Frequency drift corrected ultra-stable laser through phase-coherent fiber producing a quantum channel

Phase coherent fibers (PCF) are essential to distribute nearly monochromatic photons, ultra-stable in their frequency and phases, which have demanding requirements for state-of-the-art networked experiments, quantum as well as very high-speed communications. We report the development of a novel system that produces PCF links, also actively corrects the unavoidable slow frequency drift of the source laser. The PCF follows white phase noise limited $σ_o \times τ^{-1}$ stability behavior having $σ_o$ values $1.9(2) \times 10^{-16}$ and $2.6(1) \times 10^{-16}$ for a 3.3 km field-deployed and 71 km spool fibers, respectively, with up to 47.5 dB suppression of the phase noise compared to a normal fiber. Additionally, the system is featured to correct the source laser’s 33.8 mHz/s frequency drift to as low as $\simeq 0.05$ mHz/s. Therefore, this all-in-one solution producing a quantum link can potentially enhance the effectiveness of the twin field quantum key distribution (TF-QKD) by nearly a 73-fold reduction of the QBER that arises from using unstabilized fiber links, as well as relaxes the laser frequency drift correction constraints by severalfold.

💡 Research Summary

The paper presents a comprehensive solution that simultaneously stabilizes an ultra‑stable laser’s slow frequency drift and suppresses phase noise in a phase‑coherent fiber (PCF) link, thereby delivering a quantum‑grade optical channel. The experimental platform uses a 1550 nm ultra‑stable laser with ≈1 Hz linewidth. The laser output is split 90:10; the 10 % branch serves as a near‑unperturbed reference (out‑of‑loop beat), while the 90 % branch traverses two acousto‑optic modulators (AOM1 and AOM2) and the transmission fiber (either a 3.3 km field‑deployed link or a 71 km laboratory spool). Light reflected from a Faraday‑mirror‑terminated path interferes with the original laser, producing an in‑loop beat signal that contains three contributions: the laser’s frequency drift Δf_dL, the fiber‑induced phase noise f_nF, and the correction frequencies applied to the AOMs (f_cL for laser drift, f_cF for fiber noise).

The in‑loop beat is digitized by a 125 MSa/s ADC and mixed with a locally generated oscillator (f_LO) derived from a GPS‑disciplined rubidium clock. After low‑pass filtering (≤113 kHz), the residual signal equals –