Structured photons take it slow

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Science  20 Feb 2015:
Vol. 347, Issue 6224, pp. 828
DOI: 10.1126/science.aaa6931

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Light may behave both as a particle and as a wave. Newton conjectured the particle aspects and observed wave aspects (“Newton's rings”). Young showed wave interference in his double-slit experiments, and Einstein formulated light as particles (photons) to explain the photoelectric effect. This dual description leads to two different speeds for light: the wave speed, which for a plane wave in vacuum is the now fixed constant, c; and the particle or “group” speed at which energy or information propagates, which can be less than c—even zero, in some cases. On page 857 of this issue, Giovannini et al. (1) convincingly show that individual photons can have a speed less than c. In order to show this, the individual photons were prepared with transverse structure, so they cannot be represented as truly plane waves.