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

The pulsed optically pumped (POP) atomic clock has demonstrated unexpected performance in terms of frequency stability and drift. However, it remains a huge challenge to make this type of atomic clock more compact. Herein, we report the design of a miniaturized physics package, which is equipped with a magnetron microwave cavity holding a vapor cell of 1.3 cm internal diameter. The Zeeman transition spectrum reveals that the microwave cavity resonates in TE011-like mode. Based on a low-noise testbed, we also quantitatively analyze the relaxation time, linewidth, and noise sources of the resulting POP atomic clock. The population and coherence relaxation time are measured to be 3.16(0.16) and 2.97(0.03) ms under the temperature of 333 K, which are compatible well with the theoretical calculation. The Ramsey signal shows a contrast of 35% and a linewidth of 192 Hz. The total volume of the physics package is about 44 cm3, including a layer of magnetic shielding. The short-term frequency stability is measured to be 4.8 × 10-13τ-1/2 (where τ is the averaging time), which is mainly limited by the relative intensity noise of the laser system.