Noise characteristics of an ideal shunted Josephson junction

Detailed computer simulations have been made of the effect of intrinsic noise current on the I-V characteristic and voltage noise spectral density S_v(f) of an ideal Josephson element shunted by a capacitor and resistor. The minimum value of the parameter _c at which hysteresis appears in the quasistatic I-V characteristic is greater than the noise-free value of 1. Moreover, as _c is increased, a region of large differential resistance R_d appears in the I-V characteristic that is associated with a large increase in S_V(0). In this regime the noise current is seen to cause random switching between the superconducting and nonzero voltage states. For all bias conditions, S_V(f) > R_d²S_I(f) where S_I(f) is the spectral density of the intrinsic current noise. A comparison is made between these results and previous calculations. The implications for low-noise superconducting devices are discussed.