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Deterministic sources of high-fidelity entangled qubit pairs encoded in thedual-rail photonic basis, i.e., presence of a single photon in one of twoorthogonal modes, are a key enabling technology of many applications of quantuminformation processing, including high-rate high-fidelity quantumcommunications over long distances. The most popular and mature sources of suchphotonic entanglement, e.g., those that leverage spontaneous parametricdown-conversion (SPDC) or spontaneous four-wave mixing (sFWM), generate anentangled (so-called, continuous-variable) quantum state that containscontributions from high-order photon terms that lie outside the span of thedual-rail basis, which is detrimental to most applications. One often uses lowpump power to mitigate the effects of those high-order terms. However thatreduces the pair generation rate, and the source becomes inherentlyprobabilistic. We investigate a cascaded source that performs a linear-opticalentanglement swap between two SPDC sources, to generate a heralded photonicentangled state that has a higher fidelity (to the ideal Bell state) comparedto a free-running SPDC source. Further, with the Bell swap providing aheralding trigger, we show how to build a multiplexed source, which despitereasonable switching losses and detector loss and noise, yields a Fidelityversus Success Probability trade-off of a high-efficiency source ofhigh-fidelity dual-rail photonic entanglement. We find however that there is athreshold of $1.5$ dB of loss per switch, beyond which multiplexing hurts theFidelity versus Success Probability trade-off.

Heralded-Multiplexed High-Efficiency Cascaded Source of Dual-Rail Polarization-Entangled Photon Pairs using Spontaneous Parametric Down Conversion