  In previous quantum key distribution (QKD) protocols, information is encoded
on either the discrete-variable of single-photon signal or continuous-variables
of multi-photon signal. Here, we propose a new QKD protocol by encoding
information on continuous-variables of a single photon. In this protocol, Alice
randomly encodes her information on either the central frequency of a
narrow-band single photon pulse or the time-delay of a broadband single photon
pulse, while Bob randomly chooses to do either frequency measurement or time
measurement. The security of this protocol rests on the energy-time uncertainty
relation, which prevents Eve from simultaneously determining both frequency and
time information with arbitrarily high resolution. In practice, this scheme may
be more robust against various channel noises, such as polarization and phase
fluctuations.
