Entanglement distribution: To herald or not to herald

TL;DR

This paper compares entanglement distribution rates of three spectral-island-based systems, highlighting trade-offs between heralded and unheralded approaches for quantum internet applications.

Contribution

It provides a detailed comparison of entanglement distribution rates and equipment requirements for three islands-based SPDC systems, emphasizing the impact of heralding efficiency.

Findings

ZALM outperforms signal-path erasure at low heralding efficiencies.

Unheralded operation surpasses heralded systems when using multiple spectral islands.

Equipment complexity varies significantly between heralded and unheralded approaches.

Abstract

High-rate, high-fidelity entanglement distribution is essential for the creation of a quantum internet, and spontaneous parametric downconverters (SPDCs) are, at present, the preferred sources of entangled signal-idler photon pairs for transmission to Alice and Bob's quantum nodes. SPDCs using phase-matched spectral islands are especially attractive, in this regard, because they provide wavelength-division multiplexed signal-idler pairs with single-mode temporal behavior. This paper compares the entanglement distribution rates of three islands-based systems. Two use idler detections for heralding: islands-based zero-added-loss multiplexing (ZALM), and an islands-based Sagnac SPDC source with signal-path erasure. The third employs an unheralded Sagnac SPDC source. For 90% or lower heralding efficiencies, ZALM's per-pump-pulse entanglement distribution rate exceeds that of the signal-path erasure source, and both rates are inferior to unheralded operation's when all three systems employ $N_I$ spectral islands and allocate $N_M = N_I$ quantum memories to each pump pulse. These behaviors, however, must be weighed against the three systems' differing equipment requirements, e.g., ZALM requires a pair of perfectly-matched Sagnac sources, which is a significant burden not incurred by the signal-path erasure approach, and both heralded systems will suffer, in comparison with unheralded operation, if they cannot realize high heralding efficiencies.