FIGURE 1. Sending photons faster than light.

a, How light absorption and refractive index of a dispersive material change rapidly with wavelength when the wavelength of the light pulse is near an atomic absorption band. The anomalous-dispersion region (where the group velocity of light can be negative) coincides with a region of strong light absorption. b, How the gain and refractive index of caesium gas changes with wavelength when there is a 'gain doublet' (two closely spaced peaks) in the amplification of light. Wang et al.⁵ show that in this case the anomalous-dispersion region can be used to make a pulse of light travel faster than c.