How fast is a twisted photon?

Recent measurements have highlighted that even in vacuum spatially shaped photons travel slower than 푐, 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.