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Measuring the subluminal group velocity of structured single photons


The speed of light in vacuum is a universal physical constant. It is the speed at which all massless particles and their corresponding fields, such as light, travel in vacuum. An often overlooked part of this principle is that it only applies to plane-wave perturbations of the field. Ordinary light beams have finite transverse size, which leads to a modification of their wavevectors, that in turn results in a change to their phase and group velocities. This change–which can be many times greater than the optical wavelength and as such should not be mistaken with the Gouy phase–is typically very small but readily observable. I will discuss a recent experiment that studies how the group velocity of photon pairs from parametric down-conversion is affected by altering the beam's transverse spatial structure, by measuring the photons' relative arrival times with high precision. A clear and measurable reduction of the group velocity of single photons is attained; this otherwise classical effect can be best explored by combining structured light, single-photon measurements and quantum interference. These findings, supported by a full theoretical treatment, extend beyond the intuitive case of a diffraction-limited Bessel beam and have interesting consequences in different families of structured light, such as modes carrying orbital angular momentum.