Seminar: Yu-Shiba-Rusinov subgap excitations in hybrid superconductor/ semiconductor nanowires containing quantum dots

An interacting quantum dot coupled to a superconducting contact is an artificial analogue of a quantum impurity in a superconductor. The physics of such system is governed by the fermionic parity and spin of the two possible ground states, doublet or singlet, and their corresponding Yu-Shiba-Rusinov subgap excitations. Changes in the fermionic parity of the ground state occur as a quantum phase transition where the energy of the subgap excitations crosses zero energy. After an introduction about such excitations and their Zeeman splitting in hybrid superconductor/semiconductor nanowires [1-4], I will focus on full shell nanowires. In this geometry, the role of the external magnetic flux needs to be taken into account. Specifically, the Little-Parks modulation of the pairing and the winding of the superconducting phase around the shell (fluxoid) may result in robust zero-bias peaks owing to subgap levels close to zero energy and flattening of fermionic parity crossings. Such features could be easily misinterpreted as originating from Majorana zero modes [5,6].

In the second part of the talk, I will explain how microwave spectroscopy in a transmon circuit QED setup allows to probe the ground state parity of a quantum dot Josephson junction as a function of gate voltages, external magnetic flux, and magnetic field [7]. The measured parity phase diagram is in very good agreement with that predicted by the single-impurity Anderson model. Furthermore, continuous time monitoring of the circuit allows to resolve the quasiparticle dynamics of the quantum dot Josephson junction across the phase boundaries. Away from the transition, in either the singlet or doublet phase, the lifetime in the ground state sector is of the order of several milliseconds, exceeding that of the excited state by more than an order of magnitude. These results hold promise towards realizing Andreev qubits, a spin qubit located inside a superconductor [8,9].

References:

[1]  Lee, Jiang, Katsaros, Aguado, Lieber, De Franceschi, Phys. Rev. Lett. 109, 186802 (2012).

[2]  Žitko, Lim, López, Aguado, Phys. Rev. B 91, 045441 (2015).

[3]  Lee, Jiang, Zitko, Aguado, Lieber, De Franceschi, Phys. Rev. B 95, 180502 (2017).

[4]  Lee, Jiang, Houzet, Aguado, Lieber, De Franceschi, Nat. Nanotech., 9, 79  (2014).

[5]  Valentini, Peñaranda, Hofmann, Brauns, Hauschild, Krogstrup, San-Jose, Prada, Aguado, Katsaros, Science 373, 82 (2021).

[6] Escribano, Levy-Yeyati, Aguado, Prada, San-Jose, Phys. Rev. B 105, 045418 (2022).

[7] Bargerbos, Pita-Vidal, Žitko, Ávila, Splitthoff, Grünhaupt, Wesdorp, Andersen, Liu, Kouwenhoven, Aguado, Kou, van Heck, Phys. Rev. X Quantum 3, 030311 (2022).

[8] Bargerbos, Pita-Vidal, Žitko, Splitthoff, Grünhaupt, Wesdorp, Liu, Kouwenhoven, Aguado, Andersen, Kou, van Heck, arXiv: 2208.09314 (2022).

[9] Pita-Vidal, Bargerbos, Žitko, Splitthoff, Grünhaupt, Wesdorp, Liu, Kouwenhoven, Aguado, van Heck, Kou, Andersen, arXiv: 2208.10094 (2022).