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European Frequency and Time Forum

Progress towards a strontium optical lattice clock at NPL

Elizabeth M. Bridge, Ian R. Hill, Yuri B. Ovchinnikov, G. P. Barwood, E. A. Curtis, Patrick Robert Gill

Published Apr 1, 2010• 0 citations
Publisher

Abstract

The1S0-3P0transition at 698 nm in neutral strontium has the potential to be an extremely precise optical frequency standard [1] and is one of several candidates for a future redefinition of the second. Recent measurements of this transition at three national laboratories around the world have fractional frequency uncertainties in the 1015range and agree to within a hertz [2,3,4]. To realise fully the potential accuracy of this frequency standard, in depth studies of systematic effects need to be performed in order to characterise them at the level necessary for a redefinition of the second. At the National Physical Laboratory we are developing a compact, diode laser based optical lattice clock. We intend to study the clock transition in both88Sr and87Sr with focus on the uncertainties in the frequency shifts due to blackbody radiation. The setup incorporates a novel permanent magnet based Zeeman slower [5].

DOI
10.1109/eftf.2010.6533688

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Accurate time keeping is critical to the function and advancement of many human endeavours. Atomic clocks based on a microwave transition in atomic Cs have served as primary frequency standards to realise the SI second since their formal adoption in 1967. State-of-the-art, laser cooled, Cs fountains now routinely operate with fractional frequency uncertainties below the 10⁻¹⁵ level in laboratories throughout the world. Over the last decade, efforts to build frequency standards based on optical transitions have come to fruition and potential inaccuracies below the 10⁻¹⁷ level have been demonstrated. Such standards point to the future of time keeping and much development is being pursued to realise the potential of these 'next generation' optical clocks. One such approach, on which this work focusses, relies on a 'magic' wavelength optical lattice trap to provide (almost) Doppler- and recoil-free confinement of atoms during clock interrogation. At this 'magic' wavelength, light induced perturbation of the clock states is equal and so cancels precisely. This thesis describes the design and implementation of an optical lattice frequency standard apparatus based on 88Sr held in a 1D optical lattice. A review of the development of atomic frequency metrology is given from a historical viewpoint and the technology and limitations of current methods discussed. To tackle the blackbody radiation shift, which currently presents the greatest limitation to the Sr lattice clock, imposing at the 10⁻¹⁶ level, a scheme to measure the blackbody radiation shift coefficient at the 10⁻¹⁷ level is developed. Techniques to laser cool and trap atomic strontium are described and the implementation detailed. All laser systems required for clock operation are described including the design and implementation of extended-cavity diode lasers for precision spectroscopy. Details relevant to the eventual operation of the apparatus are discussed.

•Jun 1, 2012•3 citations
The1S0-3P0transition at 698 nm in neutral strontium has the potential to be an extremely precise optical frequency standard [1] and is one of several candidates for a future redefinition of the second. Recent measurements of this transition at three national laboratories around the world have fractional frequency uncertainties in the 1015range and agree to within a hertz [2,3,4]. To realise fully the potential accuracy of this frequency standard, in depth studies of systematic effects need to be performed in order to characterise them at the level necessary for a redefinition of the second. At the National Physical Laboratory we are developing a compact, diode laser based optical lattice clock. We intend to study the clock transition in both88Sr and87Sr with focus on the uncertainties in the frequency shifts due to blackbody radiation. The setup incorporates a novel permanent magnet based Zeeman slower [5].
The1S0-3P0transition in neutral strontium is being considered as one of the choices for the future redefinition of the SI second. At the National Physical Laboratory we are designing and implementing an optical clock based on this transition, holding the atoms in an optical lattice during the clock interrogation cycle, which greatly increases the potential accuracy of the clock. Our focus is on building a compact experimental system (including a novel permanent-magnet-based Zeeman slower) designed with the ability to assess systematic frequency shifts in mind, especially those related to blackbody radiation. Further study of systematic effects and additional intercomparisons of experimental results from different laboratories are being pursued with a view to gaining support for a future redefinition.
Strontium optical lattice clocks can measure the passage of time with extraordinary precision. Capitalising on this precision, we can anticipate the deployment of lattice clocks to search for new physics beyond the standard model, to build new technologies for geodesy and navigation, and potentially to underpin a future redenition of the SI second. This thesis reports on the construction and evaluation of a robust Sr optical lattice clock at NPL. We describe the apparatus needed to capture, cool, and load samples of neutral strontium atoms into a magic-wavelength, far off-resonant lattice trap at 813 nm. We provide details of our optical local oscillator - the "clock laser" - and how it is used to realise an Sr-referenced optical frequency standard. We rigorously characterise the various contributing factors which limit the performance of the clock, focusing on the standard measures of (1) frequency instability, and (2) systematic frequency uncertainty. Finally, we introduce new innovations for improving the accuracy of the 88Sr lattice clock, including methods of "modified hyper-Ramsey" and multi-photon spectroscopy of the clock transition.
We report on recent realization of an optical lattice clock operating on the1S0-3P0transition in88Sr, at LENS laboratories. The clock transition is excited with the technique of magnetic field-induced spectroscopy. New experimental techniques has also been developed in order to simplifies the clock spectroscopy in Sr atoms. The first new method helps in the first search of the clock transition without the use of extensive frequency metrology hardware and in particular optical frequency combs. This technique exploits a near coincidence in the atomic wavelengths of the1S0-3P0clock and1S0-3P1second stage cooling transitions in Sr, which are only 5 THz far apart. This coincidence enables the use of an optical (transfer) cavity to reference the frequency of the clock transition relative to that of the much stronger cooling transition. Secondly we reduce the complexity of the experimental setup by using only semiconductor laser sources in the apparatus. With this setup, about 10488Sr atoms are trapped in a 1D lattice formed by 200 mW of radiation tuned near the magic wavelength at 813 nm. We will present a first uncertainty budget for our optical lattice clock based on88Sr with particular attention to density dependent collisions, which led to an unexpectedly high signal contrast for long interaction times. In view of an absolute frequency measurement of clock transition we'll also present recent experimental results on optimization of an home made Ti:Sa optical frequency comb. Additionally, we anticipate that the simplification of the experimental setup presented will help address a wider range of applications including those requiring transportable devices. Along these lines we will report progress on the realization of the first transportable Sr optical lattice clock.
Optical atomic clocks have proven to be the most stable clocks with the lowest systematic uncertainty which has now reached a record level of (9.4 X10/(^{19}/)). Thus optical clocks now surpass the best fountain Cs atomic clocks by three orders of magnitude which opens the prospect of using optical clocks as frequency standards for timekeeping in the near future. \n \nThis thesis describes the progress that has been made towards a stationary optical lattice clock based on strontium atoms, and reports the results of an ongoing characterization. \n \nAn experimental apparatus was designed and built to efficiently slow, cool, and trap the strontium atoms which represent the heart of the strontium optical lattice clock discussed in this thesis. The novel aspects of the experimental development described in the thesis comprise a frequency doubling cavity for producing 461 nm laser light employed for cooling and trapping the strontium atoms in the first stage cooling. This cavity provided sufficient power for cooling and trapping the strontium atoms for up to 300 mW of 461 nm laser light. Furthermore, a new design of Zeeman slower based on spherical permanent magnets was produced, built and characterized. \n \nThe slower is compact, tunable, easy to assemble and maintain, and cost-effective as the permanent magnets do not require any power supply or water cooling. It is suggested that this kind of Zeeman slower constitutes an ideal solution for portable optical clock systems. A blue magneto-optical trap was realized for the first time with such a Zeeman slower. With about 30 mW of a 461 nm laser, up to 6X10/(^8/) atoms were captured.
Accurate time keeping is critical to the function and advancement of many human endeavours. Atomic clocks based on a microwave transition in atomic Cs have served as primary frequency standards to realise the SI second since their formal adoption in 1967. State-of-the-art, laser cooled, Cs fountains now routinely operate with fractional frequency uncertainties below the 10⁻¹⁵ level in laboratories throughout the world. Over the last decade, efforts to build frequency standards based on optical transitions have come to fruition and potential inaccuracies below the 10⁻¹⁷ level have been demonstrated. Such standards point to the future of time keeping and much development is being pursued to realise the potential of these 'next generation' optical clocks. One such approach, on which this work focusses, relies on a 'magic' wavelength optical lattice trap to provide (almost) Doppler- and recoil-free confinement of atoms during clock interrogation. At this 'magic' wavelength, light induced perturbation of the clock states is equal and so cancels precisely. This thesis describes the design and implementation of an optical lattice frequency standard apparatus based on 88Sr held in a 1D optical lattice. A review of the development of atomic frequency metrology is given from a historical viewpoint and the technology and limitations of current methods discussed. To tackle the blackbody radiation shift, which currently presents the greatest limitation to the Sr lattice clock, imposing at the 10⁻¹⁶ level, a scheme to measure the blackbody radiation shift coefficient at the 10⁻¹⁷ level is developed. Techniques to laser cool and trap atomic strontium are described and the implementation detailed. All laser systems required for clock operation are described including the design and implementation of extended-cavity diode lasers for precision spectroscopy. Details relevant to the eventual operation of the apparatus are discussed.