How fast is a twisted photon?

TL;DR

This study investigates how adding orbital angular momentum to photons affects their speed, revealing that twisting photons can accelerate them compared to untwisted photons, with experimental and theoretical support.

Contribution

The paper demonstrates experimentally and theoretically that introducing OAM to photons reduces their delay, effectively accelerating them, which advances understanding of photon propagation with OAM.

Findings

Adding OAM reduces photon delay, indicating acceleration.

Experimental confirmation using Hong-Ou-Mandel interferometry.

Theoretical calculations support the observed OAM acceleration.

Abstract

Recent measurements have highlighted that spatially shaped photons travel slower than c, the speed of monochromatic, plane waves in vacuum. Here we investigate the intrinsic delay introduced by `twisting' a photon, i.e. by introducing orbital angular momentum (OAM). In order to do this we use a Hong-Ou-Mandel interferometer to measure the change in delay of single photons when we introduce OAM on a ring-shaped beam that is imaged through a focusing telescope. Our findings show that when all other parameters are held constant the addition of OAM reduces the delay (accelerates) with respect to the same beam with no OAM. We support our results using a theoretical method to calculate the group velocity and gain an intuitive understanding of the measured OAM acceleration by considering a geometrical ray-tracing approach.