title: Optics. Spatially structured photons that travel in free space slower than the speed of light.
abstract: That the speed of light in free space is constant is a cornerstone of modern physics. However, light beams have finite transverse size, which leads to a modification of their wave vectors resulting in a change to their phase and group velocities. We study the group velocity of single photons by measuring a change in their arrival time that results from changing the beam's transverse spatial structure. Using time-correlated photon pairs, we show a reduction in the group velocity of photons in both a Bessel beam and photons in a focused Gaussian beam. In both cases, the delay is several micrometers over a propagation distance of ~1 meter. Our work highlights that, even in free space, the invariance of the speed of light only applies to plane waves.
authors.affiliation: School of Physics and Astronomy, Scottish Universities Physics Alliance (SUPA), University of Glasgow, Glasgow G12 8QQ, UK.
authors.forename: Daniel
authors.initials: D
authors.lastname: Giovannini
authors.affiliation: School of Physics and Astronomy, Scottish Universities Physics Alliance (SUPA), University of Glasgow, Glasgow G12 8QQ, UK.
authors.forename: Jacquiline
authors.initials: J
authors.lastname: Romero
authors.affiliation: School of Physics and Astronomy, Scottish Universities Physics Alliance (SUPA), University of Glasgow, Glasgow G12 8QQ, UK. Faculty of Nuclear Sciences and Physical Engineering, Czech Technical University in Prague, Brehová 7, 115 19 Praha 1, Czech Republic.
authors.forename: Václav
authors.initials: V
authors.lastname: Potoček
authors.affiliation: School of Physics and Astronomy, Scottish Universities Physics Alliance (SUPA), University of Glasgow, Glasgow G12 8QQ, UK.
authors.forename: Gergely
authors.initials: G
authors.lastname: Ferenczi
authors.affiliation: School of Physics and Astronomy, Scottish Universities Physics Alliance (SUPA), University of Glasgow, Glasgow G12 8QQ, UK.
authors.forename: Fiona
authors.initials: F
authors.lastname: Speirits
authors.affiliation: School of Physics and Astronomy, Scottish Universities Physics Alliance (SUPA), University of Glasgow, Glasgow G12 8QQ, UK.
authors.forename: Stephen M
authors.initials: SM
authors.lastname: Barnett
authors.affiliation: School of Engineering and Physical Sciences, SUPA, Heriot-Watt University, Edinburgh EH14 4AS, UK.
authors.forename: Daniele
authors.initials: D
authors.lastname: Faccio
authors.affiliation: School of Physics and Astronomy, Scottish Universities Physics Alliance (SUPA), University of Glasgow, Glasgow G12 8QQ, UK. miles.padgett@glasgow.ac.uk.
authors.forename: Miles J
authors.initials: MJ
authors.lastname: Padgett