Extendable optical phase synchronization of remote and independent quantum network nodes over deployed fibers

TL;DR

This paper presents a robust, extendable optical phase synchronization scheme for remote quantum network nodes operating over deployed telecom fibers, enabling stable entanglement generation in metropolitan quantum networks.

Contribution

It introduces a multi-task stabilization scheme for optical phase synchronization in telecom-band quantum networks, addressing challenges like delays and power limitations.

Findings

Achieved a short-term phase stability with a standard deviation of about 30 degrees.

Demonstrated stable optical phase over 10 hours with degrees-level accuracy.

Enabled a key technology for NV-center based quantum links.

Abstract

Entanglement generation between remote qubit systems is the central tasks for quantum communication. Future quantum networks will have to be compatible with low-loss telecom bands and operate with large separation between qubit nodes. Single-click heralding schemes can be used to increase entanglement rates at the cost of needing an optically phase-synchronized architecture. In this paper we present such a phase synchronization scheme for a metropolitan quantum network, operating in the low-loss telecom L-band. To overcome various challenges such as communication delays and optical power limitations, the scheme consists of multiple tasks that are individually stabilized. We characterize each task, identify the main noise sources, motivate the design choices and describe the synchronization schemes. The performance of each of the tasks is quantified by a transfer-function measurement that investigates the frequency response and feedback bandwidth. Finally we investigate the resulting optical phase stability of the fully deployed system over a continuous period of 10 hours, reporting a short-term stability standard deviation of ${\sigma} \approx 30 \deg$ and a long-term stability of the average optical phase to within a few degrees. The scheme presented served as a key enabling technology for an NV-center based metropolitan quantum link. This scheme is of interest for other quantum network platforms that benefit from an extendable and telecom compatible phase synchronization solution.