Signatures of phase-coherent transport and the role of quantum fluctuations in the dynamical Coulomb blockade regime

Josephson junctions operated in the dynamical Coulomb blockade regime recently gained an significant amount of attention as central building block in concepts to demonstrate the non-Abelian character of Majorana fermions. Its physical properties are strongly affected by the intimate interplay of intrinsic quantum fluctuations and environmentally-induced quantum fluctuations each of which promoting different Cooper pair transport mechanisms at small voltages around zero. To shed light on the detailed transport mechanisms occurring in this type of junction, we performed voltage-biased measurements on the small-capacitance Josephson junction of a scanning tunneling microscope at milli-Kelvin temperatures. The low voltage-regime of experimental current-voltage characteristics can be modeled by the two complementary descriptions of phase coherent and incoherent Cooper pair transport, signaling the occurrence of qualitatively different transport mechanisms. This observation receives further support from analyzing the calculated Fano factor of the current noise as a probe for correlations in Cooper pair transport, following a theoretical proposal. Together our experimental observations and related data analysis provide a clear signature of coherent Cooper pair transport along with the absence of perfect charge quantized transport around zero voltage, as well as of incoherent Cooper pair transport towards higher voltages.