It is now possible to fabricate metallic contacts of atomic dimension separating two metallic wires. Our nanocontacts are fabricated from a 100 nm wide metallic bridge which is suspended above a flexible substrate. In a dilution refrigerator, a mechanical constraint is applied to the substrate and breaks the bridge. The two remaining wires can then be reassembled to form a nanocontact of variable dimension. This atomic-level metal cold working is possible in the ultra-high vacuum of the refrigerator can.
Previous experiments in our lab had shown that the non-linearities of the I(V) can be used to access the individual transmission of each conduction channel, which characterize all its properties. The superconducting atomic contacts is thus a test-bed for mesoscopic physics. We conducted experiments to measure the shot noise emitted at finite bias voltage, the critical current of current or phase biased contacts between superconducting electrodes, and the effect of microwave excitations. Quantitatively agreement is found with the predictions of the mesoscopic Josephson effect, which consider that the supercurrent is carried by doublets of localized states within the contacts, called the Andreev states.
Whereas previous experiments could be understood with the sole ground Andreev state, we probed in 2012-2013 transitions between Andreev states, and since 2014 superpositions of Andreev states by coupling atomic contacts with a microwave resonator, in a circuit QED geometry. The Andreev doublet appears as a new type of superconducting Qubit, based on microscopic states. We are currently exploring the limits in the lifetime and coherence time of Andreev Qubits.
In parallel, we start exploring situations where the spin degeneracy of single quasiparticles in Andreev states can be lifted. This requires using as a weak link a material with strong spin-orbit interaction: InAs nanowires. We have shown with DC measurements that the number of transport channels can be tuned and that high transparencies can be achieved. Experiments in the microwave domain are under developpement.