  We demonstrate that the performance of threshold detectors based on Al
Josephson junctions can be significantly improved by exploiting the phase
diffusion regime. When the escape dynamics of the detector switches to this
regime, a decrease in both - dark count rate and the standard deviation of
switching current is simultaneously observed. However, this effect is essential
for (i) critical currents below 100 nA, and (ii) temperatures of the order of
several hundreds millikelvin. Importantly that for such detectors optimal
performance occurs at finite temperatures, making the microwave single photon
detection feasible even in the sub-K range. Possible explanation of these
findings is discussed.
