Pulsed optically pumped atomic clock with a medium- to long-term frequency stability of 10-15.

Herein, we report a significant improvement in the medium- to long-term frequency stability of our pulsed optically pumped (POP) vapor-cell rubidium clock. Such an achievement is established with the better control of our system and the environment. An integrated optical module, including a distributed Bragg reflector laser and an acousto-optic modulator, is developed to improve the stability of the laser. The physics package is sealed in a vacuum chamber with a vacuum of 4 × 10-4 Pa to significantly reduce the impacts of the barometric effect. An AC-driven heater is placed much closer to the cell to enable a better temperature control. The resolution of the servo control voltage is also optimized. With all these improvements, a frequency stability of 4.7 × 10-15 at 104 s in terms of the Allan deviation is obtained. We also estimate the main noise sources that limit the frequency stability of the POP atomic clock.

Note: Pulsed optically pumped atomic clock based on a paraffin-coated cell.

We report on the implementation of a pulsed optically pumped atomic clock based on a paraffin-coated cell. The relaxation times are measured, with the longitudinal relaxation time, T1 = 9.7 ± 0.4 ms, and the transversal relaxation time, T2 = 0.40 ± 0.03 ms. We demonstrated that the measured frequency stability of the clock is 3.9 × 10-13 τ-1/2 (1 s ≤ τ ≤ 100 s) and reaches a value of 3.1 × 10-14 for τ = 1000 s, where τ is the averaging time. This is an unprecedented result for a paraffin-coated vapor cell clock, and it makes significant contributions toward improving the performance of the wall-coated vapor cell atomic clock.

Frequency stability of a pulsed optically pumped atomic clock with narrow Ramsey linewidth.

In general, the linewidth of the Ramsey central fringe (RCF) is equal to 1/(2T), where T is the Ramsey free-evolution time. We demonstrate that the RCF linewidth of a pulsed optically pumped (POP) atomic clock with orthogonal polarization detection based on the magneto-optical rotation effect can be narrowed down to 1/(4T). The Allan deviation of the POP atomic clock decreases from 2.4×10-13τ-1/2 to 1.4×10-13τ-1/2. This corresponds to an improvement in the frequency stability by about 60%. We also estimate the main noise sources that limit the short-term frequency stability of the POP atomic clock.

Pulsed optically pumped atomic clock with zero-dead-time.

By alternatively operating two pulsed optically pumped (POP) atomic clocks, the dead time in a single clock can be eliminated, and the local oscillator can be discriminated continuously. A POP atomic clock with a zero-dead-time (ZDT) method is then insensitive to the microwave phase noise. From τ = 0.01 to 1 s, the Allan deviation of the ZDT-POP clock is reduced as nearly τ-1, which is significantly faster than τ-1/2 of a conventional clock. During 1-40 s, the Allan deviation returns to τ-1/2. Moreover, the frequency stability of the ZDT-POP clock is improved by one order of magnitude compared with that of the conventional POP clock. We also analyze the main factors that limit the short-term frequency stability of the POP atomic clock.

High-contrast and narrow-linewidth resonant profile for continuous operation atomic clock.

We report a high-contrast and narrow-linewidth resonant line profile by measuring the magneto-optical rotation of the transmitted light in a forward-scattering arrangement. We also report the splitting of the transmitted line profile at a strong microwave excitation. This profile may provide a good competitive scheme for the passive Rb frequency standard.

Detection of ultrahigh resonance contrast in vapor-cell atomic clocks.

We propose and demonstrate a novel detection scheme of clock signals and obtain an ultrahigh resonance contrast up to 90%, which leads to the remarkable improvement of the precision of the signal-to-noise ratio. The frequency stability in terms of Allan deviation of the proposed detection scheme is improved by an order of magnitude under equivalent conditions.

Numerical modeling of optical levitation and trapping of the "stuck" particles with a pulsed optical tweezers.

We present the theoretical analysis and the numerical modeling of optical levitation and trapping of the stuck particles with a pulsed optical tweezers. In our model, a pulsed laser was used to generate a large gradient force within a short duration that overcame the adhesive interaction between the stuck particles and the surface; and then a low power continuous-wave(cw) laser was used to capture the levitated particle. We describe the gradient force generated by the pulsed optical tweezers and model the binding interaction between the stuck beads and glass surface by the dominative van der Waals force with a randomly distributed binding strength. We numerically calculate the single pulse levitation efficiency for polystyrene beads as the function of the pulse energy, the axial displacement from the surface to the pulsed laser focus and the pulse duration. The result of our numerical modeling is qualitatively consistent with the experimental result.