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Optical lattice clocks and frequency comparisons

Hidetoshi Katori

Published Jun 1, 2010• 0 citations
Publisher

Abstract

To date, optical clocks based on singly trapped ions and ultracold neutral atoms trapped in the Stark-shift-free optical lattices are regarded as promising candidates for future atomic clocks. Since 2006, “optical lattice clocks” have been evaluated close to the Cs clocks' uncertainty limit internationally. One of the essential experimental challenges in the development of “optical lattice clocks” is to find out the better lattice geometries as well as interrogated atom species (including their quantum statistics) that bring out the potential performance of the clock scheme, taking into account the collisional frequency shift, the Black body radiation (BBR) shift, the atomic multipolar and hyperpolarizability effects.

DOI
10.1109/cpem.2010.5544504

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To date, optical clocks based on singly trapped ions and ultracold neutral atoms trapped in the Stark-shift-free optical lattices are regarded as promising candidates for future atomic clocks. Since 2006, “optical lattice clocks” have been evaluated close to the Cs clocks' uncertainty limit internationally. One of the essential experimental challenges in the development of “optical lattice clocks” is to find out the better lattice geometries as well as interrogated atom species (including their quantum statistics) that bring out the potential performance of the clock scheme, taking into account the collisional frequency shift, the Black body radiation (BBR) shift, the atomic multipolar and hyperpolarizability effects.
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.
We experimentally investigate an optical clock based on 171^{171}171Yb (I=1/2I=1/2I=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 3.4×10−163.4 \times 10^{-16}3.4×10−16, 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.
We experimentally investigate an optical clock based on 171^{171}171Yb (I=1/2I=1/2I=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 3.4×10−163.4 \times 10^{-16}3.4×10−16, 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.
In 2013, for the first time ever, a many-atom optical lattice clock surpassed single ion clocks as the most stable, and accurate frequency standard in existence. Since this first result, continued improvements to the clock's accuracy and stability have been made. This work will focus on two of these major improvements. Firstly the frequency uncertainty was brought down to 2.5*10^-18, thanks in part to an accurate determination of the black body radiation shift, based on better characterization of the level structure of 87Sr. Secondly, a control modification will be demonstrated which provides 20% reduction in the standard deviation for the clock's frequency under differential measurement schemes. This represents a new record for clock accuracy, pushing performance towards the 10^-19 level.
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
We consider designs of optical lattice clocks in view of the quantum statistics, relevant atomic spins, and atom-lattice interactions. The first two issues lead to two optimal constructions for the clock: a one-dimensional (1D) optical lattice loaded with spin-polarized fermions and a 3D optical lattice loaded with bosons. By taking atomic multipolar interactions with the lattice fields into account, an "atomic motion insensitive" wavelength is proposed to provide a precise definition of the "magic wavelength." We then present a frequency comparison of these two optical lattice clocks: spin-polarized fermionic 87 Sr and bosonic 88 Sr prepared in 1D and 3D optical lattices, respectively. Synchronous interrogations of these two optical lattice clocks by the same probe laser allowed canceling out its frequency noise as a common mode noise to achieve a relative stability of 3 × 10 −17 for an averaging time of τ = 350 s. The scheme, therefore, provides us with a powerful means to investigate intrinsic uncertainty of the clocks regardless of the probe laser stability. We discuss prospects of the synchronous operation of the clocks on the measurement of the geoid height difference and on the search of constancy of fundamental constants.