Frequency-bin encoding represents a promising yet underexplored approach for Quantum Key Distribution (QKD) despite its compatibility with existing fiber-optic infrastructure. This paper presents the first complete demonstration of entanglement-based QKD using frequency-bin encoding, addressing previous limitations in this field.
The researchers implemented the BBM92 protocol using photon pairs generated by two independent, high-finesse ring resonators on a silicon photonic chip. Their setup achieved both a high pair generation rate (27 MHz/mW²) and a small frequency-bin spacing (15 GHz), making it compatible with standard electro-optic modulation devices. The system employs passive basis selection and simultaneously records all sixteen projective measurements using six superconducting detectors.
A critical discovery was the sensitivity of frequency-bin encoding to random phase noise caused by environmental thermal fluctuations, which significantly impairs transmission beyond a few kilometers of fiber. To overcome this challenge, the team developed a real-time phase compensation system that maintains a low and stable Quantum Bit Error Rate (QBER). Through systematic investigation of phase noise, they correlated phase drift between frequency-bins with variations in the fiber’s optical path and temperature.
Using this adaptive phase rotation technique for the measurement basis, they achieved stable transmission over fiber spools extending up to 26 kilometers with a secure key rate of at least 4.5 bits per second. This represents a significant advancement in practical quantum communication technology.
The work combines several key innovations: high-brightness integrated photon pair sources, wide emission bandwidth, and the ability to generate entangled photons across multiple degrees of freedom. Unlike previous studies that lacked random basis selection, simultaneous detection of all measurement outcomes, or proper RF clock distribution between remote users, this implementation incorporates all essential elements for implementing frequency-bin QKD in telecom networks.
This research introduces frequency as a viable degree of freedom for entanglement-based QKD protocols in existing telecommunication infrastructure, potentially expanding the toolkit available for secure quantum communication networks while leveraging the intrinsic benefits of frequency encoding for high-dimensional quantum information processing.
Reference: Tagliavacche, N., Borghi, M., Guarda, G. et al. Frequency-bin entanglement-based Quantum Key Distribution. npj Quantum Inf 11, 60 (2025). doi:10.1038/s41534-025-00991-5
