Quantum jumps and quasiparticle trapping in superconducting one-atom contacts

Contact: Hugues POTHIER, hugues.pothier@cea.fr, +33 1 69 08 55 29

Summary: We propose to measure in real time the occupation of a single quasiparticle state within a superconductor.

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Can one measure in real time the changes in the occupation of a single quasiparticle state within a superconductor, and understand this dynamics? We have shown recently that although bulk superconductors consist in an assembly of delocalized and overlapping pairs of electrons, the Cooper pairs, localized states arise at atomic-size contacts [1]. The occupation of these fermionic states with 0, 1, or 2 quasiparticles was detected using circuit quantum electrodynamics techniques [2], with a measurement almost at the quantum non-demolition (QND) limit. One observes quantum jumps between states with 0 and 2 quasiparticles, which correspond to the two possible states of a localized Cooper pair. In addition, frequent changes in the parity (transitions between 0 and 1, or 1 and 2) are found in the experiment, due to the trapping or the un-trapping of single quasiparticles. We propose to design an experiment in which this dynamics is slowed down, with an improved detection scheme reaching the QND limit. This will allow characterizing the individual processes from a continuous measurement of the occupation.

During the internship, the student will be integrated in an active research group on quantum electronics and will acquire several techniques: nanofabrication, low temperatures, low-noise and microwave measurements.

A thesis is proposed after the internship.

[1] L. Bretheau et al., “Exciting Andreev pairs in a superconducting atomic contact”
Nature 499, 312 (2013). arXiv:1305.4091

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



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