Field Demonstration of a Multi-User Continuous-Variable Quantum Access Network for Quantum-to-the-Home

Abstract: Realizing scalable Quantum-to-the-Home (QTTH) faces a bottleneck: link asymmetry in broadcast continuous-variable quantum access networks (CV-QANs) hinders the selection of a globally optimal modulation variance. We demonstrate a downstream broadcast CV-QAN connecting a Quantum Line Terminal (QLT) to multiple Quantum Network Units (QNUs) over commercial fiber. Operating within a trusted local network domain, we establish a multi-user utility model to select the optimal shared variance, balancing network efficiency and user fairness. Supported by robust digital signal processing, our 1:16 field trial achieves Mbit/s-level asymptotic secure key rates, bridging theoretical protocols with Fiber-to-the-Home reality and guiding future scalable access architectures.

• The paper demonstrates a 1:16 downstream broadcast continuous-variable quantum access network over commercial China Telecom fiber, extending CV-QKD from laboratory links to a realistic field environment.
• Using a shared modulation variance of 4.3 SNU and a max-min fairness strategy, six representative users achieved asymptotic secret key rates of 0.97–2.20 Mbit/s across 15.00–16.78 dB channel attenuation.
• The study shows that aggregate throughput and worst-user fairness require different modulation settings, while composable-security simulations indicate that block lengths near 10¹¹ symbols may be needed for positive finite-size key rates.

Context and motivation

Quantum key distribution (QKD) has matured from point-to-point laboratory links into backbone and metropolitan infrastructures, but the access segment—the "last mile" connecting a central office to individual end users—remains comparatively underdeveloped. Continuous-variable QKD (CV-QKD) is an attractive candidate for this segment because coherent detection is naturally compatible with classical wavelength-division multiplexing and passive optical network (PON) architectures. Prior work on broadcast CV-QANs, including a 16-node laboratory demonstration with Mbit/s-level per-user secret key rates (SKRs), established the protocol foundations but operated under symmetric, controlled channel conditions (2606.18840).

The paper under discussion addresses the defining practical obstacle of downstream broadcast architectures: because a single Gaussian-modulated coherent state (GMCS) stream is passively split by a $1{:}N$ optical distribution network (ODN), the modulation variance $V_A$ is necessarily a system-wide shared parameter, while real field deployments exhibit strongly asymmetric per-user channel transmittances due to unequal drop-fiber lengths. The authors demonstrate a 1:16 downstream broadcast CV-QAN over commercial China Telecom fiber in Minhang District, Shanghai, and develop an analytical framework for selecting the shared $V_A$ under such asymmetry.

System model: fairness–efficiency trade-off

The security analysis adopts the strict non-collaboration assumption—coexisting users do not assist Eve—which the authors justify by noting that all 16 quantum network units (QNUs) reside within a single administrative domain (one building). Under this assumption, the broadcast SKR for user $i$ reduces to the familiar form $$\mathrm{SKR}_{AB_i} = f(\beta I(A;B_i) - \chi_{B_iE}^{(N)})$$, where the Holevo term accounts for Eve's information given the multi-user covariance structure.

The central theoretical contribution is a unified $\alpha$-fair system utility over the shared modulation variance:

$$\text{U}_{sys}(V_A,\alpha)=\sum_j \frac{\text{SKR}_j(V_A)^{1-\alpha}}{1-\alpha}$$

(with the logarithmic form at $\alpha=1$). Two limits are analyzed explicitly. In the $\alpha \to 0$ limit, maximizing sum-SKR yields an optimum $V_{A1}$ shifted toward larger modulation, dominated by high-quality links; in the $V_A$0 limit (max-min), the optimum $V_A$1 shifts toward smaller modulation because Eve's accessible information grows faster than the mutual information for low-transmittance users. The paper's key structural claim is that no single shared $V_A$2 simultaneously optimizes both objectives—the two optima diverge under asymmetric channels—so the operator must make an explicit efficiency-versus-fairness choice. The experiments adopt the max-min criterion as the operational baseline to guarantee a minimum service level.

Field deployment and experimental setup

The deployed tree-topology network connects a Quantum Line Terminal (QLT) at the Yongde telecommunication room to the XunTai building via a 4.08 km feeder fiber, followed by a $V_A$3 splitter and 30–50 m indoor distribution fibers. At the QLT, a ~100 Hz linewidth laser at 1550 nm feeds a commercial IQ modulator driven by a 5 GSa/s AWG; a 500 MHz Gaussian-modulated RRC-shaped signal (roll-off 0.2, 600 MHz bandwidth) is frequency-shifted to −500 MHz and multiplexed with a pilot tone at −1.25 GHz. A 1490 nm amplitude-modulated clock provides WDM-multiplexed hardware synchronization. Each QNU performs heterodyne detection with a local local oscillator (LLO) scheme using an integrated coherent receiver, digitized at 5 GHz.

The DSP chain comprises pilot-tone-based frequency offset recovery, phase drift compensation exploiting co-propagation of signal and pilot through the same fiber, and coherent demodulation with matched RRC filtering. Parameter estimation uses 15 frames of $V_A$4 symbols per user.

Results

The shared modulation variance was set to $V_A$5 SNU, selected by the max-min criterion. Per-user channel parameters span 15.00–16.78 dB attenuation and excess noise between 0.031 and 0.042 SNU, with electronic noise near 0.16 SNU and reconciliation efficiency $V_A$6:

User	Attenuation (dB)	Excess noise (SNU)	SKR (Mbit/s)
1	15.00	0.0361	1.80
2	14.67	0.0306	2.20
3	15.92	0.0404	1.28
4	16.16	0.0348	1.37
5	16.78	0.0421	0.97
6	16.48	0.0397	1.12

All six representative users achieve Mbit/s-level asymptotic SKRs despite asymmetric links, which constitutes the paper's principal empirical claim: broadcast CV-QANs remain viable under realistic FTTH-style channel disparity. A composable-security analysis following the finite-size framework of Wang et al., with $V_A$7, FER = 0.2, and 8-bit ADC quantization, indicates that under the theoretical 12 dB splitting loss of the 1:16 architecture, a block length of approximately $V_A$8 is required for a positive composable key rate—a demanding requirement relative to the $V_A$9-symbol frames used experimentally.

Limitations and open questions

The paper is candid about several constraints. First, the Mbit/s results are asymptotic; the composable finite-size analysis is presented only as a simulation, and the authors state that rigorous finite-size verification under field noise conditions "remains a nontrivial task," deferring it to future work with larger block sizes and prolonged stable operation. Second, the security proof relies on the non-collaboration assumption among users; the alternative worst-case model in which Eve exploits inter-user leakage would yield lower rates, so the reported figures are conditional on the single-administrative-domain trust model. Third, the experimental evaluation covers six of the sixteen deployed QNUs, chosen as representative of the channel asymmetry rather than the full population. Finally, the paper identifies large-scale resource allocation, system control, and long-term stability as unresolved engineering challenges for widespread deployment.

Conclusion

This work extends multi-user broadcast CV-QKD from laboratory conditions to a commercial telecommunication environment, demonstrating a 1:16 downstream access network in which representative users achieve 0.97–2.20 Mbit/s asymptotic SKRs over asymmetric field channels. Its analytical contribution—an $V_A$0-fair utility framework showing that the shared modulation variance cannot be globally optimal for both aggregate throughput and worst-user performance—provides concrete deployment guidance for QTTH-style architectures. The gap between the demonstrated asymptotic rates and the block lengths required for composable security remains the principal open question raised by the results.