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At sufficiently low temperatures, superconductors cannot absorb microwave radiation of energy smaller than the superconducting energy gap D.  In Josephson weak links instead, where two superconductors (S) are weakly coupled through a long diffusive metallic wire (N), radiation can be absorbed in N because the induced gap in the density of states or minigap is considerably smaller than D.

In a recent article the researchers’ team 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 which allows to access the harmonic content of the current-phase relation under microwave radiation.

With this approach, which does not require a specialized on-chip circuitry, they could see that a strong anharmonicity of the current-phase relation arises under illumination, especially at high frequency when inelastic transitions across the induced minigap 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 shed light on the complex mechanisms involved in irradiated mesoscopic superconductors and has important implications in Andreev-based quantum computing prospects.

Link to the article in Physical Review Research 1, 032009 (2019):

For further information:
Professor Tero Heikkilä, Ä¢¹½Ö±²¥, Finland,  tero.t.heikkila@jyu.fi, tel. +358408054804
University Researcher Pauli Virtanen, pauli.t.virtanen@jyu.fi
Communications officer Tanja Heikkinen, tanja.s.heikkinen@jyu.fi, tel. +358 50 581 8351
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