High-fidelity, quasi-deterministic entanglement generation using phase-matched spectral islands in a zero-added-loss multiplexing architecture

TL;DR

The paper addresses the probabilistic nature of SPDC entanglement sources by proposing islands-based ZALM, a spectral multiplexing scheme that avoids source-switching losses. By engineering $N_I$ phase-matched spectral islands and employing both same-island and cross-island heralding (SPCI), the authors derive analytic expressions for herald probabilities, Bell-state fidelity, and Bell-state fraction under both lossless and lossy conditions, including partial BSM and propagation losses. They demonstrate that high-fidelity (≥99%), near-unity Bell-state fraction (≥99.98%) and quasi-deterministic per-pulse herald probabilities (≥25%) can be achieved with a modest number of islands (tens to a few hundred), yielding entanglement-delivery rates on the order of 2.5×10^5 s^-1 for realistic pump rates and link losses. The approach significantly reduces the hardware burden compared to prior 800-channel schemes, with practical implications for satellite-to-ground and fiber-based quantum networks. The work also outlines avenues for further optimization and integration with quantum memories and alternative heralding architectures.

Abstract

Spontaneous parametric down-converters (SPDCs) are the best available entanglement sources for distributing entanglement in a quantum internet. However, their intrinsically probabilistic nature, and their need to operate at low brightness to suppress multipair events, dictate that multiplexed SPDC arrays are required for high-rate distribution in that application. Early SPDC multiplexing proposals involved path switching, whose switching losses significantly degrade performance. The present paper proposes and analyzes a scheme for spectral multiplexing that provides entanglement-distribution rates well in excess of the state of the art. It builds on zero-added-loss multiplexing (ZALM)~[Phys. Rev. Appl. {\bf 19}, 054029 (2023)] for high-rate heralded entanglement generation, which does not require a switched array of SPDCs. Our ZALM's SPDCs rely on nonlinear crystals with $N_I$ phase-matched spectral islands, each generating two-mode squeezed-vacuum states. Also, our ZALM's multiplexing protocol uses both same-island and cross-island heralding, which allows the entanglement-delivery rate to approximately scale as $N_I^2$ in the realistic weak-squeezing regime. As a result, our scheme uses an order of magnitude fewer spectral channels than the original ZALM proposal, which may enable near-term implementations of satellite-to-ground or fiber-optic based ZALM architectures.

Figures (10)

• Figure 1: Schematic of a Sagnac-configured SPDC source Wong2006 of signal-idler biphotons suitable for use in islands-based ZALM. A periodically-poled lithium niobate (PPLN) crystal with $N_I$ phase-matched spectral islands footnote3 is bidirectionally pulse-pumped for type-0 nondegenerate phase matching. $D$, $H$, and $V$: diagonal, horizontal, and vertical polarizations. HR: high reflector. $\lambda$: wavelength. PBS: polarizing beam splitter. HWP: half-wave plate.
• Figure 2: Schematic of islands-based ZALM's partial Bell-state measurement for heralding polarization-entangled photon pairs. Here $S_k$ and $I_k$ for $k = 1,2$ denote the signal ($S$) and idler ($I$) beams from the $k$th Sagnac source. $I_\pm$ denote the idler-beam outputs from the 50--50 beam splitter (BS); $I_{\pm P}$ for $P= H, V$ denote the horizontally ($H$) and vertically ($V$) polarized outputs from the polarizing beam splitter (PBS) illuminated by $I_\pm$; CWDM$_I$ denotes the idler-beam coarse wavelength-division multiplexing filter. SPD, single-photon detector.
• Figure 3: Sketch of the frequency-domain wave function for a biphoton produced by 6 identical, spectrally-factorable phase-matched spectral islands, with signal-idler center frequencies $\{(\omega_{S_n},\omega_{I_n}): n=1,2,\ldots,6\}$.
• Figure 4: Performance of ideal islands-based ZALM with same-island heralding: per-pump-pulse probability of an $n$th-island true herald, $\Pr(\mathcal{H}_{\rm true})$, versus average number of signal-idler pairs per SPDC island per pump pulse, $G-1$, for $N_I = 2,4,6,\ldots, 16$ islands.
• Figure 5: Performance of islands-based ZALM with a lossy partial BSM: Bell-state fraction, $\mathcal{B}$, of the $S_1S_2$ output state versus average number of signal-idler pairs per SPDC island per pump pulse, $G-1$, for (top to bottom) $\eta_T= 1,0.9,0.8,\ldots, 0.5$.