Improved Quantum State Tomography for the Systems with XX+YY Couplings and Z Readouts

TL;DR

This paper presents an improved quantum state tomography method for systems with XX+YY couplings and Z readouts, significantly reducing measurement requirements while maintaining accuracy, applicable to superconducting quantum computing systems.

Contribution

The work introduces a novel QST scheme using 2-qubit evolutions and integer programming optimization, reducing measurements by over 60% for certain quantum systems.

Findings

Reduces measurement count by over 60% compared to traditional QST.

Maintains or improves accuracy and robustness against errors.

Demonstrates good implementability for superconducting quantum systems.

Abstract

Quantum device characterization via state tomography plays an important role in both validating quantum hardware and processing quantum information, but it needs the exponential number of the measurements. For the systems with XX+YY-type couplings and Z readouts, such as superconducting quantum computing (SQC) systems, traditional quantum state tomography (QST) using single-qubit readout operations at least requires $3^n$ measurement settings in reconstructing an $n$-qubit state. In this work, I proposed an improved QST by adding 2-qubit evolutions as the readout operations and obtained an optimal tomographic scheme using the integer programming optimization. I respectively apply the new scheme on SQC systems with the Nearest-Neighbor, 2-Dimensional, and All-to-All connectivities on qubits. It shows that this method can reduce the number of measurements by over 60% compared with the traditional QST. Besides, comparison with the traditional scheme in the experimental feasibility and robustness against errors were made by numerical simulation. It is found that, the new scheme has good implementability and it can achieve comparable or even better accuracy than the traditional scheme. It is expected that the experimentalist from the related fields can directly utilize the ready-made results for reconstructing quantum states involved in their research.