  Since the invention of Bennett-Brassard 1984 (BB84) protocol, many quantum
key distribution (QKD) protocols have been proposed and some protocols are
operated even in field environments. One of the striking features of QKD is
that QKD protocols are provably secure unlike cryptography based on
computational complexity assumptions. It has been believed that, to guarantee
the security of QKD, Alice and Bob have to monitor the statistics of the
measurement outcomes which are used to determine the amount of the privacy
amplification to generate a key. Recently a new type of QKD protocol, called
round robin differential phase shift (RRDPS) protocol, was proposed, and
remarkably this protocol can generate a key without monitoring any statistics
of the measurement outcomes. Here we report an experimental realization of the
RRDPS protocol. We used a setup in which Bob randomly chooses one from four
interferometers with different pulse delays so that he could implement phase
difference measurements for all possible combinations with five-pulse time-bin
states. Using the setup, we successfully distributed keys over 30 km of fiber,
making this the first QKD experiment that does not rely on signal disturbance
monitoring.
