We focus on the escape process from superconducting to normal states in a mesoscopic superconducting device, e.g., nanowire, biased at a current close to the critical depairing current. Our interest is to understand switching events which are not initiated by external perturbations, such as photons striking the wire. Such switching events are called 'dark counts' in the photon detection community. We will argue that the probable origin of dark counts is quantum tunneling of phase slips of Little type. Little's phase slips (LPS) are topological fluctuations that carry the superconducting order parameter between distinct current-carrying states . Owing to LPS, superconducting nanowires acquire electrical resistance and the supercurrent dissipates. In such wires, it is well known that at higher temperatures LPS occur through the process of thermal barrier-crossing by the superconducting order parameter. At low temperatures, the general expectation is that LPS should proceed through macroscopic quantum tunneling events, which are also called quantum phase slips (QPS) in nanowires (or 'dark counts' in photon detection devices based on nanowires). QPS in nanowire differ from macroscopic quantum tunneling events in Josephson junction because Little's phase slips have normal (non-superconducting) cores, which are filled with normal electrons. Linear resistive measurements have produced evidence both for and against QPS. Qualitatively different evidence for QPS events in thin wires is obtained through the statistical analysis of the switching current, proposed developed by Kurkijarvi . At higher temperatures, we observe that the width of the distribution of the switching current follows the Kurkijarvi power law , i.e. the variance it is proportional to the temperature to the power of 2/3 [i.e., sigma~T^(2/3)]. At lower temperatures the width of the distribution saturates, while the average switching current keeps increasing with cooling. These and other related facts provide strong evidence for the reality of macroscopic quantum tunneling in thin superconducting wires [3,4], which is the leading explanation for mysterious 'dark counts' in photon detectors.
1. W. A. Little, Phys. Rev. 156, 396 (1967)
2. J. Kurkij'arvi, Phys. Rev. B 6, 832 (1972)
3. M. Sahu et al., Nature Physics 5, 503 (2009)
4. T. Aref, et al., Phys. Rev. B 86, 024507 (2012)