Characterizing an Entangled-Photon Source with Classical Detectors and Measurements

TL;DR

This paper demonstrates a method called stimulated-emission tomography (SET) that uses classical detectors to characterize entangled-photon sources, providing an efficient alternative to traditional quantum state tomography.

Contribution

The authors experimentally validate SET as a practical technique to reconstruct entangled photon states without single-photon measurements, matching the accuracy of quantum state tomography.

Findings

SET accurately reconstructs polarization density matrices.

SET predicts purity and concurrence of photon pairs.

Classical detectors suffice for entangled-photon source characterization.

Abstract

Quantum state tomography (QST) is a universal tool for the design and optimization of entangled-photon sources. It typically requires single-photon detectors and coincidence measurements. Recently, it was suggested that the information provided by the QST of photon pairs generated by spontaneous parametric down-conversion could be obtained by exploiting the stimulated version of this process, namely difference frequency generation. In this protocol, so-called "stimulated-emission tomography" (SET), a seed field is injected along with the pump pulse, and the resulting stimulated emission is measured. Since the intensity of the stimulated field can be several orders of magnitude larger than the intensity of the corresponding spontaneous emission, measurements can be made with simple classical detectors. Here, we experimentally demonstrate SET and compare it with QST. We show that one can accurately reconstruct the polarization density matrix, and predict the purity and concurrence of the polarization state of photon pairs without performing any single-photon measurements.