Compact two-photon optical clock with a long-term stability of 8.1 × 10-15 without linear drift removal

Area of Science:

• Atomic, Molecular, and Optical Physics
• Metrology and Measurement Science
• Laser Physics and Technology

Background:

• Atomic clocks are crucial for navigation, communication, and fundamental science.
• Developing compact and stable atomic clocks is essential for space-borne applications.
• Two-photon transitions offer advantages for atomic clock design, including reduced sensitivity to magnetic fields and first-order Doppler shifts.

Purpose of the Study:

• To stabilize a 1556.2 nm fiber laser to the 5S1/2 → 5D5/2 two-photon transition in 87Rb.
• To evaluate the frequency stability of a compact rubidium two-photon optical clock.
• To analyze systematic effects limiting clock performance for potential space applications.

Main Methods:

• Utilized a compact physics package for laser stabilization to the 87Rb two-photon transition.
• Employed a commercial fiber frequency comb to generate a 200 MHz microwave output.
• Performed detailed analysis of systematic effects influencing short-term and long-term stability.

Main Results:

• Achieved a fractional frequency stability of 2.2 × 10-13 at τ = 1 s and 8.1 × 10-15 at τ = 4000 s without linear drift removal.
• Identified shot noise as the primary limitation for short-term stability.
• Identified ac Stark shift as the primary limitation for long-term stability.

Conclusions:

• The stabilized fiber laser and rubidium two-photon transition form a highly stable compact atomic clock.
• The rubidium two-photon optical clock demonstrates excellent performance suitable for next-generation space-borne applications.
• Further optimization can mitigate systematic effects for even greater stability.