An interferometric device is proposed in order to analyze the quartet mode in biased three-terminal Josephson junctions (TTJs), and to provide experimental evidence for the emergence of a single stationary phase, the so-called quartet phase. In such a quartet superconducting quantum interference device (quartet SQUID), the flux sensitivity exhibits period $hc/4e$, which is the fingerprint of a transient intermediate state involving two entangled Cooper pairs. The quartet SQUID provides two pieces of information: an amplitude that measures a total “quartet critical current,” and a phase lapse coming from the superposition of the following two current components: the quartet supercurrent which is odd in the quartet phase, and the phase-sensitive multiple Andreev reflection (phase MAR) quasiparticle current, which is even in the quartet phase. This makes a TTJ a generically “$\theta$ junction.” Evidence for phase MARs plays against conservative scenarios involving synchronization of AC Josephson currents, based on “adiabatic” phase dynamics and resistively shunted junction–like models.

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• Electrical properties
• Josephson junctions
• Superconducting devices
• Superconductors