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

Ko-Fan Huang; Yuval Ronen; Régis Mélin; Denis Feinberg; Kenji Watanabe; Takashi Taniguchi; Philip Kim

doi:10.1038/s41467-022-30732-7

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

Ko-Fan Huang ; Yuval Ronen ; Régis Mélin ; Denis Feinberg ; Kenji Watanabe (National Institute for Materials Science) ; Takashi Taniguchi (National Institute for Materials Science) ; Philip Kim

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Ko-Fan Huang, Yuval Ronen, Régis Mélin, Denis Feinberg, Kenji Watanabe, Takashi Taniguchi, Philip Kim. Evidence for 4e charge of Cooper quartets in a biased multi-terminal graphene-based Josephson junction. Nature Communications. 2022, 13 (1), 3032. https://doi.org/10.1038/s41467-022-30732-7

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In a Josephson junction (JJ) at zero bias, Cooper pairs are transported between two superconducting contacts via the Andreev bound states (ABSs) formed in the Josephson channel. Extending JJs to multiple superconducting contacts, the ABSs in the Josephson channel can coherently hybridize Cooper pairs among different superconducting electrodes. Biasing three-terminal JJs with antisymmetric voltages, for example, results in a direct current (DC) of Cooper quartet (CQ), which involves a four-fermion entanglement. Here, we report interference of CQ formed in graphene based multi-terminal (MT) JJs with a magnetic flux loop. We observe that the critical quartet supercurrent exhibits magneto- oscillations associated with a charge of 4e, thereby presenting an evidence for interference between different CQ processes. The CQ critical current shows non-monotonic bias dependent behavior, which can be modeled by transitions between Floquet-ABSs. Our experimental observation for voltage-tunable nonequilibrium CQ-ABS in flux-loop-JJs significantly extends our understanding of MT-JJs, enabling future design of topologically unique ABS spectrum.

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