Quantum interferometer for quartets in superconducting three-terminal Josephson junctions

Abstract

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 socalled 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 "θ 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.

Quartet currents in a biased three-terminal diffusive Josephson junction

“…Provided phase-MARs are not negligible, such a phase shift could be observed in a phase-sensitive experiment as proposed in Ref. [31].…”

“…Probing the sign changes of the quartet current, more generally the ϕ-TTJ, requires a full quartet SQUID geometry [31].…”

“…First, dissipative currents generalize [13,14,16,17] the MAR currents of a two-terminal junction. As a novel feature, those currents depend on the quartet phase, due to interfering scattering paths within the node [16,31,32]. Second, the special condition V 1 = −V 2 drives a DC Cooper pair component from terminal S 0 to terminals S 1,2 , as a current of quartets (entangled pairs of Cooper pairs) I Q .…”

“…Another striking signature of the voltage commensurability is the presence of phase-dependent "phase-MAR" currents, where quasiparticle transport is accompanied by quartets instead of single pairs like in the two-terminal case [16,31,32].…”

“…They open new phase-coherent channels in a dissipative structure, that cannot be deduced in a phenomenological way from independent two-terminal Josephson currents. Discriminating classical downmixing from mesoscopic multipair resonances and separating quartets from phase-MARs remains an experimental challenge for the future that could be tackled with a suitable SQUID geometry [31].…”