One-photon measurement of two-photon entanglement

TL;DR

This paper presents a novel method to measure entanglement in two-photon mixed states using only single-photon detection and interference patterns, simplifying the process especially when one particle cannot be detected.

Contribution

The authors introduce a quantum interference-based technique that enables entanglement measurement with detection of only one photon in a two-photon mixed state.

Findings

Entanglement can be characterized by interference visibility.

The method works with mixed states and requires only single-photon detection.

Experimental validation demonstrates effective entanglement measurement.

Abstract

Entanglement is a fundamental feature of quantum mechanics, considered a key resource in quantum information processing. Measuring entanglement is an essential step in a wide range of applied and foundational quantum experiments. When a two-particle quantum state is not pure, standard methods to measure the entanglement require detection of both particles. We introduce a method in which detection of only one of the particles is required to characterize the entanglement of a two-particle mixed state. Our method is based on the principle of quantum interference. We use two identical sources of a two-photon mixed state and generate a set of single-photon interference patterns. The entanglement of the two-photon quantum state is characterized by the visibility of the interference patterns. Our experiment thus opens up a distinct avenue for verifying and measuring entanglement, and can allow for mixed state entanglement characterization even when one particle in the pair cannot be detected.