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Demonstration of an LLO CV-QKD system over 12 km of optical fiber

This paper demonstrates a secure Gaussian continuous-variable quantum key distribution (CV-QKD) system over 12 km of optical fiber using fully independent local oscillator lasers, achieving secret key rates of 5.11 Mbit/s in the asymptotic regime and 4.67 Mbit/s in the finite-size regime to establish a foundation for metropolitan deployment under strict security constraints.

Original authors: Christiano M. S. Nascimento, Artur A. Matoso, Gustavo C. Amaral, Guilherme P. Temporão

Published 2026-08-10

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CC BY 4.0

Original authors: Christiano M. S. Nascimento, Artur A. Matoso, Gustavo C. Amaral, Guilherme P. Temporão

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Imagine the internet as a giant, bustling city where everyone is constantly sending secret letters to one another. For decades, the locks on these letters have been made of math puzzles. They are incredibly hard to solve, but if a super-smart computer with enough time and power comes along, it can eventually pick the lock. This is why scientists are looking for a new kind of lock, one that doesn't rely on math puzzles but on the weird, magical rules of the very small world of atoms and light. This field is called quantum physics, and the specific technology for making unbreakable locks is known as Quantum Key Distribution (QKD).

Think of QKD like sending a message written on a soap bubble. If a spy tries to peek at the bubble to read the message, the bubble pops, and the message is ruined. This means the sender and receiver know immediately if someone is eavesdropping. One popular version of this technology, called Continuous-Variable QKD (CV-QKD), is special because it can send these secret quantum messages right alongside the regular, everyday internet traffic on the same fiber-optic cable, like a secret note hidden inside a package of newspapers. However, for a long time, the way these systems worked had a tiny crack in their security: they relied on a shared "flashlight" (called a local oscillator) to read the message. If a spy tampered with that flashlight, they could trick the system into thinking it was safer than it really was.

This paper presents a clever solution to that problem. The researchers built a system where the sender and the receiver each have their own independent flashlights, rather than sharing one. This is called a "Local-Local-Oscillator" (LLO) system. By using two separate light sources, they closed the security loophole, making it much harder for a spy to sneak in. The team tested this new setup over a 12-kilometer stretch of standard optical fiber, which is about the distance of a long commute or a trip across a medium-sized city. They managed to successfully generate secret keys at a speed of 5.11 million bits per second in an ideal scenario, and a very solid 4.67 million bits per second when accounting for real-world imperfections. This proves that it is possible to build a secure, high-speed quantum network using the existing fiber cables that already run under our streets, paving the way for a future where our digital secrets are protected by the laws of physics itself.

Technical Summary: Demonstration of an LLO CV-QKD System over 12 km of Optical Fiber

Problem Statement
Continuous-variable quantum key distribution (CV-QKD) offers high secret key rates and seamless integration with classical communication channels within a single optical fiber. However, early implementations relied on transmitting a local oscillator (LO) alongside the quantum signal to mitigate phase noise. This approach introduces a critical security vulnerability known as the "local oscillator attack," where an eavesdropper (Eve) can manipulate the LO to force legitimate parties (Alice and Bob) to underestimate the information bound they can achieve. While the Local-Local-Oscillator (LLO) scheme—where Alice and Bob use independent lasers and transmit only a phase reference—addresses this loophole, full implementation is challenging. It requires robust phase reference recovery and the integration of complex classical post-processing steps (parameter estimation, reconciliation, privacy amplification) that many prior experimental works have not completed, often stopping at parameter estimation.

Methodology
The authors present an end-to-end experimental demonstration of an LLO CV-QKD system operating over a 12 km single-mode fiber (SMF) spool in a laboratory setting.

Experimental Setup: The system utilizes Gaussian-modulated coherent states. Alice employs a 1550 nm continuous-wave (CW) laser with a 1 kHz linewidth, modulated via an in-phase and quadrature (IQ) modulator driven by a 16-bit arbitrary waveform generator (AWG) at 3.125 GSa/s. A variable optical attenuator (VOA) ensures the few-photon regime, and a Faraday isolator prevents back-reflections.
Receiver Architecture: Bob uses an independent 1550 nm CW laser (also 1 kHz linewidth) as his local oscillator, creating a frequency offset of approximately 800 MHz relative to Alice's laser. The incoming signal and Bob's LO are combined at a balanced beam splitter and detected via a balanced heterodyne detector (1 GHz bandwidth). The signal is digitized by a 12-bit oscilloscope synchronized to the transmitter clock.
Digital Signal Processing (DSP): An offline DSP pipeline, adapted from the QOSST framework, processes the data. This routine identifies pilot tones to estimate carrier frequency offset and sampling clock mismatch, locates subframes using CAZAC preambles, and performs phase tracking over 100 segments to correct residual fluctuations. The signal is then downsampled and phase-corrected.
Post-Processing and Security Model: The system operates under a trusted-device security model. Data is organized into logical frames of $\approx 10^7$ coherent states, divided into ten subframes. Information reconciliation uses multidimensional reconciliation with multi-edge-type low-density parity-check (MET-LDPC) codes (rate $\approx 0.128$ ). Parameter estimation quantifies Eve's information via the Holevo bound, utilizing conservative confidence bounds (Student's t-distribution for transmittance, chi-squared for excess noise) for the finite-size regime. Privacy amplification is performed using Toeplitz matrices.

Key Results
The system successfully generated secret keys over the 12 km link with the following performance metrics:

Secret Key Rates (SKR): The system achieved an asymptotic SKR of 5.11 Mbit/s and a finite-size SKR of 4.67 Mbit/s. These results align closely with theoretical predictions (5.09 Mbit/s and 4.64 Mbit/s, respectively).
Channel Parameters: The measured average channel transmittance was $T = 0.6$ , with an excess noise of $\xi_B = 15.99$ mSNU (asymptotic) and a detector electronic noise of 43.18 mSNU.
Reconciliation Efficiency: The reconciliation efficiency ( $\beta$ ) was 95.45%, with a frame error rate (FER) of 0.623.
Finite-Size Analysis: In the finite-size regime, the confidence bounds were $\xi_{max}^B = 18.37$ mSNU and $T_{min} = 0.596$ . Despite these conservative worst-case estimates, positive key rates were maintained, indicating the system remained within the secure operating region.

Significance and Claims
The paper claims to establish a foundation for the metropolitan fiber deployment of CV-QKD under strict security constraints. By demonstrating a full-fledged implementation that covers the entire processing chain—from quantum-state transmission and coherent detection to secret-key extraction—the work addresses the gap between theoretical LLO protocols and practical, secure systems.

The authors emphasize that while the current results serve as a proof of concept conducted over a fixed link with offline processing, the successful generation of positive finite-size secret keys validates the integration of the optical system, DSP, and classical post-processing. The results suggest a potential cutoff distance of approximately 125 km for this system in metropolitan environments, motivating future integration into the Rede Rio Quântica Network in Brazil. The work does not claim universal composability but notes that the current treatment incorporates several underlying principles of such frameworks, including explicit security parameters and statistical confidence bounds. Future work is identified as focusing on reducing excess noise, extending transmission distance, increasing symbol rates, and moving toward real-time implementation.

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