An Andreev quantum dot (AQD) is a phase-biased superconducting weak link in which discrete Andreev bound states develop. In particular, a single-channel AQD accommodates one Andreev state that can be occupied by either zero, one or two quasiparticles. In a recent cQED experiment on one-atom weak links we demonstrated the coherent manipulation of the two-level system formed by the even states [1]. Moreover, as a common feature to other superconducting devices, the single-occupied state of the AQD was also observed. We have focused on the role of the odd state in the dynamics of the AQD and I will present our results on the time-domain study of the parity jumps observed due to quasiparticle poisoning. Although the odd states are spin-degenerate, the fact that they are long-lived states makes them appealing for a qubit. I will present our recent progress in the realization of a spin-AQD using a gated InAs-nanowire where degeneracy can be lifted by the combination of strong spin-orbit coupling and a Zeeman field. We show that quasi-ballistic weak links can be obtained, an important requirement to achieve single-spin manipulation.

Work done in collaboration with P. Senat, P. F. Orfila, L. Tosi, P. Bertet, P. Joyez, D. Vion, D. Esteve, H. Pothier and C. Urbina from the Quantronics group and P.Krogstrup and J. Nygård from Center for Quantum Devices, Niels Bohr Institute, University of Copenhagen, Denmark.

[1] C. Janvier et al., “Coherent manipulation of Andreev states in superconducting atomic contacts” Science 349, 1199 (2015), arXiv:1509.03961