Frequency reproducibility of solid-state thorium-229 nuclear clocks

Area of Science:

• Nuclear physics
• Metrology
• Solid-state physics

Background:

• Solid-state thorium-229 (229Th) nuclear clocks are emerging as powerful tools for precision metrology and fundamental physics research.
• Their low sensitivity to environmental factors and potential for high emitter density in solid-state crystals offer advantages over current atomic clocks.
• Solid-state systems requiring only simple thermal control are crucial for developing compact, field-deployable clocks.

Purpose of the Study:

• To explore and characterize the frequency reproducibility of the 229Th:CaF2 nuclear clock transition.
• To investigate the impact of doping concentration, temperature, and time on the transition's linewidth and center frequency.
• To establish a foundation for controlling coherent nuclear excitation of 229Th in solid-state hosts.

Main Methods:

• Measurement of the transition linewidth and center frequency under varying doping concentrations, temperatures, and over time.
• Characterization of the concentration-dependent inhomogeneous linewidth, identifying limitations due to intrinsic host crystal properties.
• Determination of an optimal working temperature (196(5) K) where first-order thermal sensitivity vanishes.

Main Results:

• The concentration-dependent inhomogeneous linewidth of the nuclear transition was reported, limited by the CaF2 crystal properties.
• An optimal working temperature of 196(5) K was identified, at which the first-order thermal sensitivity becomes negligible.
• At 195 K, a frequency reproducibility of 220 Hz (1.1 × 10^-13 fractional uncertainty) was achieved for two differently doped 229Th:CaF2 crystals over seven months.

Conclusions:

• The study provides foundational data for understanding and controlling the coherent nuclear excitation of 229Th in solid-state hosts.
• The identified optimal temperature and achieved reproducibility pave the way for developing highly stable, compact nuclear clocks.
• These advancements hold potential for applications in constraining temporal variations of fundamental constants and enhancing precision metrology.