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Research supported by the Spanish Ministry of Science, Innovation and Universities through Grants No. PGC2018097018-B-I00, No. FIS2016-80434-P (AEI/FEDER, EU), No. BES-2016-078122 (FPI program), No. RYC-2011-09345 (Ramon y Cajal program), and the Maria de Maeztu Program for Units of Excellence in R&D (No. CEX2018-000805-M). We also acknowledge support from the EU Horizon 2020 research and innovation program under the FETOPEN Grant Agreement No. 828948 and from the CSIC Research Platform on Quantum Technologies PTI-001.

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Superconducting islands with topological Josephson junctions based on semiconductor nanowires

Publicated to:PHYSICAL REVIEW B. 102 (9): - 2020-09-25 102(9), DOI: 10.1103/PhysRevB.102.094518

Authors: Avila, J.; Prada, E.; San-Jose, P.; Aguado, R.;

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Abstract

We theoretically study superconducting islands based on semiconductor-nanowire Josephson junctions and take into account the presence of subgap quasiparticle excitations in the spectrum of the junction. Our method extends the standard model Hamiltonian for a superconducting charge qubit and replaces the Josephson potential by the Bogoliubov-de Gennes Hamiltonian of the nanowire junction, projected onto the relevant low-energy subgap subspace. This allows us to fully incorporate the coherent dynamics of subgap levels in the junction. The combined effect of spin-orbit coupling and Zeeman energy in the nanowires forming the junction triggers a topological transition, where the subgap levels evolve from finite-energy Andreev bound states into near-zero-energy Majorana bound states. The interplay between the microscopic energy scales governing the nanowire junction (the Josephson energy, the Majorana coupling, and the Majorana energy splitting), with the charging energy of the superconducting island, gives rise to a great variety of physical regimes. Based on this interplay of different energy scales, we fully characterize the microwave response of the junction, from the Cooper pair box to the transmon regimes, and show how the presence of Majoranas can be detected through distinct spectroscopic features. In split-junction geometries, the plasma mode couples to the phase-dispersing subgap levels resulting from Majorana hybridization via a Jaynes-Cummings-like interaction. As a consequence of this interaction, higher order plasma excitations in the junction inherit Majorana properties, including the 4 pi effect.

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