Realization of a pulsed optically pumped Rb clock with a frequency stability below [Formula: see text].

We present the frequency stability performances of a vapor cell Rb clock based on the pulsed optically pumping (POP) technique. The clock has been developed in the frame of a collaboration between INRIM and Leonardo SpA, aiming to realize a space-qualified POP frequency standard. The results here reported were obtained with an engineered physics package, specifically designed for space applications, joint to laboratory-grade optics and electronics. The measured frequency stability expressed in terms of Allan deviation is [Formula: see text] at 1s and achieves the value of [Formula: see text] for integration times of 40000 s (drift removed). This is, to our knowledge, a record result for a vapor-cell frequency standard. In the paper, we show that in order to get this result, a careful stabilization of microwave and laser pulses is required.

Kr-Based Buffer Gas for Rb Vapor-Cell Clocks.

Optically pumped Rb vapor cell clocks are by far the most used devices for timekeeping in all ground and space applications. The compactness and the robustness of this technology make Rb clocks extremely well fit to a large number of applications, including GNSS, telecommunication, and network synchronization. Many efforts are devoted to improve the stability of Rb clocks and reduce their environmental sensitivity. In this article, we investigate the use of a novel mixture of buffer gas based on Kr and N2, capable of reducing by more than one order of magnitude the barometric and temperature sensitivities of the clock, with possible improvement of their long-term stability.

Loaded Microwave Cavity for Compact Vapor-Cell Clocks.

Vapor-cell devices based on microwave interrogation provide a stable frequency reference with a compact and robust setup. Further miniaturization must focus on optimizing the physics package, containing the microwave cavity and atomic reservoir. In this article, we present a compact cavity-cell assembly based on a dielectric-loaded cylindrical resonator. The loaded cavity resonating at 6.83 GHz has an external volume of only 35 cm3 and accommodates a vapor cell with 0.9-cm3 inner volume. The proposed design aims at strongly reducing the core of the atomic clock, maintaining, at the same time, high-performing short-term stability ( σy(τ) ≤ 5×10-13 τ-1/2 standard Allan deviation). The proposed structure is characterized in terms of microwave field uniformity and atom-field coupling with the aid of finite-element calculations. The thermal sensitivity is also analyzed and experimentally characterized. We present preliminary spectroscopy results by integrating the compact cavity within a rubidium clock setup based on the pulsed optically pumping technique. The obtained clock signals are compatible with the targeted performances. The loaded-cavity approach is, thus, a viable design option for miniaturized microwave clocks.

Intensity Detection Noise in Pulsed Vapor-Cell Frequency Standards.

Laser intensity noise is currently recognized as one of the main factors limiting the short-term stability of vapor-cell clocks. In this article, we propose a signal theory approach to estimate the contribution of the laser intensity fluctuations to the short-term stability of vapor-cell clocks working in a pulsed regime. Specifically, given the laser intensity noise spectrum, an analytical expression is derived to evaluate its impact on the clock Allan deviation (ADEV). The theory has been tested for two intensity noise spectra of interest in clock applications: white frequency noise and flicker noise. The predicted results turn out to be in good agreement with experiments performed with a prototype of pulsed optically pumped Rb-cell clock, and can be extended to other compact clocks.

Reducing Cavity-Pulling Shift in Ramsey-Operated Compact Clocks.

We describe a method to stabilize the amplitude of the interrogating microwave field in compact atomic clocks working in a Ramsey approach. In this technique, we take advantage of the pulsed regime to use the atoms themselves as microwave amplitude discriminators. Specifically, in addition to the dependence on the microwave detuning, the atomic signal after the Ramsey interrogation acquires a dependence on the microwave pulse area (amplitude times duration) that can be exploited to implement an active stabilization of the microwave field amplitude, in a similar way in which the Ramsey clock signal is used to lock the local oscillator frequency to the atomic reference. The stabilization allows us to reduce the microwave field-amplitude fluctuations, which in turn impact the clock frequency through cavity pulling. The proposed technique has shown to be effective to improve our clock frequency stability on medium and long term. We demonstrate the method for a vapor-cell clock working with a hot sample of atoms, but it can be extended to cold-atom compact clocks.