[Paper Review] Silicon-based decoder for polarization-encoding quantum key distribution

Yongqiang Du, Xun Zhu+6 more|arXiv (Cornell University)|Dec 8, 2022

Quantum Information and Cryptography4 citations

TL;DR

This paper presents a fully integrated silicon photonic decoder for polarization-encoding quantum key distribution (QKD), enabling on-chip polarization state analysis and automatic compensation for fiber-induced polarization drift. The chip uses a polarization-to-path conversion method via a polarization splitter-rotator and achieves a 100-km fiber QKD secret key rate of 240 bps with a 0.56% quantum bit error rate over 10 hours of continuous operation.

ABSTRACT

Silicon-based polarization-encoding quantum key distribution (QKD) has been widely studied, owing to its low cost and robustness. However, prior studies have utilized off-chip devices to demodulate the quantum states or perform polarization compensation, given the difficulty of fabricating polarized independent components on the chip. In this paper, we propose a fully chip-based decoder for polarization-encoding QKD. The chip realizes a polarization state analyzer and compensates for the BB84 protocol without requiring additional hardware. It is based on a polarization-to-path conversion method that uses a polarization splitter-rotator. The chip was fabricated using a standard silicon photonics foundry; it has a compact design and is suitable for mass production. In the experimental stability test, an average quantum bit error rate of $0.56\%$ was achieved through continuous operation for 10 h without any polarization feedback. Furthermore, using the developed feedback algorithm, the chip enabled the automatic compensation of the fiber polarization drift, which was emulated by a random fiber polarization scrambler. In the case of the QKD demonstration, we obtained a finite-key secret rate of 240 bps over a fiber spool of 100 km. This study represents an important step toward the integrated, practical, and large-scale deployment of QKD systems.

Motivation & Objective

• To eliminate the need for off-chip components in polarization-encoding QKD by developing a fully chip-integrated decoder.
• To enable on-chip polarization state analysis and automatic compensation for fiber-induced polarization drift.
• To demonstrate a compact, mass-producible silicon photonic solution compatible with standard CMOS fabrication processes.
• To achieve stable, low-error QKD performance over long-haul fiber links without external polarization feedback.

Proposed method

• The decoder uses a polarization-to-path conversion method based on a polarization splitter-rotator (PSR) to map polarization states to distinct output paths.
• The chip integrates a polarization splitter-rotator, thermal phase shifters, multimode interferometers (MMI), and variable optical attenuators (VOA) on a standard silicon photonics platform.
• A feedback algorithm dynamically adjusts the voltages on thermal phase shifters (PS1–PS4) to compensate for polarization drift by minimizing error rates in X and Z bases.
• The feedback algorithm uses finite-difference approximation to estimate partial derivatives of error rates with respect to voltage changes, enabling gradient-based correction.
• The system continuously monitors error rates in the X and Z bases and applies corrective voltage updates in a cyclic, iterative manner to maintain low QBER.
• The chip is packaged in a compact 3.95 × 2.19 × 0.90 cm³ form factor and fabricated using a standard silicon photonics foundry process.

Experimental results

Research questions

• RQ1 Can a fully chip-integrated decoder replace off-chip polarization demodulation and compensation in polarization-encoding QKD systems?
• RQ2 What is the performance of a silicon photonic decoder in maintaining low quantum bit error rate (QBER) over extended operation without external feedback?
• RQ3 Can an on-chip feedback algorithm effectively compensate for random fiber polarization drift in real time?
• RQ4 What is the achievable secret key rate of a fully integrated silicon-based QKD system over long-haul fiber links?

Key findings

• The chip achieved an average quantum bit error rate (QBER) of 0.56% over 10 hours of continuous operation without any external polarization feedback.
• The developed feedback algorithm successfully compensated for fiber polarization drift emulated by a random fiber polarization scrambler, maintaining stable QKD performance.
• Over a 100-km fiber spool, the system demonstrated a finite-key secret key rate of 240 bps with a QBER of 2.61×10⁻².
• The chip’s compact size (1.6 × 1.7 mm²) and compatibility with standard silicon photonics fabrication enable mass production and integration into practical QKD systems.
• The system maintained a secret key rate of 4.94×10⁴ bps at 25 km, decreasing to 2.40×10² bps at 100 km, demonstrating scalability over long distances.
• The method achieved high stability and low error rates across all tested fiber lengths, with the QBER increasing from 0.53% at 25 km to 2.61% at 100 km, remaining within acceptable thresholds for secure key generation.