Reports

The speed of light is trivially given as /c n, where c is the speed of light in free space and n is the refractive index of the medium. In free space, where 1,n = the speed of light is simply c. We show that the introduction of transverse structure to the light beam reduces the group velocity by an amount depending upon the aperture of the optical system. The delay corresponding to this reduction in the group velocity can be greater than the optical wavelength and consequently should not be confused with the ≈π Gouy phase shift (1, 2). To emphasize that this effect is both a linear and intrinsic property of light, we measure the delay as a function of the transverse spatial structure of single photons. The slowing down of light that we observe in free space should also not be confused with slow, or indeed fast, ligh