Source-linked AI summary
Coexistence of high-bit-rate quantum key distribution and data on optical fiber
K. A. Patel, J. F. Dynes, I. Choi, A. W. Sharpe, A. R. Dixon, Z. L. Yuan, R. V. Penty, A. J. Shields
TL;DR
The paper addresses secure-key-rate evaluation for QKD systems operating with decoy states and fiber-induced Raman noise. It combines security-proof-based rate estimation with measurements and calculations of Raman scattering, transmittance, QBER, and synchronization performance.
Problem
Estimating secure key rates requires accounting for single- and zero-photon contributions, error rates, transmittance, and Raman-noise counts in the quantum channel.
Method
The system uses BB84 with decoy states, estimates single-photon parameters through linear programming, and models Raman-scattered light by integrating its fiber-length dependence.
Results
The Raman-scattering calculations agree well with measurements, while recovered-clock timing jitter is approximately 10 ps over the tested operating range.
Takeaways & Limitations
The reported modeling and synchronization performance support evaluation of high-speed single-photon detection in fiber-based QKD systems.
Abstract
from arXiv · showhide
Quantum key distribution (QKD) uniquely allows distribution of cryptographic keys with security verified by quantum mechanical limits. Both protocol execution and subsequent applications require the assistance of classical data communication channels. While using separate fibers is one option, it is economically more viable if data and quantum signals are simultaneously transmitted through a single fiber. However, noise-photon contamination arising from the intense data signal has severely restricted both the QKD distances and secure key rates. Here, we exploit a novel temporal-filtering effect for noise-photon rejection. This allows high-bit-rate QKD over fibers up to 90 km in length and populated with error-free bidirectional Gb/s data communications. With high-bit rate and range sufficient for important information infrastructures, such as smart cities and 10 Gbit Ethernet, QKD is a significant step closer towards wide-scale deployment in fiber networks.
A. Quantum subsystem.
The quantum subsystem implements decoy-state BB84 using a pulsed, phase-encoded optical transmitter and matched interferometric single-photon detection. Secure key rates are calculated from decoy-state single-photon estimates using Koashi’s security proof.
- The transmitter uses a 1550 nm pulsed laser, intensity modulator, asymmetric Mach–Zehnder interferometer, and optical attenuator.
- The receiver uses a matched asymmetric Mach–Zehnder interferometer with two self-differencing single-photon detectors.
- Decoy-state BB84 uses signal and weaker decoy pulses, with photon fluxes of 0.5, 0.1, and 0.0007 photons per pulse.Signal states generate secure keys, while weaker decoys protect against photon-number-splitting attacks.
- Secure key rates are determined from Koashi’s security proof by estimating single-photon parameters from decoy states.The rate expression includes single-photon contributions, error correction, QBER, zero-photon counts, and session duration.
B. Clock subsystem.
The clock subsystem synchronizes the QKD system with a low-rate pulsed laser and regenerates the 1 GHz system clock at Bob. Its operating power balances timing jitter against photon-scattering noise.
- A 10 MHz clock laser is used instead of the 1 GHz system rate, reducing the launch power needed for synchronization.Bob detects the received clock with a standard small-form-pluggable receiver and regenerates the 1 GHz clock using a frequency synthesizer.
- At -47.6 dBm received clock power, the clock laser operates at approximately 30 times less power than either data laser.The authors state that this makes the clock laser’s impact on the quantum channel negligible.
- Approximately 10 ps of Alice–Bob timing jitter is sufficiently small for efficient self-differencing detection, while Bob’s cycle-cycle jitter is 1 ps.The 10 ps figure includes fiber drift and represents the worst case for the recovered clock.
C. Raman-scattered light intensity
The Raman-scattered-light analysis integrates scattering from data lasers across the fiber to estimate contamination entering the quantum receiver. Calculations based on the Raman coefficient agree well with measurements.
- Raman-scattered light entering the quantum receiver is calculated from data-laser power, wavelengths, channel bandwidth, fiber length, and attenuation.The model integrates the scattered contribution over the entire fiber length.
- The Raman coefficient is measured from the back-scattered Raman spectrum and used to calculate Raman-power dependence on fiber length.
- The calculated Raman powers agree well with the actual measurements.
D. Simulation of the secure key rate
The secure-key-rate simulation uses measurable decoy-protocol quantities, including QBER and transmittance, to evaluate the rate expression. Transmittance is defined as Bob’s detection probability for transmitted pulse classes.
- The simulation calculates QBER and transmittances for the different pulse classes used in the decoy protocol.
- Transmittance is Bob’s detection probability for a given class of pulses transmitted by Alice.
- The signal-pulse QBER is approximated using the expression introduced for the secure-key-rate simulation.