Projecting onto any two-photon polarization state using linear optics

TL;DR

This paper introduces a linear optics method to project two-photon polarization states onto any desired state, enabling flexible quantum measurements without nonlinearities, demonstrated experimentally with high confidence.

Contribution

It presents a novel, experimentally feasible scheme for arbitrary two-photon state projection using Hong-Ou-Mandel interference and post-selection, without requiring nonlinear optics.

Findings

Successfully implemented projection onto any two-photon polarization state.

Achieved measurement efficiency between 50% and 100%.

Verified entangling nature with six standard deviations of confidence.

Abstract

Projectors are a simple but powerful tool for manipulating and probing quantum systems. For instance, projecting two-qubit systems onto maximally entangled states can enable quantum teleportation. While such projectors have been extensively studied, partially-entangling measurements have been largely overlooked, especially experimentally, despite their important role in quantum foundations and quantum information. Here, we propose a way to project two polarized photons onto any state with a single experimental setup. Our scheme does not require optical non-linearities or additional photons. Instead, the entangling operation is provided by Hong-Ou-Mandel interference and post-selection. The efficiency of the scheme is between 50% and 100%, depending on the projector. We perform an experimental demonstration and reconstruct the operator describing our measurement using detector tomography. Finally, we flip the usual role of measurement and state in Hardy's test by performing a partially-entangling projector on separable states. The results verify the entangling nature of our measurement with six standard deviations of confidence.