  Quantum key distribution (QKD) enables two distant users, Alice and Bob, to
share secret keys. In existing QKD protocols, an eavesdropper's intervention
will inevitably disturb the quantum signals; thus, Alice and Bob must monitor
the signal disturbance to place a bound on the potential information leakage.
However, T. Sasaki et al. proposed a quite different protocol, named
round-robin differential phase shift (RRDPS), in which the amount of
eavesdropped information is bounded without monitoring the signal disturbance.
Here, we present the first active implementation of the RRDPS protocol. In our
experiment, Alice prepares packets of pulses, with each packet being a train
with 65 pulses, and the global phase of each packet is randomized. Bob uses a
1-GHz, 1-64-bit actively controlled variable-delay interferometer to realize
random switching of the different delays. Benefiting from the large pulse
number of each packet, the high stability and low insertion loss of the
interferometer, the system can distribute secret key over a distance of 90 km.
Our results confirm the feasibility of performing practical QKD with this novel
protocol.
