Electrical current in normal metals (or semiconductors) is carried by electrons or holes. When one introduces a large barrier for the transmission of current along a normal metal, the transport happens via tunneling of electrons across the barrier. The quanta of charge that goes through the barrier in each tunneling event, i.e., the electron charge, can be extracted from the Shot Noise that is introduced in the transmitted current [1]. In a superconductor-normal conductor junction the current - at energy lower than the band gap of the superconductor - proceeds through the process of Andreev Reflection (AR) in which the charge is transferred in shots of twice the electron charge[2].

In a Josephson device made of two superconductors weakly connected by a so called ‘weak link’ (that can be a thin insulator, a normal conductor, or another “weak” superconductor), the charge transport is richer and involves processes of Multiple Andreev Reflections (MAR). If the transmission through the weak link is small enough, the charge shots are quantized and are equal to n*e, where e is the electron charge, and n is an integer varying with the bias[3].

We fabricated a Josephson device made of a short segment of InAs nanowire contacted by two superconducting leads (Aluminum) and observed the sub-harmonic gap structure in the differential conductance arising from the MAR processes. We then measured the Shot Noise and deduced the effective transported charge up to 5e. Further investigations of the Shot Noise at the superconductor gap (at bias of) are being made and may reveal information about the superconductor quasiparticle charge at this important energy regime.

[1] Schottky, Annalen der Physik 57: 541-567 (1918)

[2] Das et al., Nature Communications 3, 1165 (2012)

[3] Y. Naveh, D.V. Averin, Phys. Rev. Lett. 82, 4090 (1999)