On the measurement of two-photon single-mode coupling efficiency in parametric down-conversion photon sources

Photon-based quantum information schemes have increased the need for light sources that produce individual photons, with many such schemes relying on optical parametric down-conversion (PDC). Practical realizations of this technology require that the PDC light be collected into a single spatial mode defined by an optical fibre. In this paper, we present two possible models to describe single-mode fibres coupling with PDC light fields in a non-collinear configuration, leading to two different results. These approaches include factors such as crystal length and walk-off, non-collinear phase-matching and also transverse pump field distribution. We propose an experimental test to distinguish between the two models. The goal is to help clarify open issues, such as how to extend the theory beyond the simplest experimental arrangements and, more importantly, to suggest ways to improve the collection efficiency.