  In conventional quantum key distribution (QKD) protocols, security is
guaranteed by estimating the amount of leaked information through monitoring
signal disturbance, which, in practice, is generally caused by environmental
noise and device imperfections rather than eavesdropping. Such estimation
therefore tends to overrate the amount of leaked information in practice, leads
to a fundamental threshold of the bit error rate. The threshold becomes a
bottleneck of the development of practical QKD systems. In classical
communication, according to Shannon's communication theory, information can
transform through a noisy channel even if the background noise is very strong
compare to the signal and hence the threshold of the bit error rate tends to
50%. One might wonder whether a QKD scheme can also tolerate error rate as high
as 50%. The question is answered affirmatively with the recent work of
round-robin differential phase-shift (RRDPS) protocol, which breaks through the
fundamental threshold of the bit error rate and indicates another potential
direction in the field of quantum cryptography. The key challenge to realize
the RRDPS scheme lies on the measurement device, which requires a
variable-delay interferometer. The delay needs to be chosen from a set of
predetermined values randomly. Such measurement can be realized by switching
between many interferometers with different delays at a high speed in
accordance with the system repetition rate. The more delay values can be chosen
from, the higher error rate can be tolerated. By designing an optical system
with multiple switches and employing an active phase stabilization technology,
we successfully construct a variable-delay interferometer with 128 actively
selectable delays. With this measurement, we experimentally demonstrate the
RRDPS QKD protocol and obtain a final key rate of 15.54 bps via a total loss of
18 dB and 8.9% error rate.
