Marcelo Goffman : A quantum circuit harnessing both the bosonic and the fermionic side of the Josephson effect

Abstract

Superconducting artificial atoms rely on Josephson junctions, whose quantum variable is the phase difference φ, a collective electromagnetic degree of freedom of the condensate. Yet, every Josephson weak link also hosts a fermionic degree of freedom: the occupation of Andreev bound states by quasiparticles.This second quantum variable can encode qubits when the weak link is highly transparent and a magnetic flux threading a small-inductance loop imposes a classical phase φ across it. Up to now, every explored circuit exploits the quantumness of just one of these degrees of freedom, while the other stays frozen. What happens when a transparent weak link is embedded in a large-inductance circuit, so that both degrees of freedom are dynamical? Bosonic and fermionic excitations hybridize, opening regimes where the circuit low-energy states are protected against relaxation and, at sweet spots, against dephasing. I will discuss this circuit, which we call FerBo, and present a first implementation based on an InAs/Al nanowire weak link and a Josephson-junction-array superinductance.

Marcelo Goffman, Quantronics Group, Service de Physique de l’Etat Condensé, CEA-CNRS, CEA-Saclay, 91191 Gif-sur-Yvette Cedex, France