Tomography in the presence of stray inter-qubit coupling

TL;DR

This paper introduces a coupling compensated tomography protocol that accurately reconstructs quantum states in systems with unavoidable stray inter-qubit couplings, overcoming limitations of standard methods.

Contribution

The authors develop a novel tomography protocol that corrects for parasitic couplings in software, enabling accurate quantum state reconstruction in realistic coupled qubit systems.

Findings

Successfully demonstrated on two transmon qubits with ZZ coupling

Can be applied to large systems with various stray couplings

Allows use of arbitrary and non-orthogonal tomography pulses

Abstract

Tomography is an indispensable part of quantum computation as it enables diagnosis of a quantum process through state reconstruction. Existing tomographic protocols are based on determining expectation values of various Pauli operators which typically require single-qubit rotations. However, in realistic systems, qubits often develop some form of unavoidable stray coupling making it difficult to manipulate one qubit independent of its partners. Consequently, standard protocols applied to those systems result in unfaithful reproduction of the true quantum state. We have developed a protocol, called coupling compensated tomography, that can correct for errors due to parasitic couplings completely in software and accurately determine the quantum state. We demonstrate the performance of our scheme on a system of two transmon qubits with always-on $\textit{ZZ}$ coupling. Our technique is a generic tomography tool that can be applied to large systems with different types of stray inter-qubit couplings and facilitates the use of arbitrary tomography pulses and even non-orthogonal axes of rotation.