system. Optical differential measurements were conducted on Laguerre-Gaussian beams (L-G beams) with$l$= 5 and$l$= 8 to validate the feasibility of this approach. This approach mitigates the impact of disturbances on OAM measurement, enabling direct detection through a simplified system, which serves as a valuable reference for utilizing OAM as an information carrier in free-space optical communication.

G. Alzetta, E. Arimondo, C. Ascoli

Khoi-Nguyen Huynh-Vu, Lin Htoo Zaw, Valerio Scarani

We introduce entanglement witnesses for spin ensembles which detect genuine multipartite entanglement using only measurements of the total angular momentum. States that are missed by most other angular-momentum-based witnesses for spin ensembles, which include Greenberger-Horne-Zeilinger states and certain superpositions of Dicke states, can be effectively detected by our witness. The protocol involves estimating the probability that the total angular momentum is positive along equally spaced directions on a plane. Alternatively, one could measure along a single direction at different times, under the assumption that the total spins undergo a uniform precession. Genuine multipartite entanglement is detected when the observed score exceeds a separable bound. Exact analytical expressions for the separable bound are obtained for spin ensembles ${j}_{1}\ensuremath{\bigotimes}{j}_{2}\ensuremath{\bigotimes}\ensuremath{\cdots}\ensuremath{\bigotimes}{j}_{N}$ such that the total spin is a half-integer and numerical results are reported for the other cases. Finally, we conjecture an expression for the separable bound when the total spin is not known, which is well supported by the numerical results.

Stephen Mark Barnett, Fiona C. Speirits, Mohamed Babiker

Abstract Stated simply the paradox is as follows: it is clear that the orbital angular momentum of a light beam in its direction of propagation is an intrinsic quantity, and therefore has the same value everywhere in the beam. How then can a Gaussian beam, with precisely zero orbital angular momentum, drive a (single-photon) quadrupole transition which requires the transfer of angular momentum 2ℏ to an absorbing atom?

Jonathan Leach, Miles J. Padgett, Stephen Mark Barnett +2 more