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arXiv:1812.01658 · Physical Review Applied

Compact Cold-Atom Clock for Onboard Timebase: Tests in Reduced Gravity

Mehdi Langlois, L. De Sarlo, David Holleville, N. Dimarcq, Jean-François Schaff, Simon Bernon

Published Dec 4, 2018• 26 citations
arXivHTMLPDF

Abstract

Improving global navigation satellite systems (GNSS) requires a more stable basis of timekeeping. Cold-atom clocks are currently the most accurate time references, but are large. This article presents an atomic clock, ``Rubiclock'', that is compact and portable, thanks to a fiber-based laser bench, isotropic light cooling, and atoms that remain motionless during the operating sequence. This configuration gives the clock an important benefit in microgravity; measurements performed on 0-$g$ flights show better stability than on the ground. The stability that Rubiclock could achieve in a satellite makes it a good candidate for the next generation of GNSS.

DOI
10.1103/physrevapplied.10.064007

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Improving global navigation satellite systems (GNSS) requires a more stable basis of timekeeping. Cold-atom clocks are currently the most accurate time references, but are large. This article presents an atomic clock, ``Rubiclock'', that is compact and portable, thanks to a fiber-based laser bench, isotropic light cooling, and atoms that remain motionless during the operating sequence. This configuration gives the clock an important benefit in microgravity; measurements performed on 0-ggg flights show better stability than on the ground. The stability that Rubiclock could achieve in a satellite makes it a good candidate for the next generation of GNSS.
Highly stable atomic frequency standards are of increasing importance for a variety of space applications, ranging from communication to navigation and time transfer to tests of fundamental science. The requirements for an atomic clock vary significantly depending on the application, and for many space systems, compactness and robust design are at a premium, and stability dominates over absolute accuracy. We report on progress with our design for a compact laser-cooled Cs- beam atomic clock suitable for satellite applications such as GPS. The basic design features a continuous, cold atomic beam extracted from a magneto-optic trap (MOT). This cold atomic beam is then to be used in a laser-pumped Ramsey clock, with the clock signal derived from either a microwave C-field or, alternatively, by Raman resonance between the Ramsey fields. In order to reduce light shifts from the MOT light and improve signal-to-noise, the atomic beam will be optically deflected and transversely cooled upon extraction from the MOT. We estimate that the shot-noise-limited stability achievable with this physics package can be 2 to 3 orders of magnitude better than current Cs-beam clocks used in satellite applications. We present our experimental progress towards a working frequency standard, including characterization of our six-beam magneto-optic cold atom trap and production and characterization of a cold atomic beam.
Precision navigation and timing is an important focus in the Air Force Research Laboratory's experimental efforts. The development and testing of an optical frequency standard is suitable for an array of both terrestrial and space based applications. An optical atomic clock based upon a two-photon transition at 778 nm in rubidium, is an ideal candidate to meet the requirements of Global Satellite Navigation System (GNSS), while being a viable option for other applications. Here we demonstrate that a simple architecture, using off-the-shelf components and an external cavity diode laser system that is confined to less than 3 ft2 of breadboard, exhibits a frequency instability of 7 x 10-13/??, for times less than 100 seconds.
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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.
The technique of laser cooling of atoms gives an opportunity to improve the performance of atomic clocks by using laser-cooled atoms. The most successful cold atom clock, called the atomic fountain, is now widely used as the primary frequency standard in many labs. The cold atom clock for satellite applications, however, has not been reported so far due to special requirements of space applications. Here, we report the development of an engineering model of a satellite-borne cold atom clock, which satisfied all requirements of in-orbit operation. The core of the clock’s principle is the laser cooling of atoms by diffuse laser lights inside the microwave cavity. The structure of the physics package is presented, and its main parameters are also given. The principle and design of the optical bench are described. The initial test results are presented, and the possible improvements are also discussed.
In the opening paragraphs of his 1905 paper introducing special relativity, Einstein defines time as “the position of the small hand of my watch,” with the validity of this definition restricted to the immediate vicinity of the watch. He then goes on to discuss the possibility of conventionally synchronizing distant clocks using the exchange of electromagnetic signals and thus of realizing a network of clocks that can be used to provide space and time information. Such networks of clocks are today a reality, the best known ones being the global satellite navigation systems (GNSS) like GPS or Galileo. These consist of atomic clocks on the ground and onboard satellites, together with the quartz clocks that are inside the everyday GNSS receivers we use in our cars and smartphones. In recent years,