Superconducting Weak Links

Superconducting Atomic Contacts

Atomic contacts are metallic contacts of atomic dimension between two metallic wires. They are fabricated from a 100-nm-wide metallic bridge suspended above a flexible substrate. In a dilution refrigerator, the substrate is bent till the bridge breaks. The two remaining wires are then brought close, till a contact forms. This atomic-level metal cold working is possible in the ultra-high vacuum of the refrigerator can.

We have 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 those experiments could be understood with the sole ground Andreev state, in 2010 we started looking at the excited states using a tunneling spectroscopy technique, and subsequently, using microwave spectroscopy. 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.

Andreev States in InAs nanowires

We have explored situations where the spin degeneracy of Andreev states is lifted. This requires using as a weak link a material with strong spin-orbit interaction as InAs nanowires. By studying the MAR contribution to the current, we have shown that the number of transport channels can be tuned and that high transparencies can be achieved. In a recent experiment we have revealed the spin-orbit splitting of Andreev states by microwave spectroscopy.

We are now developping a new type of qubit based on Andreev states and circuit modes.

See our publications

Persons involved:
Joan Caceres,
Marcelo Goffman, Hugues Pothier, Cristian Urbina

To learn more about this project click here.

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