Session P4b: Timekeeping and Quantum Networking

Quantum-Enhanced Phase-Locking of a Metropolitan Scale Deployed Fiber Link with Faint Light
M.V. Jabir, N. Fajar R. Annafianto, National Institute of Standards and Technology (NIST); I. A. Burenkov, NIST & Joint Quantum Institute & University of Maryland, College Park; A. Battou, NIST; S. V. Polyakov, NIST & Department of Physics, University of Maryland, College Park
Location: Seaview A/B
Date/Time: Thursday, Jan. 30, 11:26 a.m.

Coherence is a crucial aspect of quantum systems, especially for quantum communication, where preserving the phase reference in photonic quantum states is vital for many protocols. Optical fiber serves as an effective medium for photonic quantum networks, allowing light to travel long distances with high resistance to environmental noise and moderate loss. However, these long fiber links can experience optical path length instabilities caused by temperature fluctuations and other environmental factors. Homodyne or heterodyne detection with classical light is typically employed to stabilize the phase of optical signals propagating in a fiber link. However, relying on classical signals for phase stabilization can be problematic in multiplexed quantum networks, where both quantum and classical signals coexist in the same fiber. Classical signals can introduce noise because of the Raman scattering into the quantum channel, imposing strict power limitations [1]. Therefore, keeping all signal power at low, few-photon levels is essential to prevent cross-talk between quantum and classical channels. This necessitates phase stabilization using faint coherent states, which allows for the coexistence of different types of signals in a scalable quantum network.
We propose a phase stabilization method that utilizes single-photon detection and operates with only a few hundred thousand photons per second. This marks the first implementation of a quantum-enhanced phase tracking and stabilization algorithm for long-distance fiber links (over 100 km). By leveraging optical displacement and photon counting, we achieved a timing jitter below 0.07 fs using just 650,000 photons per second and a 50% on/off duty cycle to accommodate other faint light signals like photonic qubits. This single-photon-compatible interferometric stabilization enhances the potential for all quantum communication and entanglement distribution protocols, paving the way for rapid advancements in quantum networks. The time multiplexing inherent in our stabilization protocol ensures compatibility with a wide range of quantum and quantum-inspired networking protocols, including quantum receivers for classical communication.
Our approach employs a strongly attenuated coherent signal from an ultrastable laser operating in the C-band (1550 nm) to stabilize the long optical fiber link through single-photon detection. The signal is divided using a 99:1 fiber beam splitter, where 99 % is directed to the fiber link, and 1 % serves as a local oscillator. Acousto-optic modulators (AOMs) in both arms of the interferometer modulate the optical phase, controlled by a field-programmable gate array (FPGA). The signals are then recombined at another 99:1 fiber beam splitter, with the output sent to a superconducting nanowire single photon detector (SNSPD). Each photon detection yields an electrical pulse sent to the FPGA for phase estimation and tracking. The local oscillator is set to displace beamsplitter output to the vacuum state, and the FPGA’s adaptive algorithm computes feedback for the AOMs to maintain this state. We transmit the phase stabilization signal during half of the cycle, while the other half is reserved for the quantum signal. The SNSPD output informs the feedback algorithm, ensuring that the stabilization signal remains low, preventing saturation, and allowing effective detection of the quantum payload.
We analyzed phase noise across fiber spools of varying lengths, finding that longer fibers exhibit increased phase noise. Using a laser with a coherence length much greater than the most extended link, we minimized phase noise from the source. We report integrated phase noise below 0.09 radians and achieved stabilization with residual instability below 20 mrad for fibers up to 100 km and below 50 mrad for a 120 km deployed fiber loop over 1 second. This stability was achieved using attenuated laser light averaging only 650,000 photons per second at the receiver, leveraging a quantum-enhanced phase estimation method that demonstrated a verifiable advantage in Fisher information over classical methods. Our system exhibited an average quantum advantage of up to 2 dB compared to the best classical phase estimation techniques.
This work supports phase-sensitive measurement, communication, and entanglement distribution protocols, including those for quantum repeaters. Until now, the quantum networking community had avoided phase-sensitive protocols even when that meant significant complications in practical use, including unnecessary consumption of entanglement and nestled transduction steps. With our demonstration that stable phase coexistence in long fiber links is achievable, future protocols can be streamlined, accelerating progress in the field.
[1] I. A. Burenkov, A. Semionov, Hala, T. Gerrits, A. Rahmouni, D. Anand, Y.-S. Li-Baboud, O. Slattery, A. Battou, and S. V. Polyakov, “Synchronization and coexistence in quantum networks,” Opt. Express 31, 11431–11446 (2023).