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Evidence for 4e charge of Cooper quartets in a biased multi-terminal graphene-based Josephson junction

Theorists in Néel Institute have predicted that phase-coherent DC quartets, made of two entangled Cooper pairs, form in a three-terminal Josephson junction. After experiments in metallic (CEA Grenoble) and nanowire (Weizmann) junctions, now the Harvard group uses a graphene junction, with a two-branch grounded terminal enclosing a magnetic flux, providing the first evidence for the 4e charge of quartets. The nonmonotonous voltage dependence and the « inverted » flux dependence of the quartet current point towards transitions between Andreev levels triggered by the voltage. This interpretation is supported by model calculations using Keldysh Green’s functions.