Quantum Coherence of the Quartet Scheme Observed by Shapiro Resonance Under Radio-Frequency Irradiation in Three Terminal Josephson Junctions

We have investigated three-terminal superconductor (S)-normal metal (N)-superconductor (S) Josephson junctions. In a geometry where a T-shaped normal metal (Cu) is connected to three superconducting reservoirs (Al), new subgap structures appear in the differential resistance when the bias voltages on two of the three terminals compensate each other exactly. These features correspond to the correlated motion of Cooper pairs within the structure. They are consistent with the prediction of quartets formed by the simultaneous splitting of two Cooper pairs at one of the superconducting contacts and the emission of two phase correlated Cooper pairs in the two other electrodes. To test the quantum coherence of such mechanism, we have studied the effect of a radio-frequency irradiation at 14 GHz down to 100 mK. Well known Shapiro steps involving two terminals are measured, and in particular, half-integer steps are observed. This is consistent with the transfer of two Cooper pairs between two terminals. This paper reveals that the quartet resonances also show Shapiro steps in agreement with the quartet mechanism. Our findings confirm the quartet scenario and demonstrate its coherent nature.