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PROGRESS ON A PORTABLE RUBIDIUM FOUNTAIN FREQUENCY STANDARD

Paul D. Kunz, Thomas P. Heavner, Steven R. Jefferts

Published Aug 22, 2015• 0 citations
OpenAlex

Abstract

We are developing a transportable laser-cooled rubidium (Rb) atomic fountain frequency standard. We have a complete optical package, and have recently shown cooled, launched, and detected atoms. Additionally, a new physics package has been designed and is currently under construction. Here we describe the recent progress made on this project, including refinements to our optical package and its evaluation, and our design choices for the physics package.

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We are developing a transportable laser-cooled rubidium (Rb) atomic fountain frequency standard. We have a complete optical package, and have recently shown cooled, launched, and detected atoms. Additionally, a new physics package has been designed and is currently under construction. Here we describe the recent progress made on this project, including refinements to our optical package and its evaluation, and our design choices for the physics package.
Abstract : We are developing a transportable laser-cooled rubidium (Rb) atomic fountain frequency standard. Over the last year, we have built the Ramsey cavity and rubidium source oven and are preparing to assemble the physics package. We have also made numerous refinements to the laser package, including transitioning to a new style of laser diode, vastly improving the optical phase-locked loops (OPLL), and improving the optical spatial mode to get more power out of the optical fibers. These refinements have enabled the laser cooling to collect many more atoms, which will improve the fountain s signal-to-noise ratio.
We are developing a simple and robust laser and optics system suitable for a transportable laser-cooled rubidium (Rb) atomic fountain frequency standard. This system uses DFB (Distributed Feedback) diode lasers and a frequency offset-locking scheme to generate the optical frequencies needed for laser-cooling, launching, post-cooling, and detection of Rb atoms. Here, we describe the early results from this project and the anticipated performance of a compact, transportable Rb fountain frequency standard using this laser system. This system will also be useful for nontransportable laser-cooled atomic frequency standards.
Atomic interferometry is a highly precise metrological technique of interest to both fundamental and applied physics. One field in which atomic interferometry is of particular interest is gravimetry, where instruments based upon the controlled manipulation of atomic states have superseded the performance of classical devices. In such devices, a standing wave derived from laser beams with well-defined phase difference are used to impart momentum to atoms undergoing free fall, generating spatial superpositions along the axis of gravity, before returning the atoms to an overlapped final state for recombination. This thesis presents the design and experimental progress towards a gravity gradiometer based upon the interference of rubidium-87 atoms, cooled and launched in a fountain configuration. The fountain is leveraged from a decommissioned frequency standard, which has been re-purposed and redesigned for use as a combined absolute gravimeter and gravity gradiometer package. It is capable of loading 100 billion atoms in 100 ms and then cooling and launching these atoms in a moving molasses configuration, with a final temperature of 5 μK. Furthermore, the process can be quickly repeated, with a second cloud subsequently launched, reaching its apogee at the same time as the first cloud, allowing for the possibility of simultaneous measurement at different heights and, in the future, the inference of the gravity gradient. Control of the fountain is provided by a stable and miniaturised optical setup, allowing for use with a future transportable or field-based device. A Raman beam system to perform the interferometry sequence has been constructed and characterised, likewise designed in a minaturised and stable manner. The Raman beams simultaneously generated Ramsey interference fringes on two independent clouds of atoms -- one launched to a height of 0.7 m and the other to 1 m. Such fringes were clearly observable after 200 ms of evolution time. Mach-Zehnder interference fringes were also demonstrated for dropped and launched atoms, providing the capability for gravimetric measurements to be made once the system has been fully characterised. The principle causes of noise in the Mach-Zehnder cases were identified as arriving from a lack of atoms and the phase noise of the Raman beams. To overcome these challenges, a new Raman beam system has been preliminary designed and tested and a new iteration of the fountain, moving towards quantum degeneracy, has been planned.