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Conference on Precision Electromagnetic Measurements

Two-photon single mode coupling efficiency: definition, measurement, optimization.

Stefania Castelletto, Ivo Pietro Degiovanni, Alan L. Migdall, M. Ware

Published Jun 1, 2004• 0 citations
Publisher

Abstract

The authors investigate the physical meaning of coupling a two-photon field (non-classical state of light) into two single mode optical fibers, studying the limits to this due to a non-coherent emission of photons in parametric down conversion, an important starting point for many single photon source schemes
DOI
10.1109/cpem.2004.305343

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We investigate the coupling efficiency of parametric downconversion light (PDC) into single and multimode optical fibers as a function of the pump beam diameter, crystal length, and walk-off. We outline two different theoretical models for the preparation and collection of either single-mode or multimode PDC light (defined by, for instance, multimode fibers or apertures). Moreover, we define the mode-matching collection efficiency, important for realizing a single-photon source based on PDC output into a well-defined single mode. We also define a multimode collection efficiency that is useful for single-photon detector calibration applications.
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. We present two possible models to describe single-mode fibres coupling with PDC light fields in a non-collinear configuration, that lead to two different results. These approaches include factors such as 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 ways to improve the collection efficiency.
We investigate the coupling efficiency of parametric downconversion light (PDC) into single and multi-mode optical fibers as a function of the pump beam diameter, crystal length and walk-off. We outline two different theoretical models for the preparation and collection of either single-mode or multi-mode PDC light (defined by, for instance, multi-mode fibers or apertures, corresponding to bucket detection). Moreover, we define the mode-matching collection efficiency, important for realizing a single-photon source based on PDC output into a well-defined single spatial mode. We also define a multimode collection efficiency that is useful for single-photon detector calibration applications. 1.
We investigate the coupling efficiency of parametric downconversion light (PDC) into single and multi-mode optical fibers as a function of the pump beam diameter, crystal length and walk-off. We outline two different theoretical models for the preparation and collection of either single-mode or multi-mode PDC light (defined by, for instance, multi-mode fibers or apertures, corresponding to bucket detection). Moreover, we define the mode-matching collection efficiency, important for realizing a single-photon source based on PDC output into a well-defined single spatial mode. We also define a multimode collection efficiency that is useful for single-photon detector calibration applications.
We develop and apply an effective analytic theory of a noncollinear, broadband type-I parametric down-conversion to study a coupling efficiency of the generated photon pairs into single mode optical fibers. We derive conditions necessary for highly efficient coupling for single and double type-I crystal producing polarization entangled states of light. We compare the obtained approximate analytic expressions with the exact numerical solutions and discuss the results for a case of beta barium borate crystals.
Many quantum computation and communication schemes require, or would significantly benefit from, true sources of single photon on-demand (SPOD). Unfortunately, such sources do not exist. It is becoming increasingly clear that coupling photons out of a SPOD source will be a limiting factor in many SPOD implementations. In particular, coupling these source outputs into optical fibers (usually single mode fibers) is often the preferred method for handling this light. We investigate the practical limits to this coupling as relates to parametric downconversion, an important starting point for many SPOD schemes. We also explored whether it is possible to optimize the engineering of the downconversion sources to improve on this coupling. We present our latest results in this area.