Selecting the pre-detection characteristics for effective fiber coupling of entangled photon sources

TL;DR

This paper investigates how pump beam focusing and crystal thickness influence biphoton mode coupling efficiency in SPDC, providing experimental insights to optimize entangled photon sources for quantum information.

Contribution

It offers a combined numerical and experimental analysis of pump focusing effects on biphoton modes, highlighting optimal conditions for efficient entangled photon generation.

Findings

Coupling efficiency decreases asymptotically with pump focusing.

Ellipticity and asymmetry in biphoton modes are influenced by focusing.

Optimal focusing conditions improve entangled photon source efficiency.

Abstract

Photon modes have an important role in characterizing the quantum sources of light. Proper coupling of various photon modes obtained in spontaneous parametric down conversion (SPDC) process in optical fibers is essential to generate an effective source of entangled photons. The two main pre-detection factors affecting the biphoton mode coupling in SPDC are the pump beam focusing parameter and the crystal thickness. We present the numerical and experimental results on the effect of pump focusing on conditional down-converted photon modes for a Type-I BBO crystal. We experimentally verify that biphoton coupling efficiency decreases asymptotically with pump beam focusing parameter. We attribute this behaviour to (a) the asymmetry in the spatial distribution of down-converted photons with the pump beam focusing parameter and (b) the ellipticity of biphoton modes introduced due to the focusing of the pump beam. We also show experimentally the ellipticity as well as quantify the asymmetry with the pump focusing parameter. These results may be useful in selecting optimum conditions in the down-conversion process for generating efficient sources of entangled photons for quantum information applications.