  To increase dramatically the distance and the secure key generation rate of
quantum key distribution (QKD), the idea of quantum decoys--signals of
different intensities--has recently been proposed. Here, we present the first
experimental implementation of decoy state QKD. By making simple modifications
to a commercial quantum key distribution system, we show that a secure key
generation rate of 165bit/s, which is 1/4 of the theoretical limit, can be
obtained over 15km of a Telecom fiber. We also show that with the same
experimental parameters, not even a single bit of secure key can be extracted
with a non-decoy-state protocol. Compared to building single photon sources,
decoy state QKD is a much simpler method for increasing the distance and key
generation rate of unconditionally secure QKD.
