  In quantum key distribution (QKD), the bit error rate is used to estimate the
information leakage and hence determines the amount of privacy amplification
--- making the final key private by shortening the key. In general, there
exists a threshold of the error rate for each scheme, above which no secure key
can be generated. This threshold puts a restriction on the environment noises.
For example, a widely used QKD protocol --- BB84 --- cannot tolerate error
rates beyond 25%. A new protocol, round-robin differential phase shifted
(RRDPS) QKD, essentially removes this restriction and can in principle tolerate
more environment disturbance. Here, we propose and experimentally demonstrate a
passive RRDPS QKD scheme. In particular, our 500 MHz passive RRDPS QKD system
is able to generate a secure key over 50 km with a bit error rate as high as
29%. This scheme should find its applications in noisy environment conditions.
