  Since several papers appeared in 2000, the quantum key distribution (QKD)
community has been well aware that photon number splitting (PNS) attack by Eve
severely limits the secure key distribution distance in BB84 QKD systems with
Poissonian photon sources. In attempts to solve this problem,
entanglement-based QKD, single-photon based QKD, and entanglement
swapping-based QKD, have been studied in recent years. However, there are many
technological difficulties that must be overcome before these schemes can
become practical systems. Here we report a very simple QKD system, in which
secure keys were generated over >100 km fibre for the first time. We used an
alternative protocol of differential phase shift keying (DPSK) but with a
Poissonian source. We analysed the security of the DPSK protocol and showed
that it is robust even against hybrid attacks including collective PNS attack
over consecutive pulses, intercept-and-resend (I-R) attack and beamsplitting
(BS) attack, because of the non-deterministic collapse of a wavefunction in a
quantum measurement. To implement this protocol, we developed a new detector
for the 1.5 um band based on frequency up-conversion in a periodically poled
lithium niobate (PPLN) waveguide followed by a Si avalanche photodiode (APD).
The use of the new detectors increased the sifted key generation rate up to > 1
Mbit/s over 30 km fibre, which is two orders of magnitude larger than the
previous record.
