Non-adiabatic dynamics of strongly driven diffusive Josephson junctions

(a) Scanning electron micrograph of the measured Superconductor(Nb)/Normal metal (Ag)/ Superconductor(Nb) Josephson junction. (b) Color-coded sketch of the normalized energy-phase dependent density of states of a long diffusive SNS junction. Full (dashed) vertical arrows represent the high (low) probability inelastic transitions. Grey (blue) circles represent electron-like (hole-like) quasiparticles. Credit: University of Jyväskylä/Pauli Virtanen

Researchers from the University of Paris-Saclay, the University of Regensburg and the University of Jyvaskyla have delivered a combined experimental and theoretical work revealing the profound nature of quantum transport in strongly driven diffusive Josephson junctions.

They have studied the out-of-equilibrium dynamical state induced by the absorption of high frequency microwave photons in diffusive superconductor-normal metal-superconductor (SNS) junction. To characterize this state, the researchers pioneered a harmonic-resolved ac-Josephson spectroscopy technique to access the harmonic content of the current-phase relation under microwave radiation.

The team discovered that a strong anharmonicity of the current-phase relation arises under illumination, especially at high frequency when inelastic transitions across the induced mini-gap are favored. This novel regime goes well beyond the standard Eliashberg theory and is understood because of the modifications of the supercurrent-carrying Andreev spectrum induced by non-adiabatic transitions.

These findings have important implications in Andreev-based quantum computing prospects. (Phys.org)

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