Mediated tunable coupling of flux qubits

TL;DR

This paper demonstrates how a monostable SQUID can mediate and tune the coupling between flux qubits, allowing for a switchable transition from antiferromagnetic to ferromagnetic interactions, with both classical and quantum analyses supporting the findings.

Contribution

It introduces a method to continuously tune qubit coupling via a SQUID's flux-dependent susceptibility, with a fully quantum treatment providing precise evaluation of the coupling.

Findings

Tunable coupling from AF to FM achieved

Quantum and classical analyses agree on coupling behavior

Exact susceptibility expression applicable to qubit energy-level analysis

Abstract

It is sketched how a monostable rf- or dc-SQUID can mediate an inductive coupling between two adjacent flux qubits. The nontrivial dependence of the SQUID's susceptibility on external flux makes it possible to continuously tune the induced coupling from antiferromagnetic (AF) to ferromagnetic (FM). In particular, for suitable parameters, the induced FM coupling can be sufficiently large to overcome any possible direct AF inductive coupling between the qubits. The main features follow from a classical analysis of the multi-qubit potential. A fully quantum treatment yields similar results, but with a modified expression for the SQUID susceptibility. Since the latter is exact, it can also be used to evaluate the susceptibility--or, equivalently, energy-level curvature--of an isolated rf-SQUID for larger shielding and at degenerate flux bias, i.e., a (bistable) qubit. The result is compared to the standard two-level (pseudospin) treatment of the anticrossing, and the ensuing conclusions are verified numerically.