Fig. 1: Illustration of quartet formation and a single source voltage bias characterization of four-terminal Josephson junction including a loop.

a Schematic illustration of the three-terminal quartet process with the Andreev reflection picture. The middle superconductor \({S}_{0}\) is grounded while the other two superconductors are biased at +V, −V, respectively. The two entangled Cooper pairs (with red and blue electrons) are formed in \({S}_{0}\) through two local Andreev reflections and two crossed Andreev reflections. b False color scanning electron microscopy (SEM) image of the device with measurement configurations. Graphene (purple) is top-contacted by Ti/Al superconducting electrodes (blue) and the electrode separations typically are 80–100 nm. Here we split \({S}_{0}\) in a into to two contacts \({S}_{0a},{S}_{0b}\) connected by a loop, c \(I-V\) curve of the device from the measurement configuration in b \({I}_{c}\) is the critical current and the corresponding voltage value is labeled as \({V}_{c}\) (the blue dots). d Magnetic field dependence, dI/dV as a function of the bias voltage and magnetic field. Bright region (high conductance) is the supercurrent and the edge corresponds to the value of critical current, which is modulated by the magnetic field. The SQUID-like pattern indicates the interference between two supercurrent paths (red and blue dashed lines in b). The periodicity of the fast oscillation (white dashed curve) corresponds to the loop area and the slow oscillation (yellow dashed curve) is the first lobe of Fraunhofer pattern. e Gate dependence of the supercurrent, dI/dV as a function of the bias voltage and global back-gate voltage \({V}_{{bg}}\). The critical current reaches the minimum as graphene is tuned to the Dirac point near \({V}_{{bg}}\) = −32 V.