A Zero Added Loss Multiplexing (ZALM) Source Simulation

Zero Added Loss Multiplexing (ZALM) offers broadband, per channel heralded EPR pairs, with a rich parameter space that allows its performance to be tailored for specific applications. We present a modular ZALM simulator that demonstrates how design choices affect output rate and fidelity. Built in NetSquid with QSI controllers, it exposes 20+ tunable parameters, supports IDEAL and REALISTIC modes, and provides reusable components for Spontaneous Parametric Down Conversion (SPDC) sources, interference, Dense Wavelength Division Multiplexing (DWDM) filtering, fiber delay, active polarization gates, detectors, and lossy fiber. Physics based models capture Hong Ou Mandel (HOM) visibility, insertion loss, detector efficiency, gate errors, and attenuation. Using this tool, we map trade offs among fidelity, link distance, and entangled pairs per use, and show how SPDC bandwidth and DWDM grid spacing steer performance. Using the default configuration settings, average fidelity remains constant at 0.83 but the ebit rate decreases from 0.0175 at the source to 0.0 at 50 km; narrowing the SPDC degeneracy bandwidth increases the ebit rate significantly without affecting fidelity. The simulator enables codesign of source, filtering, and feedforward settings for specific quantum memories and integrates as a building block for end to end quantum network studies.

💡 Research Summary

The paper introduces a comprehensive simulation framework for the Zero‑Added‑Loss Multiplexing (ZALM) entanglement source, implemented in the NetSquid quantum‑network simulator and driven by the QSI controller API. ZALM is a “source‑in‑the‑middle” architecture that combines two probabilistic spontaneous parametric down‑conversion (SPDC) modules to produce heralded, per‑channel Einstein‑Podolsky‑Rosen (EPR) photon pairs suitable for loading into quantum memories. The authors model each physical component—SPDC crystals (type‑II, non‑degenerate), a 50:50 beam splitter, Hong‑Ou‑Mandel (HOM) visibility, polarising beam splitters (PBS), dense wavelength‑division multiplexing (DWDM) filters, single‑photon detectors, fiber delay loops, and active Pockels‑cell gates—using physics‑based parameterizations.

Key features of the simulator include:

• More than 20 tunable parameters covering pump power, SPDC bandwidth, central wavelength, HOM visibility, insertion loss, detector efficiency, dark count rate, DWDM channel spacing and bandwidth, fiber attenuation, and gate error probabilities.
• Two operational modes – “IDEAL” (perfect components, V = 1, zero loss, unit detection efficiency) and “REALISTIC” (experimentally measured values).
• Modular component library that can be used stand‑alone or embedded in larger quantum‑network simulations, enabling co‑design of source, filtering, and feed‑forward logic for specific quantum memories (e.g., diamond NV or SiV centers).

The simulation workflow follows the physical sequence described in the ZALM protocol. Two SPDC sources are pumped simultaneously; each emits a signal‑idler photon pair with a Gaussian spectral distribution centered around a degeneracy wavelength λ. The idler photons are directed to a beam splitter where they may interfere. The degree of interference is governed by the HOM visibility V; imperfect overlap reduces the probability of generating a four‑qubit entangled state and degrades fidelity. After the beam splitter, a PBS separates photons by polarization, and each output feeds a DWDM filter. The DWDM stage provides a coarse wavelength estimate for each idler photon; the associated detector click pattern (including phase information) heralds the Bell state of the two signal photons. A short fiber delay (≈4 m, ≈20 ns) gives enough time for the heralding information to reach a post‑processing unit, which then applies the appropriate Pauli correction (via Pockels cells) to the signal photons, converting them to the singlet |Ψ⁻⟩ state—optimal for many memory loading protocols.

The authors conduct extensive parameter sweeps to explore trade‑offs among entanglement fidelity, ebit generation rate, and link distance. With a default realistic configuration (SPDC pair generation probability ≈0.1, HOM visibility ≈0.9, detector efficiency ≈0.85, fiber loss 0.2 dB/km, DWDM channel spacing 200 GHz, channel bandwidth 100 GHz), the average fidelity remains roughly constant at 0.83 across all distances, while the ebit rate drops from ≈0.0175 ebit per source use at 0 km to essentially zero at 50 km due to fiber attenuation. Narrowing the SPDC degeneracy bandwidth (e.g., from 0.5 nm to 0.1 nm) does not affect fidelity but significantly boosts the ebit rate because the tighter spectral distribution improves the accuracy of the wavelength estimate provided by the DWDM stage, thereby increasing memory loading efficiency. Similarly, adjusting the DWDM grid spacing changes the number of usable channels and the insertion loss per channel, revealing a classic bandwidth‑versus‑loss trade‑off.

The paper’s main insight is that ZALM’s performance is highly sensitive to the interplay between source spectral properties and the classical multiplexing infrastructure. By co‑optimizing SPDC bandwidth, DWDM channel grid, and feed‑forward gate parameters, one can tailor the source to the spectral acceptance of a given quantum memory, achieving higher entanglement distribution rates without sacrificing fidelity. The simulator thus serves as a valuable design tool for end‑to‑end quantum‑network studies, allowing researchers to plug the ZALM module into larger repeater or network simulations and evaluate system‑level metrics such as secret‑key rate or network throughput.

In summary, the work delivers a flexible, physics‑accurate ZALM source simulator, demonstrates its utility through systematic performance mapping, and highlights how careful parameter selection can mitigate distance‑induced loss, making ZALM a promising candidate for scalable quantum‑internet architectures.