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Joint Conference of the European Frequency and Time Forum and IEEE International Frequency Control Symposium (EFTF/IFCS)

Integrating sphere cold atom clock with cylindrical microwave cavity

Xucheng Wang, Hua-Dong Cheng, Ben-Chang Zheng, Yan-Ling Meng, Ling Xiao, Liang Liu, Yuzhu Wang

Published May 1, 2011• 1 citation
Publisher

Abstract

The integrating sphere is used to generate the diffuse light for cooling atoms. The microwave field is produced in a cylindrical cavity on a TE011 mode. We have realized the microwave interrogation with the pulsed optically pumped (POP) scheme. The Ramsey fringe is demonstrated and some factors to affect the signal to noise ratio (SNR) and the contrast of the fringe are discussed. Optimization of the experimental parameters is still in progress.

DOI
10.1109/fcs.2011.5977307

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The integrating sphere is used to generate the diffuse light for cooling atoms. The microwave field is produced in a cylindrical cavity on a TE011 mode. We have realized the microwave interrogation with the pulsed optically pumped (POP) scheme. The Ramsey fringe is demonstrated and some factors to affect the signal to noise ratio (SNR) and the contrast of the fringe are discussed. Optimization of the experimental parameters is still in progress.
We present a scheme for a compact rubidium cold-atom clock which performs diffuse light cooling, microwave interrogation, and detection of the clock signal in a cylindrical microwave cavity. The diffuse light is produced by laser light reflection at the inner surface of the microwave cavity. The pattern of the injected laser beams is specially designed to accumulate the majority of the cold atoms in the center of the microwave cavity. Microwave interrogation of the cold atoms in the cavity leads to Ramsey fringes, which have a linewidth of 24.524.524.5 Hz with a contrast of 95.695.6%95.6 when the free evolution time is 20 ms. Recently, a frequency stability of $7.3\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}13}\phantom{\rule{0.28em}{0ex}}{\ensuremath{\tau}}^{\ensuremath{-}1/2}$ has been achieved. The scheme of this physical package can largely reduce the cold-atom clock complexity and increase clock performance.
Abstract To improve the Ramsey fringe contrast for a portable free-falling-typed cold-atom microwave clock based on a TE 013 -mode cylindrical microwave resonator, this paper innovatively designs an irregular-shaped Ramsey cavity based on the combination of TE 011 and TE 013 modes cylindrical microwave resonators. To ensure b 1 𝜏 1 = b 2 𝜏 2 during the two Ramsey microwave pulses, this study constructs a 3D electromagnetic model to analyze the relationship between the matching factor η = b 1 𝜏 1 /b 2 𝜏 2 and the cavity structural parameters. Simulation results demonstrate that the irregular-shaped Ramsey cavity can effectively generate four spatially separable microwave standing wave fields, with two of them differing in height. The cavity is designed to achieve a central Ramsey fringe with a full width at half maximum (FWHM) of about 9 Hz. By iteratively refining the cavity geometry, the optimal parameters for the cavity dimensions were obtained, with a TE 013 mode height of 100.56 mm and a TE 011 mode height of 34.09 mm. This configuration achieves an amplitude-transit time matching factor η of unity ( η = 1).
An optical-plus-microwave pumping method is implemented and optimized to pump cold atoms to a magnetically insensitive state in an integrating sphere. This method consists of several optical and microwave pulses with a proper time sequence. The relative population of cold atoms in the Zeeman sublevel $|F=1,{m}_{F}=0\ensuremath{\rangle}$ is tested by measuring the Rabi oscillations and Ramsey fringes with different numbers of optical and microwave pumping pulses, demonstrating that nearly 83% of the cold atoms can be prepared in the clock state. The number of cold atoms in the target state is up to 2.5 times more than that of the optical pumping alone. Thus, the limitation on the stability of an integrating sphere cold-atom clock from atomic noise is estimated to be from $4.6\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}13}{\ensuremath{\tau}}^{\ensuremath{-}1/2}$ to $3.0\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}13}{\ensuremath{\tau}}^{\ensuremath{-}1/2}$. This method is also suitable for other precision measurements with cold atoms.
We present a compact cold-atom clock configuration where isotropic laser cooling, microwave interrogation, and clock signal detection are successively performed inside a spherical microwave cavity. For ground operation, a typical Ramsey fringe width of 20 Hz has been demonstrated, limited by the atom cloud's free fall in the cavity. The isotropic cooling light's disordered properties provide a large and stable number of cold atoms, leading to a high signal-to-noise ratio limited by atomic shot noise. A relative frequency stability of $2.2\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}13}{\ensuremath{\tau}}^{\ensuremath{-}1/2}$ has been achieved, averaged down to $4\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}15}$ after $5\ifmmode\times\else\texttimes\fi{}{10}^{3}$ s of integration. Development of such a high-performance compact clock is of major relevance for on-board applications, such as satellite-positioning systems. As a cesium clock, it opens the door to a new generation of compact primary standards and timekeeping devices.
We present a demonstration of a cold-atom optical-microwave double resonance (DR) Ramsey clock utilising an additively manufactured loop-gap-resonator cavity and grating magneto-optical trap (GMOT). The use of additive manufacturing allows for complex cavity structures, more difficult to produce with traditional machining techniques, while the GMOT architecture significantly simplifies the optical system required to trap and cool the atomic sample. In the current demonstration a single laser is used to trap < 3 × 106 87Rb atoms, cool them to below 10 µK, optically pump and read-out state populations of the atoms after microwave interrogation. A Ramsey-type interrogation scheme is employed with an empirically evaluated optimum free evolution time of 10 ms, limited by the loss of signal due to atoms falling out of the read-out beam. We demonstrate a short-term stability of < 2×10−11τ−1/2, in reasonable agreement with the predicted short-term stability based on the signal to noise ratio of the measured Ramsey fringes. Excellent field homogeneity of the cavity microwave field is demonstrated though Rabi oscillations, while almost complete optical pumping and good field orientation is evidenced by Zeeman spectroscopy of the ground-state hyperfine energy levels. This work is a novel approach towards more compact and portable cold-atom microwave clocks with significant potential for further miniaturization of the system.