Experimental Adaptive Quantum Tomography of Two-Qubit States

TL;DR

This paper demonstrates an experimental implementation of adaptive Bayesian quantum state tomography for two-qubit polarization states, showing improved accuracy for pure states and limitations for mixed states with factorized measurements.

Contribution

It provides an experimental realization of adaptive quantum tomography using an optimal measurement strategy, addressing practical noise issues and measurement restrictions.

Findings

Adaptive protocol yields lower noise floor and better accuracy for pure states.

Factorized measurements limit adaptive advantage for mixed states.

General measurements are necessary for optimal mixed state tomography.

Abstract

We report an experimental realization of adaptive Bayesian quantum state tomography for two-qubit states. Our implementation is based on the adaptive experimental design strategy proposed in [F.Husz\'ar and N.M.T.Houlsby, Phys.Rev.A 85, 052120 (2012)] and provides an optimal measurement approach in terms of the information gain. We address the practical questions, which one faces in any experimental application: the influence of technical noise, and behavior of the tomographic algorithm for an easy to implement class of factorized measurements. In an experiment with polarization states of entangled photon pairs we observe a lower instrumental noise floor and superior reconstruction accuracy for nearly-pure states of the adaptive protocol compared to a non-adaptive. At the same time we show, that for the mixed states the restriction to factorized measurements results in no advantage for adaptive measurements, so general measurements have to be used.