We describe the Sr optical lattice clock apparatus at NPL with particular emphasis on techniques used to increase reliability and minimise the human requirement in its operation. Central to this is a clock-referenced transfer cavity scheme for the stabilisation of cooling and trapping lasers. We highlight several measures to increase the reliability of the clock with a view towards the realisation of an optical time-scale. The clock contributed 502 hours of data over a 25 day period (84% uptime) in a recent measurement campaign with several uninterrupted periods of more than 48 hours. An instability of 2 x 10 -17 was reached after 10 5 s of averaging in an interleaved self-comparison of the clock.
We describe the Sr optical lattice clock apparatus at NPL with particular emphasis on techniques used to increase reliability and minimise the human requirement in its operation. Central to this is a clock-referenced transfer cavity scheme for the stabilisation of cooling and trapping lasers. We highlight several measures to increase the reliability of the clock with a view towards the realisation of an optical time-scale. The clock contributed 502 hours of data over a 25 day period (84% uptime) in a recent measurement campaign with several uninterrupted periods of more than 48 hours. An instability of 2 x 10 -17 was reached after 10 5 s of averaging in an interleaved self-comparison of the clock.
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.
This thesis presents the latest achievements regarding the Sr optical lattice clock experiment at LNE-SYRTE, Observatoire de Paris. After having described the general principles for optical lattice clocks and the operation of the clock in question, the emphasis is put on the features that have been added to the experiment since 2007. The most important new elements are an ultra-stable reference cavity for the clock laser, the development of a non-destructive detection technique, and the construction of a second Sr lattice clock. The ultra-stable cavity is constructed from a ULE spacer and fused silica mirrors and has shown a thermal noise floor at 6.5E-16, placing it among the best in the world. The non-destructive detection is effectuated by a phase measurement of a weak probe beam that traverses the atoms placed in one arm of a Mach-Zender interferometer. The non-destructive aspect enables a recycling of the atoms from cycle to cycle which consequently increases the duty cycle, allowing for an increase of the stability of the clock. With these new tools the frequency stability is expected to be 2.2E-16/tau^(1/2) for an optimized sequence. The most recent comparisons between the two Sr clocks reach an accuracy level of 1E-16 after about 1000 s, and this way we have been able to characterize lattice related frequency shifts with an unprecedented accuracy. The measurements ensure a control of lattice related effects at the 1E-18 level even for trap depths as large as 50 Er.
Frequency metrology underpins precise measurements. At the forefront of this field are optical clocks which can perform measurements that are accurate to more digits than those in the number representing seconds elapsed from the beginning of the universe. Frequency estimation errors in the operation of optical clocks lead to frequency instability, prohibiting them from reaching high accuracies on short to medium time scales, and limiting access to interesting measurements of the atomic environment and exotic external perturbations such as gravity fields or fields of fundamentally new physical origins. Optical lattice traps are typically employed in state-of-the-art optical clocks to suppress Doppler effects. By carefully tuning the optical lattice to the magic wavelength, unwanted Stark shifts can be suppressed significantly, allowing operation with thousands of atoms and minimising the fundamental source of frequency estimation errors, namely the quantum projection noise, to around 3 × 10−17 at 1 s. In this thesis, the status of the two Strontium optical lattice clocks at NPL, Sr1 and Sr2, is presented going over a few interesting measurement campaigns from the past years involving Sr1 and the latest experimental efforts to re-enable operation of Sr2. A model of clock stability is developed by incorporating known and new frameworks to characterise the relation between the operating parameters of optical clocks and their performance, with relevance to the NPL lattice clocks and beyond. In particular, a method is identified for addressing the main technical noise for optical lattice clocks, the Dick effect, which typically has a magnitude of 10−16 at 1 s and prevents optical lattice clocks from reaching the quantum projection noise level. By taking advantage of dynamical decoupling we can engineer an optimised duty cycle with benefits in general for systems limited by the optical coherence of the clock laser. Moreover, a study of other noise sources in the NPL lattice clocks is conducted using synchronous measurements between Sr1 and Sr2, revealing a large contribution from linear Zeeman shifts, also at the 10−16 level, and a plan to tackle them is detailed. A hybrid clock architecture is discussed showing how optical lattice clocks can improve the optical coherence of ion clocks thanks to their higher servo rate, and a preliminary experimental demonstration is reported. Finally, a new type of spectroscopy is presented which completely avoids aliasing of linear Zeeman shifts by simultaneously driving transitions with opposite Zeeman sensitivities. Such a technique also enables the continuous interrogation of atoms, a crucial missing component in the Dick-free conveyor belt lattice clock concept heralded as the next standard of stable optical clocks.
This thesis presents the latest achievements regarding the Sr optical lattice clock experiment at LNE-SYRTE, Observatoire de Paris. After having described the general principles for optical lattice clocks and the operation of the clock in question, the emphasis is put on the features that have been added to the experiment since 2007. The most important new elements are an ultra-stable reference cavity for the clock laser, the development of a non-destructive detection technique, and the construction of a second Sr lattice clock. The ultra-stable cavity is constructed from a ULE spacer and fused silica mirrors and has shown a thermal noise floor at 6.5E-16, placing it among the best in the world. The non-destructive detection is effectuated by a phase measurement of a weak probe beam that traverses the atoms placed in one arm of a Mach-Zender interferometer. The non-destructive aspect enables a recycling of the atoms from cycle to cycle which consequently increases the duty cycle, allowing for an increase of the stability of the clock. With these new tools the frequency stability is expected to be 2.2E-16/tau^(1/2) for an optimized sequence. The most recent comparisons between the two Sr clocks reach an accuracy level of 1E-16 after about 1000 s, and this way we have been able to characterize lattice related frequency shifts with an unprecedented accuracy. The measurements ensure a control of lattice related effects at the 1E-18 level even for trap depths as large as 50 Er.
We report on our progress toward the realization of a compact optical frequency standard referenced to strontium intercombination lines. Our current setup allows the production of ultracold Sr atoms in hundreds of ms. For high resolution spectroscopy of the 1S0-3P0 doubly forbidden transition we have prepared a 698 nm clock laser stabilized on a high finesse, symmetrically suspended cavity and a high power 813 nm light source for the optical lattice trap at the magic wavelength. Due to their compactness, reliability, and low power consumption, semiconductor laser sources represent the best choice for the development of compact and transportable devices for application both on Earth and in Space. A new Sr trapping and cooling experimental setup is also presented.
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.