Analysis of a tuneable coupler for superconducting phase qubits

TL;DR

This paper provides a theoretical analysis of a tunable coupler for superconducting phase qubits, demonstrating how to effectively switch off coupling with minimal residual interactions, crucial for quantum computing applications.

Contribution

It offers a detailed theoretical model of the tunable coupler, showing how to achieve near-zero coupling and quantify residual errors based on experimental parameters.

Findings

Coupling can be turned off with a ratio of a few times 10^{-3}.

Residual coupling errors are on the order of 10^{-6}.

Proper parameter choice can zero the coupling exactly.

Abstract

This paper presents a theoretical analysis of the recently realized tuneable coupler for superconducting phase qubits (R. C. Bialczak et al., Ref.\ \protect\onlinecite{Bialczak}). The coupling can be turned off by compensating a negative mutual inductance with a tuneable Josephson inductance. The main coupling in this system is of the $XX$ type and can be zeroed exactly, while there is also a small undesired contribution of the $ZZ$ type. We calculate both couplings as functions of the tuning parameter (bias current) and focus on the residual coupling in the OFF regime. In particular, we show that for typical experimental parameters the coupling OFF/ON ratio is few times $10^{-3}$, and it may be zeroed by proper choice of parameters. The remaining errors due to physical presence of the coupler are on the order of $10^{-6}$.