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Continuous-variable QKD over 50km commercial fiber
Yi-Chen Zhang, Zhengyu Li, Ziyang Chen, Christian Weedbrook, Yijia Zhao, Xiangyu Wang, Yundi Huang, Chunchao Xu, Xiaoxiong Zhang, Zhenya Wang, Mei Li, Xueying Zhang, Ziyong Zheng, Binjie Chu, Xinyu Gao, Nan Meng, Weiwen Cai, Zheng Wang, Gan Wang, Song Yu, Hong Guo
TL;DR
Field deployment over commercial fiber is an important step toward practical metropolitan continuous-variable QKD. The paper reports two field tests using automated control and optimized reconciliation, achieving secure key rates two orders of magnitude above previous field demonstrations and extending distribution to 50 km.
Problem
Field tests over commercial fiber are an important step toward the real-world adoption of continuous-variable QKD.
Method
The system combines feedback-based automatic control with rate-adaptive reconciliation and optimized data processing to stabilize operation and improve key extraction.
Results
Secure key rates were two orders of magnitude higher than previous field-test results, while continuous-variable QKD distribution was extended to 50 km over commercial fiber.
Takeaways & Limitations
These results move continuous-variable QKD toward a practical metropolitan setting and place secure metropolitan networks within reach of current technology.
Abstract
from arXiv · showhide
The continuous-variable version of quantum key distribution (QKD) offers the advantages (over discrete-variable systems) of higher secret key rates in metropolitan areas as well as the use of standard telecom components that can operate at room temperature. An important step in the real-world adoption of continuous-variable QKD is the deployment of field tests over commercial fibers. Here we report two different field tests of a continuous-variable QKD system through commercial fiber networks in Xi'an and Guangzhou over distances of 30.02 km (12.48 dB) and 49.85 km (11.62 dB), respectively. We achieve secure key rates two orders-of-magnitude higher than previous field test demonstrations. This is achieved by developing a fully automatic control system to create stable excess noise and by applying a rate-adaptive reconciliation protocol to achieve a high reconciliation efficiency with high success probability. Our results pave the way to achieving continuous-variable QKD in a metropolitan setting.
Results
The field tests combine automatic calibration, continuous-variable optical transmission, and high-speed postprocessing to sustain secret-key generation over commercial fibers.
- Field deployment: 30.02 km (12.48 dB) and 49.85 km (11.62 dB) commercial-fiber links were tested in Xi’an and Guangzhou, respectively.The two links had similar channel losses but different deployed lengths because of their physical networks.
- Field stabilization: Automatic feedback systems calibrated time, polarization, and phase to compensate field-induced disturbances in transmitted quantum states.The system used LO-based synchronization, dynamic polarization control, and phase-drift compensation.
- Postprocessing: 5 MHz was the system repetition rate, and GPU parallel decoding reached speeds up to 30.39 Mbits/s.Multidimensional reconciliation and multi-edge type LDPC codes were combined for efficient low-SNR error correction.
- Secret-key results: 7.57 kbps and 5.91 kbps were obtained in Xi’an under asymptotic and finite-size regimes, respectively; Guangzhou achieved 7.43 kbps and 5.77 kbps.The average Xi’an excess noise was 4% SNU, while Guangzhou had average reconciliation efficiency 0.9501 at average SNR 0.0295.
Discussion
The field tests extend CV-QKD to 50 km over commercial fiber and substantially improve secure key rates, while remaining below fundamental repeaterless bounds.
- Discussion: 50 km over commercial fiber was reached, with secure key rates two orders-of-magnitude higher than previous field-test results.The authors attribute the improvement to optimized optical-layer and postprocessing schemes.
- Discussion: The achieved secret key rates remain below the PLOB bounds for both lossy and thermal-loss channels.The figure includes bounds accounting for average excess noise in the field test.
- Discussion: A secure metropolitan CV-QKD network is described as within reach of current technology.This conclusion follows the reported commercial-fiber field tests and their improved key rates.
Appendix A: Experimental details
Three feedback subsystems support automatic operation by synchronizing timing and stabilizing polarization and phase in deployed commercial fibers.
- Appendix A: Experimental details: Three automatic feedback systems calibrate the time, polarization, and phase of transmitted quantum states.The paper identifies these feedback systems as key modules for operating CV-QKD in deployed commercial fibers.
- Appendix A: Experimental details: LO-based data synchronization uses a high-SNR reference to reduce synchronization time and promote a 100% success possibility.Data synchronization determines the signal starting point for the receiving system.
- Appendix A: Experimental details: Polarization stabilization uses an electric polarization controller, polarization beamsplitter, and photodetector, with the controller driven by high-SNR LO pulses.This arrangement is intended to handle severe polarization drift.
- Appendix A: Experimental details: Phase stabilization compensates slow drift between the LO and signal paths using an LO-path phase modulator and inserted reference data.The insertion period is determined by the frequency of phase drift.
Appendix B: Calibration model with one-time evaluation.
The one-time-evaluation calibration model estimates the shot-noise unit from a single homodyne measurement with the local-oscillator path on, while modeling detector imperfections through trusted beamsplitters. It reduces hardware and statistical requirements for practical calibration.
- Calibration procedure: The one-time-evaluation model measures the homodyne detector once with the local-oscillator path on, unlike the two-times process requiring separate measurements with both paths off and only the local-oscillator path connected.The two-times model subtracts the two measurements to calculate the shot-noise unit.
- Calibration procedure: S NUOT E = Vtot = S NUTT E + Vele, where Vtot and Vele are the homodyne-output variances with the local-oscillator path on and off, respectively.S NUTT E denotes the two-times-calibration shot-noise unit, whereas S NUOT E denotes the one-time-calibration value.
- Practical advantages: The model uses one optical switch, requires one statistical evaluation, and minimizes statistical fluctuation relative to the original two-evaluation calibration.These changes are presented as advantages for applying the calibration model in practical systems.
- Entanglement-based model: The entanglement-based model represents limited detection efficiency and electronic noise with two beamsplitters before ideal homodyne detection of mode B’.Alice heterodyne-detects mode A, while mode B is sent through the channel toward Bob.
Appendix C: Secret key rate with one-time-evaluation calibration model.
The appendix derives the secret-key-rate calculation for the one-time-evaluation model using covariance matrices, symplectic eigenvalues, and the Holevo quantity. The resulting calculation provides a secure lower bound on the actual secret key rate.
- Rate derivation: The asymptotic secret key rate for the one-time-evaluation calibration model under collective attacks and reverse reconciliation is defined by the stated rate equation.The derivation uses the covariance matrix and the Holevo quantity to bound Eve’s information.
- Covariance-matrix construction: The covariance matrix of modes A, D1, and B′ is obtained even though mode D2 is unknown because the one-time model does not measure electronic noise.The experimentally accessible Alice and Bob data suffice to obtain the parameter values appearing in the covariance matrix.
- Eavesdropper-information bound: The Holevo quantity χ(B : E) is expressed through von Neumann entropies of the relevant states and restricts the upper bound of information available to Eve.The first entropy term is calculated from the covariance matrix γAD1B′ and its symplectic eigenvalues.
- Eavesdropper-information bound: Three valid symplectic eigenvalues are obtained from γAB′D1, while two valid symplectic eigenvalues are obtained from γmB′AD1 for rewriting χ(B : E).These eigenvalues support the entropy-based calculation of the Holevo quantity.
- Final rate: The final secret key rate is obtained using the stated entropy function G(x) = (x + 1)log2(x + 1) − xlog2x and equation (5).The calculation is explicitly described as yielding a secure lower bound on the actual secret key rate.
Appendix D: Rate-adaptive reconciliation protocol.
The rate-adaptive reconciliation protocol adjusts coding to experimental channel conditions by balancing reconciliation efficiency and frame error rate, using puncturing and shortening to change the code rate.
- Rate-adaptive reconciliation maximizes the secret key rate by balancing reconciliation efficiency β and frame error rate.
- The original code rate is R_o = (n − m)/n, where n is the code length and m is the redundancy-bit length.
- Adding p punctured bits changes the code rate to R′ = (n−m)/(n−p), while adding s shortened bits changes it to R′′ = (n−m−s)/(n−s).
- Reconciliation efficiency is defined as β = R/C, where C is the classical capacity of the quantum channel.
- For practical SNR values of 0.028–0.030, the procedure selects an optimal code rate and a MET-LDPC code with rate close to that optimum.
- Bob inserts randomly generated punctured and shortened bits into his reconciliation string before computing its syndrome, and Alice reconstructs the corresponding sequence for decoding.