2e charge transport in a quantum Hall - superconductor junction revealed by shot noise

Two remarkable phenomena of condensed matter physics, the quantum Hall (QH) effect and superconductivity, when combined, can give rise to exotic topological excitations and hold great promises for future quantum computation. Andreev reflection (AR) is the underlying phenomena, which determines the quasiparticle dynamics at the QH–superconductor (SC) junction. Even decades after the discoveries of QH and SC, the AR at the QH-SC junction remains elusive. Here we have carried out conductance and shot noise measurements in a graphene QH coupled to a MoRe superconductor. The QH plateaus are observed at 4(N+1/2)e²/h, which are identical to the conductance plateaus with a normal contact. This observation is consistent with our theoretical calculation of electron-hole mixing due to repetitive AR at QH - SC junction. By measuring shot noise, we observe the Fano factor close to unity when the bias energy is less than the superconducting gap. This is direct evidence of 2e charge transport at QH - SC junction. Above the superconducting gap, the Fano factor almost vanishes, as expected due to regular charge e transport. This work will pave the way for exploration of more exotic systems, such as junction of a fractional QH and a superconductor, by shot noise measurements.