The bit‐error rate of the Josephson junction single‐flux‐quantum comparator was measured as a function of bias current offset, at clock rates up to 10 GHz. The bit‐error rate versus offset is a smooth curve, measured over 16 decades of incidence, which linearly extrapolates to 10−49for optimal bias. The lowest rate actually measured was 5×10−17, corresponding to 4 errors counted in 130 h. ©1996 American Institute of Physics.