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Optics Express

Synchronization and coexistence in quantum networks

Ivan A. Burenkov, Alexandra Semionov, Hala Hala, Thomas Gerrits, Anouar Rahmouni, DJ Anand, Ya-Shian Li-Baboud, Oliver Slattery, Abdella Battou, Sergey V. Polyakov

Published Mar 1, 2023• 56 citations
Publisher

Abstract

We investigate the coexistence of clock synchronization protocols with quantum signals in a common single-mode optical fiber. By measuring optical noise between 1500 nm to 1620 nm we demonstrate a potential for up to 100 quantum, 100 GHz wide channels coexisting with the classical synchronization signals. Both "White Rabbit" and pulsed laser-based synchronization protocols were characterized and compared. We establish a theoretical limit of the fiber link length for coexisting quantum and classical channels. The maximal fiber length is below approximately 100 km for off-the-shelf optical transceivers and can be significantly improved by taking advantage of quantum receivers.

DOI
10.1364/oe.480486

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We investigate the coexistence of clock synchronization protocols with quantum signals in a common single-mode optical fiber. By measuring optical noise between 1500 nm to 1620 nm we demonstrate a potential for up to 100 quantum, 100 GHz wide channels coexisting with the classical synchronization signals. Both "White Rabbit" and pulsed laser-based synchronization protocols were characterized and compared. We establish a theoretical limit of the fiber link length for coexisting quantum and classical channels. The maximal fiber length is below approximately 100 km for off-the-shelf optical transceivers and can be significantly improved by taking advantage of quantum receivers.
We investigate the coexistence of clock synchronization protocols with quantum signals propagating in a common optical single-mode fiber channel. By measuring optical forward and backward scattering noise in a range from 1510 nm to 1620 nm we demonstrate a potential for up to 100 quantum channels each with 100 GHz bandwidth coexisting with the classical synchronization signals using off-the-shelf optical components. Both “White Rabbit'' and a pulsed laser-based synchronization were characterized and compared.
Quantum Key Distribution (QKD) protocols offer top-tier secure communication but face challenges with noise, particularly when quantum and classical channels coexist. In fiber-optic systems, strong classical signals for data and synchronization create noise through scattering, with inelastic scattering being the main contributor. This broad-spectrum noise can overlap with quantum communication wavelengths, leading to errors and deterioration of the quantum signal. Our study examines the theoretical coexistence limit for a 1500 nm classical synchronization signal from a White Rabbit switch and a 1320 nm quantum signal from an S-Fifteen Instruments source, identifying the maximum fiber length where the quantum signal remains clear of noise. We chose the BB84 protocol for our QKD implementation, which we plan to first test in a laboratory setting without an intermediate node. In the future, adding a node (often referred to as Charlie) would improve scalability and help manage synchronization delays.
The development of prototype metropolitan-scale quantum networks is underway and entails transmitting quantum information via single photons through deployed optical fibers spanning several tens of kilometers. The major challenges in building metropolitan-scale quantum networks are compensation for polarization fluctuation, high-precision clock synchronization, and compensation for cumulative transmission time fluctuations. One approach addressing these challenges is to copropagate classical probe signals in the same fiber as the quantum signal. Thus, both signals experience the same conditions, and the changes of the fiber can therefore be monitored and compensated. Here, we demonstrate the distribution of polarization-entangled quantum signals copropagating with the White Rabbit precision time protocol classical signals in the same single-core fiber strand at metropolitan-scale distances. Our results demonstrate the feasibility of this quantum-classical coexistence by achieving high-fidelity entanglement distribution between nodes separated by 100 km of optical fiber. This advancement is a significant step towards the practical implementation of robust and efficient metropolitan-scale quantum networks.