All-optical link for direct comparison of distant optical clocks

Area of Science:

• Metrology
• Quantum Optics
• Optical Engineering

Background:

• Ultra-stable optical clocks are crucial for fundamental physics and metrology.
• Remote comparison of these clocks is challenging due to signal degradation over long distances.
• Existing methods often struggle with phase noise and polarization instability in fiber links.

Purpose of the Study:

• To develop and demonstrate an all-optical link system for remote, high-precision comparison of distant ultra-stable optical clocks.
• To overcome the limitations of phase noise and polarization fluctuations in fiber optic transmission.
• To enable reliable and stable intercomparison of optical frequency standards.

Main Methods:

• An optical carrier transfer system utilizing a fiber interferometer was developed to compensate for phase noise.
• An active polarization control system was implemented to maintain light polarization integrity.
• The system was tested over a 90-km fiber link for transfer stability measurements.
• The system was used for direct comparison of two distant Strontium (Sr) lattice clocks over a 60-km urban fiber link.

Main Results:

• Achieved transfer stabilities of 2 × 10⁻¹⁵ at 1 second and 4 × 10⁻¹⁸ at 1000 seconds over a 90-km link.
• Instabilities of the all-optical link system, including erbium-doped fiber amplifiers (EDFAs), were below 2 × 10⁻¹⁵ at 1 second and 7 × 10⁻¹⁷ at 1000 seconds.
• Successfully demonstrated direct comparison of two distant (87)Sr lattice clocks via a 60-km urban fiber link.

Conclusions:

• The developed all-optical link system provides a stable and reliable method for remote optical clock comparisons.
• The system's high transfer stability is essential for measuring the reproducibility of advanced optical frequency standards.
• This technique is a critical advancement for distributed metrology and fundamental physics experiments.