Ultra-Large-Capacity Passive Quantum Access Network Powered By Single Thermal Source

Abstract: Quantum Key Distribution (QKD) provides secure keys for classical communications through one-time-pad (OTP) encryption with physical-law security. Advanced PON-based Classical Access Networks (CANs) support up to 256 users with a total rate of 10 Gbps (10-Gbps @ 256-users). The equivalent rate demand of OTP encryption requires QKD Access Networks (QANs) to reach comparable performance, yet state-of-the-art PON-based QANs remain far from this standard. To address this gap, we propose a passive Thermal-State QAN (TS-QAN) distributing polychromatic quantum randomness from a single thermal source and supporting 304 users with an aggregate secret key rate (SKR) of 13 Gbps (13-Gbps @ 304-users). This performance is enabled by three features. First, broadband thermal states with Bose-Einstein statistics can be represented, through the Glauber-Sudarshan representation, as high-bandwidth Gaussian coherent-state ensembles across frequency modes, eliminating many active modulators and quantum random number generators (QRNGs). Second, Electro-Optic (EO) comb beacons provide time-varying polychromatic phase tracking, so each frequency-mode thermal signal can be coherently measured with a Local Local Oscillator (LLO) aided by its beacon, without large-scale phase-locking networks. Third, state broadcasting allows each user to obtain independent final keys via reverse reconciliation after accounting for residual broadcast-induced correlations, expanding network capacity with small SKR losses. Experimentally, we verify a 13-Gbps @ 304-users TS-QAN using Continuous-Variable QKD (CV-QKD) under covariance-matrix-based network security analysis including multimode Holevo leakage and broadcast correlations. This work meets the SKR and capacity demands from CAN to QAN: 13-Gbps @ 304-users satisfies the 10-Gbps @ 256-users benchmark and provides a scalable solution for modern telecommunication systems.

• The paper proposes and experimentally demonstrates a Thermal State Quantum Access Network that combines a broadband ASE source, EO-comb beacon tones, and reverse reconciliation to serve 304 users without per-user modulators or QRNGs.
• The network achieves aggregate secret key rates of 13.76 Gbps over 5 km, 7.28 Gbps over 15 km, and 2.47 Gbps over 30 km asymptotically, while finite-size analysis yields 3.60 Gbps over 5 km and 0.26 Gbps over 15 km.
• The study shows that passive broadcasting can outperform existing access-network benchmarks, but rigorous finite-size composable security, correlated-noise modeling, calibration scalability, and the high 58.70% frame-error rate remain important deployment challenges.

The gap between classical passive optical networks and quantum key distribution access networks has been a persistent obstacle to deploying information-theoretically secure encryption at metropolitan scale. The paper "Ultra-Large-Capacity Passive Quantum Access Network Powered By Single Thermal Source" (2605.20077) addresses this by proposing and experimentally demonstrating a Thermal State Quantum Access Network (TS-QAN) that distributes polychromatic quantum randomness from a single amplified spontaneous emission (ASE) source to 304 users, achieving an aggregate secret key rate (SKR) of 13.76 Gbps over 5 km — exceeding the performance benchmark of advanced Classical Access Networks (CANs), which operate at 10 Gbps across 256 users.

Motivation and architectural positioning

Existing QKD access networks (QANs) fall into two families, each with a structural deficiency. DWDM-based QANs distribute polychromatic quantum states but require per-channel active modulators and quantum random number generators (QRNGs), making hardware complexity scale linearly with network capacity. Beam-splitter (BS)-based QANs scale favorably in hardware but suffer SKR collapse under splitting loss; their best reported results are 16.80 Mbps over 8 users and 33.38 Mbps over 16 users, while the strongest DWDM-QAN reaches only 8.75 Gbps over 19 users. Neither approaches the CAN requirement of 10-Gbps @ 256-users demanded by one-time pad encryption at matched rates.

TS-QAN resolves this tension through three mechanisms. First, a broadband thermal state — via the Glauber–Sudarshan $P$ representation — is equivalent to an ensemble of Gaussian coherent states across frequency modes, so the thermal source itself supplies both the polychromatic resource and the high-bandwidth Bose–Einstein randomness, eliminating per-user modulation hardware and QRNGs. Second, an electro-optic (EO) comb provides polychromatic beacon tones enabling time-varying phase tracking for local-local-oscillator (LLO) coherent reception at every user, removing large-scale phase-locking infrastructure. Third, state broadcasting with reverse reconciliation lets all users sharing a broadcast signal extract independent final keys, provided residual broadcast-induced correlations are charged against the key rate.

Prepare-measure model

The prepare-measure (PM) description divides the network into QLT, channel, and QNUs. At the QLT, the multimode thermal state is written as a tensor product of Fock-basis mixtures with Bose–Einstein photon-number statistics $P_i(n)=\bar{n}_i^n/(1+\bar{n}_i)^{n+1}$, then re-expressed as a Gaussian ensemble of coherent-state realizations $\bigotimes_i |\gamma_{0,i}\rangle$ sampled from variance $V_{0,i}$. A single-frequency laser generates a multi-temporal-mode beacon converted to the frequency domain by a time-lens Fourier transform, establishing a common wave-packet basis $\bm{\xi}$ shared by signal and beacon. Each PPM splits both fields via unbalanced beam splitters: high-power portions are heterodyned locally to yield Alice's passive-preparation data, while low-power portions are coupled into tensor-product signal-plus-beacon states transmitted downstream.

The channel applies three successive operations: a passive frequency-allocation unitary $\hat U_D$ (matrix $D_{\rm full}$), a cascaded power-splitting operation $\hat U_{\rm CBS}$ with branch ratios $r_h$, and residual user channels $$\mathcal N_{i'}(T_{d,i'},\varepsilon_{i'},\theta_{p,i'})$$ carrying transmittance, excess noise, and phase rotation. Crucially, finite mode isolation means each user receives weighted contributions from all source-side frequency modes rather than a single clean component. Each QNU runs a DSP pipeline — beacon beat filtering, time-varying phase recovery, DC-offset removal, downsampling — whose kernel defines the effective receiver mode through overlaps $P_i(n)=\bar{n}_i^n/(1+\bar{n}_i)^{n+1}$0 with the incident frequency components. The authors treat DSP as part of the optimal receiver model, applying the identical pipeline in calibration and data acquisition, consistent with continuous-mode security analyses.

Entanglement-based security analysis

The entanglement-based (EB) formulation models the whole network as a single Gaussian state tracked by covariance matrices. Alice prepares $P_i(n)=\bar{n}_i^n/(1+\bar{n}_i)^{n+1}$1 EPR pairs; Eve's channel implements frequency allocation, cascaded splitting, and residual attenuation. The resulting Alice–Bob covariance blocks become dense after frequency allocation ($P_i(n)=\bar{n}_i^n/(1+\bar{n}_i)^{n+1}$2), and Bob–Bob blocks acquire nonzero off-diagonal entries because split branches share a common input mode. The security evaluation supports four levels — asymptotic (AC), finite-size (FS), composable (CS), and finite-size composable (FC) — and three receiver-security partitions (untrusted, trusted, all-measured), with the measured block size $P_i(n)=\bar{n}_i^n/(1+\bar{n}_i)^{n+1}$3 ranging from 1 to $P_i(n)=\bar{n}_i^n/(1+\bar{n}_i)^{n+1}$4 depending on policy. The single-user SKR charges both Eve's Holevo information and the residual classical correlation $P_i(n)=\bar{n}_i^n/(1+\bar{n}_i)^{n+1}$5 induced by broadcasting:

$P_i(n)=\bar{n}_i^n/(1+\bar{n}_i)^{n+1}$6

with aggregate rates obtained by summation or via a joint all-measured partition. Per-user and network rates are bounded respectively by the PLOB and PLOB-$P_i(n)=\bar{n}_i^n/(1+\bar{n}_i)^{n+1}$7 limits.

Experimental implementation

The QLT comprises one ASE source (>5 THz bandwidth centered at 1550 nm), a home-built EO comb with 20 GHz spacing, two WaveShapers providing 30 dB mode isolation, and 19 passive preparation modules, each containing a fiber polarizer, three beam splitters, a VOA, and an integrated coherent receiver (electronic noise 0.31 SNU, detection efficiency 56%). The channel consists of nineteen $P_i(n)=\bar{n}_i^n/(1+\bar{n}_i)^{n+1}$8 cascaded splitters feeding 304 fiber links at 0.17 dB/km. Each QNU is a single-frequency laser, polarization controller, and integrated coherent receiver executing the beacon-aided LLO DSP chain. Reverse reconciliation uses MET-LDPC codes improved by noise whitening.

Spectrum measurements confirm a flat thermal spectrum (flatness 1.24 dB, average power −29.10 dBm) and a comb flatness of 2.63 dB with sub-kHz linewidth. Parameter estimation over 95 sampled users (5 per frequency mode, block size $P_i(n)=\bar{n}_i^n/(1+\bar{n}_i)^{n+1}$9) yields mean excess noise of 0.07 SNU, mean modulation variance of 4.47 SNU, reconciliation efficiency of 95.86%, and frame error rate of 58.70%. The authors attribute the slightly elevated excess noise relative to coherent-state networks to high-bandwidth operation — dispersion impairment and bandwidth-limited receiver noise — rather than to the passive-preparation mechanism itself. Covariance-matrix reconstruction from pairwise calibrated measurements reveals the expected structural evolution: diagonal EPR correlations before allocation, dense non-diagonal blocks after allocation, a 19×304 Alice–user correlation structure, and a generally non-diagonal 304×304 Bob-side matrix.

Performance results

The headline numbers are strong. Under the asymptotic covariance-matrix analysis, TS-QAN delivers an aggregate SKR of 13.76 Gbps over 5 km, 7.28 Gbps over 15 km, and 2.47 Gbps over 30 km; under finite-size accounting, 3.60 Gbps over 5 km and 0.26 Gbps over 15 km. These figures satisfy the CAN benchmark of 10-Gbps @ 256-users using only 380 GHz of the available 5 THz thermal bandwidth — 7.6% of the spectrum. Architectural comparison at equal repetition rate places TS-QAN above all alternatives except DWDM-QAN, while its finite-size result remains well above the asymptotic BS-QAN reference. Notably, positive composable and finite-size-composable SKRs are obtained only under the all-measured security policy, where receiver-side correlations are explicitly incorporated; these remain lower than the corresponding AC and FS values because cascaded splitting severely reduces per-branch transmittance, making correction terms severe at the current block size.

Limitations and open questions

The authors are explicit about several caveats. Excess noise is modeled as independent Alice-referred noise per user channel; a general attack model with explicitly correlated excess-noise covariance across passive-network modes remains unformulated. The finite-size and composable corrections are adopted from point-to-point CV-QKD analyses as indicative extensions — the rigorous result established here is the asymptotic network analysis, not a full finite-size composable proof adapted to the multimode broadcast structure, which the authors identify as necessary future work. The covariance matrices were reconstructed from pairwise measurements under stable conditions rather than a single simultaneous full-network acquisition, and 209 of the 304 QNUs rely on conservative parameter filling from calibrated frequency groups rather than long-time individual calibration. The comparison figure is also framed as architecture-level illustration rather than a controlled comparison under matched hardware and symbol-rate conditions. Finally, the frame error rate of 58.70% implies substantial post-selection overhead, and whether passive preparation can match optimized active systems in finite-size regimes is left unresolved — the authors caution against interpreting their results as evidence of fundamental sub-optimality either way.

Conclusion

This work demonstrates that a single thermal source, combined with EO-comb beacons and state broadcasting, can sustain a passive QKD access network matching the rate and capacity of dominant classical PON infrastructure: 13-Gbps @ 304-users against 10-Gbps @ 256-users. The contribution is primarily architectural — shared-resource distribution replacing per-user active preparation — validated by a network-aware covariance-matrix security analysis that accounts for mode crosstalk and broadcast-induced correlations. Extending the rigorous security treatment to finite-size composable guarantees for the full multimode protocol, and exploiting the remaining 92% of the thermal bandwidth, are the principal questions this demonstration leaves open.