Abstract
Recent measurements have highlighted that even in vacuum spatially shaped photons travel slower than c, the speed of monochromatic plane waves. Here we investigate the intrinsic delay introduced by “twisting” a photon, i.e., by introducing orbital angular momentum (OAM), and measure the photon time of flight with a Hong–Ou–Mandel interferometer. 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.
Highlights
The propagation velocity of light in vacuum is a constant, c, only for monochromatic plane waves: any deviation from the planewave constraint can lead to a propagation velocity different from c
This result is not in contradiction with previous results where the spatial profile of a classical light beam was allowed to vary as orbital angular momentum (OAM) was introduced
The main purpose of our study here is that we are investigating the intrinsic OAM delay, rather than the overall effect due to both OAM and spatial reshaping of the beam due to the OAM. This acceleration of the photon due to OAM can be understood within the framework of ray optics and is related to the fact that the longest path for a ray is given by rays that invert an image through a telescope
Summary
The propagation velocity of light in vacuum is a constant, c, only for monochromatic plane waves: any deviation from the planewave constraint can lead to a propagation velocity different from c. The propagation of light with an azimuthal phase gradient, i.e., orbital angular momentum (OAM), was examined recently [12,13] In this case, one must allow for the fact that these beams have a ring-shaped spatial profile and that for increasing OAM, the ring radius naturally increases for the typical beam types encountered in experiments, e.g., Laguerre–Gauss (LG) or hypergeometric beams. These findings are supported by a theoretical model and ray-tracing considerations providing intuitive explanations for the observed effect. Measurements of photons in cosmology could be subject to discrepancies in arrival time if these subtle effects are not properly accounted for [20]
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