On the measurement of two-photon single-mode coupling efficiency in parametric downconversion 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 downconversion (PDC). Practical realizations of this technology require that the PDC light be collected into a single spatial mode defined by an optical fibre. We present two possible models describing single-mode fibres coupling to PDC light fields in a non-collinear configuration, that lead to two different results. These approaches include factors like crystal length, and walk-off, non-collinear phase-matching, and transverse pump field distribution. We propose an experimental test to distinguish between the two. 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 a way to improve the collection efficiency.