Biphoton transmission through non-unitary objects

TL;DR

This paper investigates how non-unitary, absorptive objects affect biphoton transmission in quantum imaging, revealing the importance of single-photon contributions and proposing new imaging and entanglement characterization methods.

Contribution

It introduces a method to image both single- and two-photon parts of transmitted biphotons, accounting for losses, and demonstrates agreement with theoretical models.

Findings

Single-photon part contains object and entanglement information.

Electron multiplying CCD can image and count biphotons simultaneously.

Results align with theoretical predictions for non-unitary object transmission.

Abstract

Losses should be accounted for in a complete description of quantum imaging systems, and yet they are often treated as undesirable and largely neglected. In conventional quantum imaging, images are built up by coincidence detection of spatially entangled photon pairs (biphotons) transmitted through an object. However, as real objects are non-unitary (absorptive), part of the transmitted state contains only a single photon, which is overlooked in traditional coincidence measurements. The single photon part has a drastically different spatial distribution than the two-photon part. It contains information both about the object, and, remarkably, the spatial entanglement properties of the incident biphotons. We image the one- and two-photon parts of the transmitted state using an electron multiplying CCD array both as a traditional camera and as a massively parallel coincidence counting apparatus, and demonstrate agreement with theoretical predictions. This work may prove useful for photon number imaging and lead to techniques for entanglement characterization that do not require coincidence measurements.