Microwave spectroscopy of interacting Andreev spins
ORAL
Abstract
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.
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.
*is supported by funding from the Dutch Research Council (NWO) and the Microsoft Quantum initiative
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Publication: https://arxiv.org/abs/2208.11198
Presenters
-
Jaap J Wesdorp
- Delft University of Technology
- Qutech, Delft University of Technology