  We study photon assisted tunneling in Nb/AlOx/Nb Josephson junctions. A
quantitative calibration of the microwave field in the junction allowed direct
verification of the quantum efficiency of microwave photon detection by the
junctions. We observe that voltages of photon assisted tunneling steps vary
both with the microwave power and the tunneling current. However, this
variation is not monotonous, but staircase-like. The phenomenon is caused by
mutual locking of positive and negative step series. A similar locking is
observed with Shapiro steps. As a result the superconducting gap assumes
quantized values equal to multiples of the quarter of the photon energy. The
quantization is a manifestation of nonequilibrium tuning (suppression or
enhancement) of superconductivity by the microwave field.
