  In the Gaussian-modulated coherent state quantum key distribution (QKD)
protocol, the sender first generates Gaussian distributed random numbers and
then encodes them on weak laser pulses actively by performing amplitude and
phase modulations. Recently, an equivalent passive QKD scheme was proposed by
exploring the intrinsic field fluctuations of a thermal source [B. Qi, P. G.
Evans, and W. P. Grice, Phys. Rev. A 97, 012317 (2018)]. This passive QKD
scheme is especially appealing for chip-scale implementation since no active
modulations are required. In this paper, we conduct an experimental study of
the passively encoded QKD scheme using an off-the-shelf amplified spontaneous
emission source operated in continuous-wave mode. Our results show that the
excess noise introduced by the passive state preparation scheme can be
effectively suppressed by applying optical attenuation and secure key could be
generated over metro-area distances.
