Unconventional motional narrowing in the optical spectrum of a semiconductor quantum dot

'Motional narrowing' refers to the striking phenomenon where the resonance line of a system coupled to a reservoir becomes narrower on increasing the reservoir fluctuation. A textbook example is found in nuclear magnetic resonance, where the fluctuating local magnetic fields created by randomly oriented nuclear spins are averaged when the motion of the nuclei is thermally activated. The existence of a motional-narrowing effect in the optical response of semiconductor quantum dots remains so far unexplored. This effect may be important in this instance because the decoherence dynamics is a central issue for the implementation of quantum information processing based on quantum dots. Here we report on the experimental evidence of motional narrowing in the optical spectrum of a semiconductor quantum dot broadened by the spectral-diffusion phenomenon. Surprisingly, motional narrowing is achieved when decreasing incident power or temperature, in contrast with the standard phenomenology observed for nuclear magnetic resonance.