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Journal of the Korean Physical Society

An Yb optical lattice clock: Current status at KRISS

Dai-Hyuk Yu, Chang Yong Park, Won-Kyu Lee, Sangkyung Lee, Sang Eon Park, Jongchul Mun, Sang-Bum Lee, Taeg Yong Kwon

Published Aug 1, 2013• 12 citations
Publisher

Abstract

The current status of an Yb optical lattice clock at the Korea Research Institute of Standards and Science (KRISS) is reported. The systematic uncertainty of the Yb clock in the first accuracy evaluation was 1.5 × 10 −14 [Park et al. , Metrologia 50, 119 (2013)]. The uncertainty was dominated by the large uncertainties in the lattice ac Stark shift and the collisional shift, which were mainly limited by the large linewidth and jitter of the clock laser. Recently, a highly stable clock laser at 578 nm was developed with a short-term linewidth of 3.5 Hz and a frequency jitter of about 25 Hz at 1 s and 10 s measurement times, respectively. The long-term frequency drift showed only a linear dependence on time, confirming that the temperature of the super-cavity was maintained a zero coefficient of thermal expansion. The frequency of the lattice laser at 759 nm was phase locked to the optical frequency comb and could be stabilized at the “absolute” frequency of the “magic wavelength”, to within a 1-MHz uncertainty. This improvement greatly reduced the fractional uncertainty due to the lattice ac Stark shift down to 2 × 10 −16 . The systematic uncertainty of the clock is currently 5.3 × 10 −15 and is dominated by the collisional frequency shift.

DOI
10.3938/jkps.63.883

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The current status of an Yb optical lattice clock at the Korea Research Institute of Standards and Science (KRISS) is reported. The systematic uncertainty of the Yb clock in the first accuracy evaluation was 1.5 × 10 −14 [Park et al. , Metrologia 50, 119 (2013)]. The uncertainty was dominated by the large uncertainties in the lattice ac Stark shift and the collisional shift, which were mainly limited by the large linewidth and jitter of the clock laser. Recently, a highly stable clock laser at 578 nm was developed with a short-term linewidth of 3.5 Hz and a frequency jitter of about 25 Hz at 1 s and 10 s measurement times, respectively. The long-term frequency drift showed only a linear dependence on time, confirming that the temperature of the super-cavity was maintained a zero coefficient of thermal expansion. The frequency of the lattice laser at 759 nm was phase locked to the optical frequency comb and could be stabilized at the “absolute” frequency of the “magic wavelength”, to within a 1-MHz uncertainty. This improvement greatly reduced the fractional uncertainty due to the lattice ac Stark shift down to 2 × 10 −16 . The systematic uncertainty of the clock is currently 5.3 × 10 −15 and is dominated by the collisional frequency shift.
We report an uncertainty evaluation of an171Yb optical lattice clock with a total fractional uncertainty of 3.6 × 10-16, which is mainly limited by the lattice-induced light shift and the blackbody radiation shift. Our evaluation of the lattice-induced light shift, the density shift, and the second-order Zeeman shift is based on an interleaved measurement where we measure the frequency shift using the alternating stabilization of a clock laser to the 6s21S0-6s6p3P0clock transition with two different experimental parameters. In the present evaluation, the uncertainties of two sensitivity coefficients for the lattice-induced hyperpolarizability shift d incorporated in a widely used light shift model by RIKEN and the second-order Zeeman shift aZare improved compared with the uncertainties of previous coefficients. The hyperpolarizability coefficient d is determined by investigating the trap potential depth and the light shifts at the lattice frequencies near the two-photon transitions 6s6p3P0-6s8p3P0, 6s8p3P2, and 6s5f3F2. The obtained values are d=-1.1(4) μHz and aZ=-6.6(3) Hz/mT2. These improved coefficients should reduce the total systematic uncertainties of Yb lattice clocks at other institutes.
Atoms confined in the optical lattice can be interrogated with Doppler- and recoil-free operation. However, if not properly controlled, the optical lattice may limit clock accuracy. To improve the lattice-light-shift uncertainty, the cooling and trapping lasers' frequency stability is optimized, and the atom's signal stability is enhanced. A ring-cavity Ti:sapphire laser is locked to the optical frequency comb, which is referenced to a 578 nm ultra-stable laser, and the beat note's stability is on the order of 10−16. Using a 10 cm Fabry–Pérot cavity referenced to the Ti:sapphire laser, the optical frequency stability is transferred to the 399 nm cooling laser, creating favorable conditions for evaluating the lattice-light-shift accurately. We reevaluate lattice-light-shift in our 171Yb optical lattice clock with an uncertainty of 8.1 × 10−18, which is an order lower than our previous result, and the magic frequency is determined to be 394 798 266.6(1.3) MHz.
This paper describes the current status of an ytterbium lattice clock under development at KRISS. After the success of 399 nm magneto-optical-trap (MOT) at a temperature 1 mK, we constructed the 2ndstage cooling laser system at a wavelength of 556 nm to cool atoms further down to several muK. We are also developing an ultra-narrow linewidth laser at a wavelength of 578 nm to probe the clock transition by doubling the frequency output of a commercial 1156 nm ECDL. Efforts to reduce the linewidth are under way.
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.
We construct a power enhancement cavity to form an optical lattice in an ytterbium optical clock. It is demonstrated that the intra-cavity lattice power can be increased by about 45 times, and the trap depth can be as large as 1400E r when laser light with a power of only 0.6 W incident to the lattice cavity. Such high trap depths are the key to accurate evaluation of the lattice-induced light shift with an uncertainty down to ∼1 × 10−18. By probing the ytterbium atoms trapped in the power-enhanced optical lattice, we obtain a 4.3 Hz-linewidth Rabi spectrum, which is then used to feedback to the clock laser for the close loop operation of the optical lattice clock. We evaluate the density shift of the Yb optical lattice clock based on interleaving measurements, which is –0.46(62) mHz. This result is smaller compared to the density shift of our first Yb optical clock without lattice power enhancement cavity mainly due to a larger lattice diameter of 344 μm.
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.