Probing the Dynamics of Andreev States in Coherent Normal/Superconducting ring: Evidence for a noisy supercurrent

Most properties of a non superconducting (N) metal connected to two superconductors (an SNS junction) can be seen as resulting from the phase dependent Andreev states (AS) in N. Density of states in N is then drastically changed with the emergence of a small energy gap, the minigap. Whereas AS equilibrium properties are well understood, AS dynamics is a more complex issue [1]. We perform experiments on a phase ($\phi$) biased NS ring coupled to a superconducting resonator. The modification of the resonances ($f$ from 200 MHz up to 14 GHz) yields the complex phase dependent susceptibility $\partial_\phi I_{ring}=\chi(f,\phi)=\chi'+i\chi''$. As expected, we find a non-dissipative $\chi'$ related to the supercurrent flowing through the ring. A more striking finding [2] is the existence of a dissipative $\chi''$ revealing a noisy supercurrent, predicted [3] but never observed before. Moreover, as $f$ increases we show that the main dissipation mechanism changes from population relaxation to microwave-induced transitions across the minigap.\\[4pt] [1] F. Chiodi et al, Sci. Rep, 1 (2011) \newline [2] B. Dassonneville et al, in preparation \newline [3] A. Martin-Rodero et al, PRB, 53 (1996)