High impedance point contact Josephson junctions

We describe point contact junctions with normal state resistances of 10Ω - 10kΩ, made from electropolished Nb parts, which couple well to millimeter radiation. The response is governed by fluctuations. The principal effect of external radiation is a change in the dynamic resistance. An approximate expression is given for the differential resistance at zero current, based on a linear combination of the effect of fluctuations and of microwave radiation. Experimental data show significant deviations from the RSJ model. The results are in qualitative agreement with numerical calculations by Kalashnik et al. We argue that in our junctions zero-point fluctuations are dominant, caused by the very small capacitance of about 10^-15F.