  In the Gaussian-modulated coherent states (GMCS) quantum key distribution
(QKD) protocol, Alice prepares quantum states \emph{actively}: for each
transmission, Alice generates a pair of Gaussian-distributed random numbers,
encodes them on a weak coherent pulse using optical amplitude and phase
modulators, and then transmits the Gaussian-modulated weak coherent pulse to
Bob. Here we propose a \emph{passive} state preparation scheme using a thermal
source. In our scheme, Alice splits the output of a thermal source into two
spatial modes using a beam splitter. She measures one mode locally using
conjugate optical homodyne detectors, and transmits the other mode to Bob after
applying appropriate optical attenuation. Under normal conditions, Alice's
measurement results are correlated to Bob's, and they can work out a secure
key, as in the active state preparation scheme. Given the initial thermal state
generated by the source is strong enough, this scheme can tolerate high
detector noise at Alice's side. Furthermore, the output of the source does not
need to be single mode, since an optical homodyne detector can selectively
measure a single mode determined by the local oscillator. Preliminary
experimental results suggest that the proposed scheme could be implemented
using an off-the-shelf amplified spontaneous emission source.
