Spatially structured photons that travel in free space slower than the speed of light

Daniel Giovannini; Jacquiline Romero; Václav Potoček; Gergely Ferenczi; Fiona Speirits; Stephen M. Barnett; Daniele Faccio; Miles J. Padgett

doi:10.1126/science.aaa3035

Spatially structured photons that travel in free space slower than the speed of light

Daniel Giovannini, Jacquiline Romero, Václav Potoček, Gergely Ferenczi, Fiona Speirits, Stephen M. Barnett, Daniele Faccio, Miles J. Padgett^*

^*Corresponding author for this work

Physics

Research output: Contribution to journal › Article › peer-review

134 Citations (Scopus)

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.

Original language	English
Pages (from-to)	857-860
Number of pages	4
Journal	Science
Volume	347
Issue number	6224
DOIs	https://doi.org/10.1126/science.aaa3035
Publication status	Published - 20 Feb 2015

Keywords

light beams
photons
Bessel beam
Gaussian beam
plane waves

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10.1126/science.aaa3035

Cite this

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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.",

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AU - Speirits, Fiona

AU - Barnett, Stephen M.

AU - Faccio, Daniele

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Spatially structured photons that travel in free space slower than the speed of light

Abstract

Keywords

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