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Optical Lattice Clock with Spin-1/2 Ytterbium Atoms

N. Lemke

Published Jan 1, 2012• 7 citations
OpenAlex

Abstract

An optical lattice clock probes a spectrally narrow electronic transition in an ensemble of optically trapped, laser-cooled atoms, for use as a time and frequency standard. To date, several lattice clocks have been demonstrated with superior stability and accuracy compared to primary frequency standards based on microwave transitions. Yet, the question of which atomic system (including the element and isotope) performs best as a lattice clock remains unsettled. This thesis describes the first detailed investigation of an optical lattice clock using a spin-1/2 isotope of the ytterbium atom. A spin-1/2 system possesses several advantages over higher-spin systems, including a simplified level structure (allowing for straightforward manipulation of the nuclear spin state) and the absence of any tensor light shift from the confining optical lattice. Moreover, the ytterbium atom (Yb) stands among the leading lattice clock candidates, offering a high-performance optical clock with some degree of experimental simplicity. The frequency stability of the Yb clock is highlighted by resolving an ultra-narrow clock spectrum with a full-width at half-maximum of 1 Hz, corresponding to a record quality factor Q = ν0/Δν = 5 × 1014. Moreover, this system can be highly accurate, which is demonstrated by characterizing the Yb clock frequency at the 3 × 10-16 level of fractional uncertainty, with further progress toward a ten-fold improvement also presented. To reach this low level of uncertainty required careful consideration of important systematic errors, including the identification of the Stark-canceling wavelength, where the clock’s sensitivity to the lattice intensity is minimized, a precise determination of the static polarizability of the clock transition, and the measurement and control of atom-atom collisions.

Related papers

Realization of advanced 171Yb optical lattice frequency standard

Gianmaria Milani

Atomic clocks constitute a fundamental tool for time and frequency metrology and their application is widespread in many technological fields. In particular, the International System of units (SI) defines the second on a microwave transition of Caesium atoms. The realization is made by clocks reaching uncertainties of few parts in 10^−16, making the second to be the quantity realized with the smallest uncertainty in the SI. However, a new generation of atomic clocks, called optical clocks, have already demonstrated to surpass Caesium standards both in accuracy and stability. The research performed during my PhD activity has been focused on the development and characterization of Ytterbium (Yb) optical lattice clocks. These systems operate with a large number of ultra-cold neutral atoms having a clock transition in the visible region of the electromagnetic spectrum. The atomic sample is trapped in an periodical optical potential called optical lattice that gives the advantage to interrogate many quantum absorbers for an extended time, with small perturbations, allowing to achieve an unprecedented stability and accuracy. The main experimental work has been carried out in the laboratories of the Istituto Nazionale di Ricerca Metrologica (INRIM) in Turin, where several atomic clocks are present including the Italian primary frequency standard, the Caesium fountain ITCsF2, and where an Yb optical lattice clock is currently being developed. This thesis describes the functioning of the system along with the full characterization of systematic effects, the complete uncertainty budget and its first absolute frequency measurement against the primary frequency standard. The clock exhibited an accuracy of 1.6 × 10^−16 and the comparison with the Cs fountain resulted in a frequency of f171Yb = 518 295 836 590 863.59(31) Hz, limited by the fountain uncertainty. This measurement is in agreement with the ytterbium frequency recommended as a secondary representation of the second in the SI and constitutes the first measurement of a Yb clock in Europe and the second one in the world against a primary frequency standard. Several upgrades have been applied after the absolute measurement. In particular, the design and realization a system capable to frequency stabilize several lasers on a single optical cavity is illustrated. This cavity has been implemented to lock the lasers used to cool and trap the atomic sample at 399, 556 and 759nm using the offset sideband locking technique, a modified version of the Pound–Drever–Hall method that gives an extended frequency tunability. The system proved to be an easy-to-use and reliable tool for the experimental activity showing a linewidth below 300 Hz at 556nm, which is the wavelength that requires the most stringent performance, and a long term drift below 20 kHz per day at 759nm. That is suitable for operating the lattice laser with a light shift uncertainty below 1 × 10−18. During my PhD I have been guest researcher at the National Institute of Standards and Technology (NIST) of Boulder, Colorado, for nine months in 2016. In these laboratories two Yb optical lattice clocks are operative. I worked on the instability measurement of a composite system exploiting the two clocks to suppress the Dick effect, called zero-dead-time (ZDT) clock, which demonstrated a fractional instability of 3 × 10−17 at 1s. The two clocks can also be operated to extend the interrogation time obtaining a spectroscopic feature after 4 s of 120(20)mHz corresponding to a quality factor Q > 4 × 10^15. I also worked on the characterization of several systematic shifts that allowed to complete the uncertainty budget of the clocks at 1.6 × 10^−18. In particular, I contributed to the characterization of lattice light shifts considering the effect of atomic sample temperature and the identification of a metrological regime called operational magic frequency where frequency shifts are insensitive to changes in trap depth.

•May 2, 2018•1 citation

Hertz-Level Clock Spectroscopy of 171 Yb Atoms in a One-Dimensional Optical Lattice

Meng-Jiao Zhang, Hui Liu, Xi Zhang +4 more

An ultra-narrow spectroscopy of clock transition with high signal-to-noise ratio is crucial for a high-performance atomic optical clock. We present a detailed study about how to obtain a Hertz-level clock transition spectrum of 171 Yb atoms. About 4 × 10 4 atoms are loaded into a one-dimensional optical lattice with a magic wavelength of 759 nm, and a long lifetime of 3 s is realized with the lattice power of 1 W. Through normalized shelving detection and spin polarization, 171 Yb clock spectroscopy with a Fourier-limited linewidth of 5.9 Hz is obtained. Our work represents a key step toward an ytterbium optical clock with high frequency stability.

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Zeb W. Barber, Chad Hoyt, J. E. Stalnaker +5 more

We describe progress toward an optical lattice clock based on an even isotope of Yb. The 1S0 → 3P0 clock resonance in 174Yb is accessed through a magnetically induced spectroscopic technique. Using ≈1mT static magnetic fields and ≈10 μW of probe light power we generate Rabi frequencies of several hertz. The narrow spectroscopic features that result (< 10 Hz FWHM) require a highly stabilized laser at the clock transition wavelength of 578 nm. We describe a new all solid-state laser system that shows hertz level stability. In order to cancel the slow drift of the cavity, spectroscopy is performed on the clock transition to provide feedback to the laser. Using a Ca neutral atom frequency standard as a reference oscillator,we show high stability and an effective method for investigating clock frequency shift systematics.

Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE•Aug 29, 2007•3 citations

High Precision Spectroscopy of Strontium in an Optical Lattice: Towards a New Standard for Frequency and Time

Martin M. Boyd

Clocks based on optical transitions in ions and atoms are quickly moving to the forefront of the frequency standards field mainly because of the high spec-tral resolution, and therefore the potential stability and accuracy, which can be achieved. In this thesis a new optical clock based on neutral atoms trapped in an optical lattice is presented, demonstrating spectroscopy of the clock transition free of any lineshape or accuracy degradation due to atomic motion. The system simu-lates a single trapped ion clock, but allows use of thousands of atoms for improved signal to noise ratio, and clock stability. High accuracy can also be achieved as the lattice can be designed to shift the energy of the two atomic clock states equally, such that the transition frequency is unchanged. Strontium is a natural candidate for such a clock as it offers an extremely narrow optical transition (∼1 mHz) based on atomic states which are very insensitive to external fields. The strontium level structure allows efficient laser cooling to 1 µK with diode laser sources, and a con-venient wavelength of 813 nm for the zero-differential Stark shift optical lattice. The strontium lattice clock system has allowed observation of high signal-to-noise

•Jan 1, 2007•36 citations

Development of an atomic frequency standard in the optical domain based on ultracold Ytterbium atoms

Marco Zoppi

The work described in this thesis was done at the Istituto Nazionale di Ricerca Metrologica (I.N.Ri.M.), in the optic division, within the Ph.D. of Metrology of the Politecnico di Torino. My work consisted in developing a new generation optical frequency standard based on Ytterbium atoms: the goal of this project is to realize a clock whose performance overcomes the ones of the standard which actually realizes the Second, the atomic Cesium fountain. This means realizes an apparatus which shows stability and accuracy better then the fountain ones: a possible way adopted in several new generation clock is using optical transitions, because they offer an higher quality factor of the clock transition itself, which means a better stability and the possibility to reach the 10^-17 level in the global accuracy of the clock. The fundamental idea of a clock is to realize as close as possible the basic idea under the actual definition of the Second, that is to fix the unit if time with a particular atomic transition, when the atom is completely unperturbed. Accomplishing this prescription means a lot of work in cooling down the atoms until the 10^-6 K regime using laser cooling techniques, developing a trap where atoms can be excited by the clock laser (the optical lattice) and of course a very careful frequency and amplitude stabilization of the clock laser itself is needed to reach our goal. The work described in this thesis regards the development of the two cooling stages of the Yb atoms and it is presented as follows. Chapter 1 explains with more details why an optical clock is needed to improve the accuracy of the actual realization of the Second, then describing the theoretical framework in which the clock will be developed. Chapter 2 contains a description of the experimental apparatus used, with more attention on the instrumentation related to the laser cooling of the atoms. Chapter 3 is about the characterization of the first cooling stage, both in a static and in a dynamic way, with some attention on the theoretical model developed to perform both the characterizations. Chapter 4 focuses on the development of the second cooling stage and it describes the actual state of the art of this stage

•Jan 1, 2012•0 citations

Spin- 1/2 Optical Lattice Clock

N. Lemke, Andrew D. Ludlow, Zeb W. Barber +7 more

We experimentally investigate an optical clock based on ;{171}Yb (I = 1/2) atoms confined in an optical lattice. We have evaluated all known frequency shifts to the clock transition, including a density-dependent collision shift, with a fractional uncertainty of 3.4 x 10;{-16}, limited principally by uncertainty in the blackbody radiation Stark shift. We measured the absolute clock transition frequency relative to the NIST-F1 Cs fountain clock and find the frequency to be 518 295 836 590 865.2(0.7) Hz.

Physical Review Letters•Aug 3, 2009•274 citations
An optical lattice clock probes a spectrally narrow electronic transition in an ensemble of optically trapped, laser-cooled atoms, for use as a time and frequency standard. To date, several lattice clocks have been demonstrated with superior stability and accuracy compared to primary frequency standards based on microwave transitions. Yet, the question of which atomic system (including the element and isotope) performs best as a lattice clock remains unsettled. This thesis describes the first detailed investigation of an optical lattice clock using a spin-1/2 isotope of the ytterbium atom. A spin-1/2 system possesses several advantages over higher-spin systems, including a simplified level structure (allowing for straightforward manipulation of the nuclear spin state) and the absence of any tensor light shift from the confining optical lattice. Moreover, the ytterbium atom (Yb) stands among the leading lattice clock candidates, offering a high-performance optical clock with some degree of experimental simplicity. The frequency stability of the Yb clock is highlighted by resolving an ultra-narrow clock spectrum with a full-width at half-maximum of 1 Hz, corresponding to a record quality factor Q = ν0/Δν = 5 × 1014. Moreover, this system can be highly accurate, which is demonstrated by characterizing the Yb clock frequency at the 3 × 10-16 level of fractional uncertainty, with further progress toward a ten-fold improvement also presented. To reach this low level of uncertainty required careful consideration of important systematic errors, including the identification of the Stark-canceling wavelength, where the clock’s sensitivity to the lattice intensity is minimized, a precise determination of the static polarizability of the clock transition, and the measurement and control of atom-atom collisions.
Atomic clocks constitute a fundamental tool for time and frequency metrology and their application is widespread in many technological fields. In particular, the International System of units (SI) defines the second on a microwave transition of Caesium atoms. The realization is made by clocks reaching uncertainties of few parts in 10^−16, making the second to be the quantity realized with the smallest uncertainty in the SI. However, a new generation of atomic clocks, called optical clocks, have already demonstrated to surpass Caesium standards both in accuracy and stability. The research performed during my PhD activity has been focused on the development and characterization of Ytterbium (Yb) optical lattice clocks. These systems operate with a large number of ultra-cold neutral atoms having a clock transition in the visible region of the electromagnetic spectrum. The atomic sample is trapped in an periodical optical potential called optical lattice that gives the advantage to interrogate many quantum absorbers for an extended time, with small perturbations, allowing to achieve an unprecedented stability and accuracy. The main experimental work has been carried out in the laboratories of the Istituto Nazionale di Ricerca Metrologica (INRIM) in Turin, where several atomic clocks are present including the Italian primary frequency standard, the Caesium fountain ITCsF2, and where an Yb optical lattice clock is currently being developed. This thesis describes the functioning of the system along with the full characterization of systematic effects, the complete uncertainty budget and its first absolute frequency measurement against the primary frequency standard. The clock exhibited an accuracy of 1.6 × 10^−16 and the comparison with the Cs fountain resulted in a frequency of f171Yb = 518 295 836 590 863.59(31) Hz, limited by the fountain uncertainty. This measurement is in agreement with the ytterbium frequency recommended as a secondary representation of the second in the SI and constitutes the first measurement of a Yb clock in Europe and the second one in the world against a primary frequency standard. Several upgrades have been applied after the absolute measurement. In particular, the design and realization a system capable to frequency stabilize several lasers on a single optical cavity is illustrated. This cavity has been implemented to lock the lasers used to cool and trap the atomic sample at 399, 556 and 759nm using the offset sideband locking technique, a modified version of the Pound–Drever–Hall method that gives an extended frequency tunability. The system proved to be an easy-to-use and reliable tool for the experimental activity showing a linewidth below 300 Hz at 556nm, which is the wavelength that requires the most stringent performance, and a long term drift below 20 kHz per day at 759nm. That is suitable for operating the lattice laser with a light shift uncertainty below 1 × 10−18. During my PhD I have been guest researcher at the National Institute of Standards and Technology (NIST) of Boulder, Colorado, for nine months in 2016. In these laboratories two Yb optical lattice clocks are operative. I worked on the instability measurement of a composite system exploiting the two clocks to suppress the Dick effect, called zero-dead-time (ZDT) clock, which demonstrated a fractional instability of 3 × 10−17 at 1s. The two clocks can also be operated to extend the interrogation time obtaining a spectroscopic feature after 4 s of 120(20)mHz corresponding to a quality factor Q > 4 × 10^15. I also worked on the characterization of several systematic shifts that allowed to complete the uncertainty budget of the clocks at 1.6 × 10^−18. In particular, I contributed to the characterization of lattice light shifts considering the effect of atomic sample temperature and the identification of a metrological regime called operational magic frequency where frequency shifts are insensitive to changes in trap depth.
An ultra-narrow spectroscopy of clock transition with high signal-to-noise ratio is crucial for a high-performance atomic optical clock. We present a detailed study about how to obtain a Hertz-level clock transition spectrum of 171 Yb atoms. About 4 × 10 4 atoms are loaded into a one-dimensional optical lattice with a magic wavelength of 759 nm, and a long lifetime of 3 s is realized with the lattice power of 1 W. Through normalized shelving detection and spin polarization, 171 Yb clock spectroscopy with a Fourier-limited linewidth of 5.9 Hz is obtained. Our work represents a key step toward an ytterbium optical clock with high frequency stability.
We describe progress toward an optical lattice clock based on an even isotope of Yb. The 1S0 → 3P0 clock resonance in 174Yb is accessed through a magnetically induced spectroscopic technique. Using ≈1mT static magnetic fields and ≈10 μW of probe light power we generate Rabi frequencies of several hertz. The narrow spectroscopic features that result (< 10 Hz FWHM) require a highly stabilized laser at the clock transition wavelength of 578 nm. We describe a new all solid-state laser system that shows hertz level stability. In order to cancel the slow drift of the cavity, spectroscopy is performed on the clock transition to provide feedback to the laser. Using a Ca neutral atom frequency standard as a reference oscillator,we show high stability and an effective method for investigating clock frequency shift systematics.
Clocks based on optical transitions in ions and atoms are quickly moving to the forefront of the frequency standards field mainly because of the high spec-tral resolution, and therefore the potential stability and accuracy, which can be achieved. In this thesis a new optical clock based on neutral atoms trapped in an optical lattice is presented, demonstrating spectroscopy of the clock transition free of any lineshape or accuracy degradation due to atomic motion. The system simu-lates a single trapped ion clock, but allows use of thousands of atoms for improved signal to noise ratio, and clock stability. High accuracy can also be achieved as the lattice can be designed to shift the energy of the two atomic clock states equally, such that the transition frequency is unchanged. Strontium is a natural candidate for such a clock as it offers an extremely narrow optical transition (∼1 mHz) based on atomic states which are very insensitive to external fields. The strontium level structure allows efficient laser cooling to 1 µK with diode laser sources, and a con-venient wavelength of 813 nm for the zero-differential Stark shift optical lattice. The strontium lattice clock system has allowed observation of high signal-to-noise
The work described in this thesis was done at the Istituto Nazionale di Ricerca Metrologica (I.N.Ri.M.), in the optic division, within the Ph.D. of Metrology of the Politecnico di Torino. My work consisted in developing a new generation optical frequency standard based on Ytterbium atoms: the goal of this project is to realize a clock whose performance overcomes the ones of the standard which actually realizes the Second, the atomic Cesium fountain. This means realizes an apparatus which shows stability and accuracy better then the fountain ones: a possible way adopted in several new generation clock is using optical transitions, because they offer an higher quality factor of the clock transition itself, which means a better stability and the possibility to reach the 10^-17 level in the global accuracy of the clock. The fundamental idea of a clock is to realize as close as possible the basic idea under the actual definition of the Second, that is to fix the unit if time with a particular atomic transition, when the atom is completely unperturbed. Accomplishing this prescription means a lot of work in cooling down the atoms until the 10^-6 K regime using laser cooling techniques, developing a trap where atoms can be excited by the clock laser (the optical lattice) and of course a very careful frequency and amplitude stabilization of the clock laser itself is needed to reach our goal. The work described in this thesis regards the development of the two cooling stages of the Yb atoms and it is presented as follows. Chapter 1 explains with more details why an optical clock is needed to improve the accuracy of the actual realization of the Second, then describing the theoretical framework in which the clock will be developed. Chapter 2 contains a description of the experimental apparatus used, with more attention on the instrumentation related to the laser cooling of the atoms. Chapter 3 is about the characterization of the first cooling stage, both in a static and in a dynamic way, with some attention on the theoretical model developed to perform both the characterizations. Chapter 4 focuses on the development of the second cooling stage and it describes the actual state of the art of this stage
We experimentally investigate an optical clock based on ;{171}Yb (I = 1/2) atoms confined in an optical lattice. We have evaluated all known frequency shifts to the clock transition, including a density-dependent collision shift, with a fractional uncertainty of 3.4 x 10;{-16}, limited principally by uncertainty in the blackbody radiation Stark shift. We measured the absolute clock transition frequency relative to the NIST-F1 Cs fountain clock and find the frequency to be 518 295 836 590 865.2(0.7) Hz.