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Chinese Physics Letters

Initial Tests of a Rubidium Space Cold Atom Clock

Lin Li, Qiuzhi Qu, Bin Wang, Tang Li, Jian-Bo Zhao, Jing-Wei Ji, Wei Ren, Xin Zhao, Meifeng Ye, Yuanyuan Yao, Desheng Lü, Liang Liu

Published Jun 1, 2016• 9 citations
Publisher

Abstract

We report the initial test results of a rubidium ( 87 Rb) space cold atom clock (SCAC). The space-qualified 87 Rb SCAC is composed of the physical package, the optical bench, the microwave synthesizer and the control electronics. After the system is integrated, about 10 8 87 Rb cold atoms are captured by magneto-optical trap. The linewidth of the Ramsey fringe is about 10 Hz for the free evolution time of 50 ms on the ground, and the signal-to-noise ratio is measured to be larger than 300. We demonstrate a good medium-term fractional frequency stability of 1.5 × 10 −14 @1000 s in the closed-loop operation on the ground. The main effects of the noise on the stability are also presented, and the optimized operating parameter is analyzed for the operation of SCAC in the microgravity environment.

DOI
10.1088/0256-307x/33/6/063201

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We report the initial test results of a rubidium ( 87 Rb) space cold atom clock (SCAC). The space-qualified 87 Rb SCAC is composed of the physical package, the optical bench, the microwave synthesizer and the control electronics. After the system is integrated, about 10 8 87 Rb cold atoms are captured by magneto-optical trap. The linewidth of the Ramsey fringe is about 10 Hz for the free evolution time of 50 ms on the ground, and the signal-to-noise ratio is measured to be larger than 300. We demonstrate a good medium-term fractional frequency stability of 1.5 × 10 −14 @1000 s in the closed-loop operation on the ground. The main effects of the noise on the stability are also presented, and the optimized operating parameter is analyzed for the operation of SCAC in the microgravity environment.
Since the atomic clock was invented, its performance has been improved for one digit every decade until 90s of last century when the traditional atomic clock almost reached its limit. With laser cooled atoms, the performance can be further improved, and nowadays the cold atom based clocks are widely used as primary frequency standards. Such a kind of cold atom clocks has great applications in space. This paper presents the design and tests of a cold atom clock (CAC) operating in space. In microgravity, the atoms are cooled, trapped, launched and finally detected after being interrogated by microwave field with Ramsey method. The results of laser cooling of atoms in microgravity in orbit are presented and compared with that on ground for the first time. That the full width at half maximum (FWHM) of obtained central Ramsey fringes varies linearly with launching velocity of cold atoms shows the effects of microgravity. With appropriate parameters, a closed-loop locking of the CAC is realized in orbit and the estimated short term frequency stability of 3.0×10−13/τ3.0\times 10 ^{-13}/\sqrt{τ}3.0×10−13/τ​ has been reached.
We describe a highly reliable optical system designed for a rubidium space cold atom clock (SCAC), presenting its design, key technologies, and optical components. All of the optical and electronic components are integrated onto an optimized two-sided 300 mm×290 mm×30 mm optical bench. The compact optical structure and special thermal design ensure that the optical system can pass all of the space environmental qualification tests including both thermal vacuum and mechanical tests. To verify its performance, the optical system is carefully checked before and after each test. The results indicate that this optical system is suitably robust for the space applications for which the rubidium SCAC was built.
We reported on the progress toward an atomic clock based on a continuous cold 87Rb atomic beam. The atomic beam source is generated directly from a 2D+Magneto-Optical Trapping (MOT) with the flux of about 3×109atoms/s and mean longitudinal velocity of 10 m/s. The beam had a free evolution time of 9 ms over the separated distance of 90 mm, producing a 54-Hz-wide Raman-Ramsey fringe. The Signal-to-Noise ratio (SNR) of Raman-Ramsey fringes is effectively increased by modulation and demodulation methods, which offer a promising approach to improving the frequency stability of the atomic clock.
We demonstrate a Ramsey-type microwave clock interrogating the 6.835 GHz ground-state transition in cold [Formula: see text]Rb atoms loaded from a grating magneto-optical trap (GMOT) enclosed in an additively manufactured loop-gap resonator microwave cavity. A short-term stability of [Formula: see text] is demonstrated, in reasonable agreement with predictions from the signal-to-noise ratio of the measured Ramsey fringes. The cavity-grating package has a volume of [Formula: see text]67 cm[Formula: see text], ensuring an inherently compact system while the use of a GMOT drastically simplifies the optical requirements for laser cooled atoms. This work is another step towards the realisation of highly compact portable cold-atom frequency standards.
Abstract This article proposes a new physics package to enhance the frequency stability of the space cold atom clock with the advantages of a microgravity environment. Clock working processes, including atom cooling, atomic state preparation, microwave interrogation, and transition probability detection, are integrated into the cylindrical microwave cavity to achieve a high-performance and compact physics package for the space cold atom clock. We present the detailed design and ground-test results of the cold atom clock physics package in this article, which demonstrates a frequency stability of 1.2 × 10−12 τ −1/2 with a Ramsey linewidth of 12.5 Hz, and a better performance is predicted with a 1 Hz or a narrower Ramsey linewidth in microgravity environment. The miniaturized cold atom clock based on intracavity cooling has great potential for achieving space high-precision time-frequency reference in the future.
The performance of space cold atom clocks (SCACs) should be improved thanks to the microgravity environment in space. The microwave interrogation cavity is a key element in a SCAC. In this paper, we develop a microwave interrogation cavity especially for the rubidium SCAC. The interrogation cavity has two microwave interaction zones with a single feed-in source, which is located at the center of the cavity for symmetric coupling excitation and to ensure that the two interaction zones are in phase. The interrogation cavity has a measured resonance frequency of 6.835056471 GHz with a loaded quality factor of nearly 4200, which shows good agreement with simulation results. We measure the Rabi frequency of the clock transition of the rubidium atom in each microwave interaction zone, and subsequently demonstrate that the distributions of the magnetic field in the two interaction zones are the same and meet all requirements of the rubidium SCAC.