Entanglement Distribution over a Polarization-Stabilized Aerial Fiber

TL;DR

This work tackles the challenge of distributing polarization-entangled photons over deployed fiber, including a demanding 62-km partially aerial link, where time-varying polarization transformations must be actively compensated. It implements a pair of Automated Polarization Compensation (APC) modules to stabilize the fiber, enabling time-multiplexed stabilization and entanglement distribution between distant nodes (NIST and UMD) with a synchronized Electrical-Optical-Electrical timing setup. The results show a distributed entangled-pair rate of about 1500 per second and a time-averaged CHSH parameter of $S=2.34±0.37$, with uptime of 92.8% when stabilization is active and an overhead of 7.2% for stabilization, improving to $S=2.39±0.14$ when excluding timed-out compensation data. This demonstrates the feasibility of polarization-entangled distribution over challenging aerial-fiber channels, marking a significant step toward practical quantum networks in real-world fiber environments.

Abstract

We experimentally demonstrate the distribution of polarization-entangled photons across a 62-km, partially-aerial fiber. With polarization stabilization applied to the fiber link, we achieve a photon pair rate of approximately 1500 per second and observe a CHSH inequality violation with S=2.34.

Figures (5)

• Figure 1: Schematic of the entanglement distribution experiment over a 62 km, partially aerial fiber. Idler photons from the entangled photon pairs are transmitted across the single-photon fiber link, which is polarization-stabilized with a pair of automated polarization compensation modules. The signal photons are detected locally at NIST, and a parallel fiber from the same TX/RX pair transmits the timing information of the signal photons to UMD. The signal-idler coincidence events are recorded with a time-stamping unit located at UMD.
• Figure 2: (a) Stokes parameters of probe light measured at the output of the fiber link. (b) Fidelity of the output SOP measured over a 1-minute interval (shaded region in (a)). Fidelity is calculated relative to the SOP at the beginning of the interval ($\text{Fidelity}=1$ at $t=0$).
• Figure 3: (a) Simplified schematic illustrating the working principle of the APC injector/compensator pair. (b) Sequence of six polarization reference states generated by the APC injector (red) and the corresponding states measured by the APC compensator after fiber transmission (blue). (c) Exemplary timing sequence of the APC compensator trigger voltage level.
• Figure 4: (a) Exemplary polarization-correlation fringes measured with active polarization stabilization. For the measurement with NIST analyzer set to 90$^\circ$ (V), the solid blue curve shows the fitted result including all data points, while the dashed blue curve shows the corrected fit obtained by excluding the outlier marked by the blue arrow. (b) Polarization compensation time spent by the APC before recording each data points in (a). The prolonged compensation time (41 seconds) is likely caused by a temporary, volatile change in the fiber environment.
• Figure 5: (a) Minimum fidelity of the received APC reference states and $S$-parameter measured over 16 hours without polarization stabilization. (b) Minimum reference state fidelity, S-parameter value and polarization compensation time recorded over 24 hours with polarization stabilization enabled.