End-to-End Fidelity and Rate Analysis for Zero-Added-Loss Multiplexing

Zero-added-loss multiplexing (ZALM) [Phys. Rev. Appl. 19, 054029 (2023)] is a midpoint-source scheme for entanglement distribution. Its quantum transmitter (QTX) employs two Sagnac sources of polarization-entangled pairs plus partial Bell-state measurements (BSMs) on their idler outputs to herald entanglement in their signal outputs, thus avoiding the switching losses suffered by other multiplexing approaches. When its Sagnac sources' spontaneous parametric downconverters (SPDCs) utilize χ(2) crystals with phase-matched spectral islands, ZALM avoids the tradeoff between heralding efficiency and output-state fidelity incurred when conventional-crystal SPDCs' outputs are channelized by dense wavelength-division multiplexing filters. This paper reports an end-to-end fidelity and rate analysis for islands-based ZALM that includes: SPDC multipair emissions of all orders and partial-BSM losses in the ZALM QTX; loss in propagation from the ZALM QTX to Alice and Bob's quantum receivers (QRXs); loss and mode mismatch in the mode converters employed at Alice and Bob's QRXs; and loss in the partial-BSM loading of Alice and Bob's emissive quantum memories.