Optimizing brightness of SPDC source in Laguerre-Gaussian modes using type-0 periodically-poled nonlinear crystal

TL;DR

This paper investigates how to optimize the brightness of entangled photon pairs generated in Laguerre-Gaussian modes via type-0 SPDC with a periodically-poled nonlinear crystal, considering non-degenerate conditions and specific focusing parameters.

Contribution

It provides a detailed analysis of SPDC brightness in LG modes without relying on common simplifying assumptions, and identifies optimal focusing conditions for maximizing brightness.

Findings

Brightness optimization depends on pump, signal, and idler focal parameters.

Simultaneous optimization of multiple LG modes with a single focal parameter is not feasible.

The framework aids in developing high-brightness quantum light sources for quantum communication.

Abstract

Photon pairs generated via spontaneous parametric down-conversion (SPDC) can exhibit entanglement in the Laguerre-Gaussian (LG) mode basis, which enables high-dimensional free-space quantum communication by exploiting the high-dimensional space spanned by the LG modes. For such free-space quantum communication, the brightness of the quantum light source plays an important role due to the atmospheric turbulence and photon loss. A variety of studies have analyzed the SPDC brightness by decomposing biphoton states into LG modes, but they have often relied on a degenerate state, a narrow spectral bandwidth approximation, or a thin crystal approximation. However, these approaches are unsuitable for non-degenerate type-0 SPDC with a periodicallypoled nonlinear crystal, which offers higher brightness due to its superior nonlinear coefficients. In this study, we examine the spectrum of photon pairs in specific LG modes generated by a type-0 ppKTP crystal whileavoiding the constraints imposed by the aforementioned assumptions. In addition, we investigate the optimal focal parameters of the pump, signal, and idler to maximize the brightness for a given LG mode. Our findings show that it is not feasible to simultaneously optimize the brightness for different LG modes with a single pump focal parameter. The results of this study provide a comprehensive framework for developing highbrightness quantum light sources and contribute to the advancement of high-dimensional free-space quantum communication.