A 44-cm³ physics package for the high-performance pulsed optically pumped atomic clock.

The article focuses on the design and performance evaluation of a miniaturized pulsed optically pumped (POP) rubidium (Rb) atomic clock physics package. This compact package, with a volume of about 44 cm³, incorporates a magnetron microwave cavity resonating in a TE011-like mode and a vapor cell of 1.3 cm internal diameter and 1.4 cm length. Experimental measurements at 333 K show population and coherence relaxation times of approximately 3.16 ms and 2.97 ms, respectively, consistent with theoretical calculations that identify spin exchange, wall collisions, and buffer-gas collisions as key relaxation mechanisms. The resulting POP atomic clock achieves a short-term frequency stability of 4.8 × 10⁻¹³ τ⁻¹/² (for averaging times between 1 and 100 seconds), primarily limited by the relative intensity noise of the laser system. The study highlights the challenges and trade-offs in miniaturizing atomic clocks while maintaining frequency performance, and suggests future improvements targeting laser noise reduction, temperature sensitivity, and further size and power consumption reductions for practical outdoor applications.