We describe an experimental investigation of fluctuations in the measured value of the critical current in Josephson tunnel junctions. The lifetime $\tau$ of the zero-voltage state against spontaneous transitions to a nonzero-voltage state is derived from these measurements. The range of $\tau$ covered by these measurements runs from ${10}^{-1\mathrm{}}$ to ${10}^{-7\mathrm{}}$ sec, with $\tau$ decreasing approximately exponentially with increasing current bias. Both thermal-noise-limited and extrinsic-noise-limited situations have been observed. In associated experiments the maximum critical current of these Sn-Sn-oxide-Sn juntions is found to be reduced from the value predicted by weak-coupling theory by a factor of $\approx 0.92$, presumably by strong-coupling effects.

• Received 29 October 1973

DOI:https://doi.org/10.1103/PhysRevB.9.4760