Microwave spectroscopy of interacting Andreev spins

Andreev bound states are fermionic states localized in weak links between superconductors which

can be occupied with spinful quasiparticles. Microwave experiments using superconducting circuits

with InAs/Al nanowire Josephson junctions have recently enabled probing and coherent manipulation

of Andreev states but have remained limited to zero or small fields. Here we use a fluxtunable

superconducting circuit in external magnetic fields up to 1T to perform spectroscopy of

spin-polarized Andreev states up to ~250 mT, beyond which the spectrum becomes gapless. We

identify singlet and triplet states of two quasiparticles occupying different Andreev states through

their dispersion in magnetic field. These states are split by exchange interaction and couple via spinorbit

coupling, analogously to two-electron states in quantum dots. We also show that the magnetic

field allows to drive a direct spin-flip transition of a single quasiparticle trapped in the junction.

Finally, we measure a gate- and field-dependent anomalous phase shift of the Andreev spectrum,

of magnitude up to approximately 0.7π . Our observations demonstrate new ways to manipulate

Andreev states in a magnetic field and reveal spin-polarized triplet states that carry supercurrent.