We proposed a scheme to implement coherent population trapping (CPT) atomic clock based on the transient CPT phenomenon. We proved that the transient transmitted laser power in a typical Λ system near CPT resonance features as a damping oscillation. Also, the oscillating frequency is exactly equal to the frequency detuning from the atomic hyperfine splitting. Therefore, we can directly measure the frequency detuning and then compensated to the output frequency of microwave oscillator to get the standard frequency. By this method, we can further simplify the structure of CPT atomic clock, and make it easier to be digitized and miniaturized.

An ultrastable optical reference cavity with re-entrant fused silica mirrors and a ULE spacer structure is designed through finite element analysis. The designed cavity has a low thermal noise limit of 1 × 10 −16 and a flexible zero crossing temperature of the effective coefficient of thermal expansion (CTE). The CTE zero crossing temperature difference between a composite cavity and a pure ULE cavity can be tuned from −10 • C to 23• C, which enables operation of the designed reference cavity near room temperature without worrying about the CTE zero crossing temperature of the ULE spacer. The design can be applied to cavities with different lengths. Vibration immunity of the cavity is also achieved through structure optimization. Lett. 82, 3799-3802 (1999

We studied the relationship between pressure ratio of the buffer gases (argon and neon) and the rate of coherent population trapping resonance frequency shift with cell temperature in Rb85. We found that when the total pressure of the buffer gases varies within the range of 5–15kPa, the frequency shift rate varies along a bell shaped curve. Every curve crossed the horizontal axis at two points that are roughly symmetrical with respect to the midpoint at 1:1. This allows us to minimize the rate of frequency shift by adjusting the pressure ratio of the buffer gases to these two points.

Higher-order mode locking has been proposed to reduce the thermal noise limit of reference cavities. By locking a laser to the HG mode of a 10-cm long all ultra-low expansion (ULE) cavity and measuring its performance with the three-cornered-hat method among three independently stabilized lasers, we demonstrate a thermal-noise-limited performance of a fractional frequency instability of 4.9×10. The results match the theoretical models with higher-order optical modes. The achieved laser instability improves the all ULE short cavity results to a new low level.

We demonstrate thermal noise limited and shot noise limited performance of ultra-stable diode laser systems. The measured heterodyne beat linewidth between such two independent diode lasers reaches 0.74 Hz. The frequency instability of one single laser approaches 1.0 × 10 for averaging time between 0.3 s and 10 s, which is close to the thermal noise limit of the reference cavity. Taking advantage of these two ultra-stable laser systems, we systematically investigate the ultimate electrical noise contributions, and derive expressions for the closed-loop spectral density of laser frequency noise. The measured power spectral density of the beat frequency is compared with the theoretically calculated closed-loop spectral density of the laser frequency noise, and they agree very well. It illustrates the power and generality of the derived closed-loop spectral density formula of the laser frequency noise. Our result demonstrates that a 10 level locking in a wide frequency range is feasible with careful design.

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Al + ions optical clock is a very promising optical frequency standard candidate due to its extremely small blackbody radiation shift. It has been successfully demonstrated with indirect cooled, quantum-logic-based spectroscopy technique. Its accuracy is limited by second-order Doppler shift, and its stability is limited by the number of ions that can be probed in quantum logic processing.We propose a direct laser cooling scheme of Al + ions optical clocks where both the stability and accuracy of the clocks are greatly improved. In the proposed scheme, two Al + ions traps are utilized. The first trap is used to trap a large number of Al + ions to improve the stability of the clock laser, while the second trap is used to trap a single Al + ions to provide the ultimate accuracy.Both traps are cooled with a continuous wave 167 nm laser. The expected clock laser stability can reach 9.0 × 10 −17 / √ τ . For the second trap, in addition to 167 nm laser Doppler cooling, a second stage pulsed 234 nm two-photon cooling laser is utilized to further improve the accuracy of the clock laser. The total systematic uncertainty can be reduced to about 1 × 10 −18 . The proposed Al + ions optical clock has the potential to become the most accurate and stable optical clock.

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