Adaptive quantum state tomography improves accuracy quadratically

TL;DR

This paper presents an adaptive quantum state tomography protocol that significantly improves accuracy, reducing infidelity quadratically with minimal additional measurement settings, demonstrated experimentally with optical qubits.

Contribution

The authors develop a simple adaptive protocol that enhances quantum state tomography accuracy with only one adaptation step and one extra measurement setting.

Findings

Quadratic reduction in worst-case infidelity from O(N^{-1/2}) to O(N^{-1})

Order of magnitude decrease in infidelity in optical qubit experiments

Effective for modest sample sizes (N=3×10^4)

Abstract

We introduce a simple protocol for adaptive quantum state tomography, which reduces the worst-case infidelity between the estimate and the true state from $O(N^{-1/2})$ to $O(N^{-1})$. It uses a single adaptation step and just one extra measurement setting. In a linear optical qubit experiment, we demonstrate a full order of magnitude reduction in infidelity (from $0.1%$ to $0.01%$) for a modest number of samples ($N=3\times10^4$).