Intrinsic limitations to the quality of pulsed spontaneous parametric downconversion sources for quantum information applications

Abstract

Spontaneous parametric downconversion (SPDC) sources are an essential element of quantum communication and quantum information processing systems. Their quality can be hampered by spectral, temporal, or spatial distinguishability of the two photons of a generated pair. Even when such defects have been corrected, the ultimate quality of the SPDC source is limited by the intrinsic multiple pair generation. In this paper, the effect of transmission, filtering and detection losses and asymmetrical spectral bandwidths in the photon pair paths on the visibility of the two-photon interference delivered by the SPDC source and its useful pair rate are investigated. In this investigation, two subcases are distinguished: deterministic and statistical splittings of the generated pairs, whatever the coherence of the multiple pairs. We show that the visibility is strongly degraded by a spectral bandwidth asymmetry, while the losses mainly affect the measured coincidence probability. The deterministic splitting configuration is shown to be generally more advantageous, although statistical splitting may present some advantages in applications with a narrow bandwidth requirement, such as those involving quantum memories. Moreover, while pump-induced optical noise only produces small visibility reductions, dark counts of the single-photon detectors are shown to limit the maximum possible visibility, particularly in the case of high-loss photon pair sources. Because our model quantifies the intrinsic limitations due to double pair emission in a simple way, the results presented can be used to optimize the design of SPDC sources and choose the optimum trade-offs between several key parameters such as visibility, generation probabilities, and spectral bandwidth, in particular for small bandwidth SPDC sources in the perspective of quantum networking and computing applications.