Yb Optical Lattice Clock Research Articles

Improving lattice-light-shift uncertainty of an 171Yb optical clock with optimized cooling and trapping lasers

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.

Intercontinental frequency ratio measurement of 171 Yb and 88 Sr optical lattice clocks

We report a remote, intercontinental frequency comparison between 171Yb (NMIJ) and 88Sr (UMK) optical lattice clocks. The frequencies were compared via the Global Positioning System Precise Point Positioning technique over the period from 10 March 2020, 00:00 UTC until 15 March 2020, 00:00 UTC. The comparison of the optical clocks yields results with a relative frequency uncertainty of 6.3×10−15 .

Year-long optical time scale with sub-nanosecond capabilities

An atomic time scale is a method for marking events and the passage of time by using atomic frequency standards. Thanks to the superior performance of atomic clocks based on optical transitions, time scales generated with optical clocks have the potential to be more accurate and stable than those based on microwave clocks. In this work, we demonstrate an experimental optical time scale based on the INRiM Yb optical lattice clock and a hydrogen maser as a flywheel oscillator, showing sub-nanosecond accuracy over months-long periods and nanosecond accuracy over a 1-year period. The obtained results show that optical time scales have competitive performances even when the optical clock has a limited and non-uniformly distributed up-time. Consequently, we are working to include the Yb clock within the ensemble of clocks routinely used for the generation of the Italian time scale. Furthermore, these results represent a crucial step towards the future redefinition of the second of the International System of Units based on an optical transition.

Evaluation of the relativistic redshift in frequency standards at KRISS

Relativistic redshift correction should be accurately considered in frequency comparisons between frequency standards. In this study, we evaluated the relativistic redshift at Korea Research Institute of Standards and Science (KRISS) using three different methods, depending on whether the approach was traditional or modern or whether the geopotential model was global or local. The results of the three methods agreed well with one another, and the height of an Yb optical lattice clock (KRISS-Yb1) was determined to be 75.15 m with an uncertainty of 0.04 m with respect to the conventionally adopted equipotential surface W0CGPM , the value of which is defined to be 62 636 856.0 m2s−2. Accordingly, the relativistic redshift of KRISS-Yb1 was evaluated to be 8.193(4) × 10−15. These data are applicable to the frequency standards at KRISS, one of which regularly participates in the calibration of the International Atomic Time.

A Yb optical clock with a lattice power enhancement cavity

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.

Evaluation of systematic frequency shift and uncertainty of an optical clock based on Bayesian hierarchical model

Optical clocks are the most accurate time–frequency measurement devices currently available. In order to evaluate the performance quickly and conveniently, we build a Bayesian hierarchical model to initially evaluate the systematic frequency shift and the corresponding uncertainty of the 171Yb optical lattice clock. Significant time savings is achieved through the Bayesian model compared to the traditional method during the absolute frequency measurement. The model is implemented via the R2jags package in R, samples are drawn from the target posterior distribution of the parameters by the Markov Chain Monte Carlo simulation (MCMC). The simulation result of the total systematic frequency shift is −1.595 Hz, with an uncertainty of 6.71 × 10−16, which is close to the actual measurement value. This method provides us a new way to evaluate optical clocks.

Search for Ultralight Dark Matter from Long-Term Frequency Comparisons of Optical and Microwave Atomic Clocks.

We search for ultralight scalar dark matter candidates that induce oscillations of the fine structure constant, the electron and quark masses, and the quantum chromodynamics energy scale with frequency comparison data between a ^{171}Yb optical lattice clock and a ^{133}Cs fountain microwave clock that span 298days with an uptime of 15.4%. New limits on the couplings of the scalar dark matter to electrons and gluons in the mass range from 10^{-22} to 10^{-20} eV/c^{2} are set, assuming that each of these couplings is the dominant source of the modulation in the frequency ratio. The absolute frequency of the ^{171}Yb clock transition is also determined as 518 295 836 590 863.69(28)Hz, which is one of the important contributions toward a redefinition of the second in the International System of Units.

Evaluation of the blackbody radiation shift of an Yb optical lattice clock at KRISS

As optical clocks are improved to reach the frequency uncertainty below the 10−17 level, the frequency shift due to the blackbody radiation (BBR) has been one of the major systematic effects hindering further improvement. To evaluate the BBR shift of an Yb optical lattice clock at KRISS, we installed an in-vacuum BBR shield and made radiation thermometry using a black-coated-sphere thermal probe. After we quantitatively measured the conduction loss of the thermal probe and the effects of all the external radiation sources, we determined the temperature at the atom trap site with an uncertainty of 13 mK, which corresponds to an uncertainty of 0.22 mHz in the clock frequency (a fractional frequency of 4.2 × 10−19). The total uncertainty of the BBR shift including the atomic response is 9.5 × 10−19.

Effective sideband cooling in an ytterbium optical lattice clock

Sideband cooling is a key technique for improving the performance of optical atomic clocks by preparing cold atoms and single ions into the ground vibrational state. In this work, we demonstrate detailed experimental research on pulsed Raman sideband cooling in a 171Yb optical lattice clock. A sequence comprised of interleaved 578 nm cooling pulses resonant on the 1st-order red sideband and 1388 nm repumping pulses is carried out to transfer atoms into the motional ground state. We successfully decrease the axial temperature of atoms in the lattice from 6.5 μK to less than 0.8 μK in the trap depth of 24 μK, corresponding to an average axial motional quantum number 〈 nz 〉 < 0.03. Rabi oscillation spectroscopy is measured to evaluate the effect of sideband cooling on inhomogeneous excitation. The maximum excitation fraction is increased from 0.8 to 0.86, indicating an enhancement in the quantum coherence of the ensemble. Our work will contribute to improving the instability and uncertainty of Yb lattice clocks.

Absolute frequency measurement of the 171Yb optical lattice clock at KRISS using TAI for over a year

We report a measurement of the absolute frequency of the 1S0–3P0 transition in the 171Yb optical lattice clock at KRISS (KRISS-Yb1) for 14 months, which was referenced to the SI second by primary and secondary standards worldwide via International Atomic Time. The determined absolute frequency is 518 295 836 590 863.75(14) Hz with the relative frequency uncertainty of 2.6 × 10−16, which agrees well with other reports. This result is expected to contribute to the future update of the CIPM recommendation frequency of the secondary frequency standards.

Characterization and Suppression of Background Light Shifts in an Optical Lattice Clock

Experiments involving optical traps often require careful control of the ac Stark shifts induced by strong confining light fields. By carefully balancing light shifts between two atomic states of interest, optical traps at the magic wavelength have been especially effective at suppressing deleterious effects stemming from such shifts. Highlighting the power of this technique, optical clocks today exploit Lamb-Dicke confinement in magic-wavelength optical traps, in some cases realizing shift cancellation at the ten parts per billion level. Theory and empirical measurements can be used at varying levels of precision to determine the magic wavelength where shift cancellation occurs. However, lasers exhibit background spectra from amplified spontaneous emission or other lasing modes which can easily contaminate measurement of the magic wavelength and its reproducibility in other experiments or conditions. Indeed, residual light shifts from laser background have plagued optical lattice clock measurements for years. In this work, we develop a simple theoretical model allowing prediction of light shifts from measured background spectra. We demonstrate good agreement between this model and measurements of the background light shift from an amplified diode laser in an Yb optical lattice clock. Additionally, we model and experimentally characterize the filtering effect of a volume Bragg grating bandpass filter, demonstrating that application of the filter can reduce background light shifts from amplified spontaneous emission well below the $10^{-18}$ fractional clock frequency level. This demonstration is corroborated by direct clock comparisons between a filtered amplified diode laser and a filtered titanium:sapphire laser.

Improved frequency ratio measurement with 87Sr and 171Yb optical lattice clocks at NMIJ

We report improved frequency ratio measurement with 87Sr and 171Yb optical lattice clocks at the National Metrology Institute of Japan. The 87Sr optical lattice clock is enhanced with several major modifications and is re-evaluated with a reduced uncertainty of 1.1 × 10−16. We employed a 171Yb optical lattice clock with an uncertainty of 4 × 10−16 that was developed for contributing to International Atomic Time. The measurement result is ν Yb/ν Sr = 1.207 507 039 343 338 58(49)sys(6)stat with a fractional uncertainty of 4.1 × 10−16, which is 3.4 times smaller than our previous measurement result.

Demonstration of the nearly continuous operation of an 171Yb optical lattice clock for half a year

Optical lattice clocks surpass primary Cs microwave clocks in frequency stability and accuracy, and are promising candidates for a redefinition of the second in the International System of Units (SI). However, the robustness of optical lattice clocks has not yet reached a level comparable to that of Cs fountain clocks which contribute to International Atomic Time (TAI) by the nearly continuous operation. In this paper, we report the long-term operation of an 171Yb optical lattice clock with a coverage of 80.3% for half a year including uptimes of 93.9% for the first 24 days and 92.6% for the last 35 days. This enables a nearly dead-time-free frequency comparison of the optical lattice clock with TAI over months, which provides a link to the SI second with an uncertainty of low 10−16. By using this link, the absolute frequency of the 1SP0 clock transition of 171Yb is measured as 518 295 836 590 863.54(26) Hz with a fractional uncertainty of 5.0 × 10−16. This value is in agreement with the recommended frequency of 171Yb as a secondary representation of the second.

Robust frequency stabilization and linewidth narrowing of a laser with large intermittent frequency jumps using an optical cavity and an atomic beam.

An experimental method is developed for robust frequency stabilization using a high-finesse cavity when the laser exhibits large intermittent frequency jumps. This is accomplished by applying an additional slow feedback signal from Doppler-free fluorescence spectroscopy in an atomic beam with increased frequency locking range. As a result, a stable and narrow-linewidth 556 nm laser maintains the frequency lock status for more than a week and contributes to more accurate evaluation of the Yb optical lattice clock. In addition, the reference optical cavity is supported at vibration-insensitive points without any vibration isolation table, making the laser setup more simple and compact.

Study of optical clocks based on ultracold 171Yb atoms**Project supported by the National Key Basic Research and Development Program of China (Grant Nos. 2016YFA0302103, 2017YFF0212003, and 2016YFB0501601), the Municipal Science and Technology Major Project of Shanghai, China (Grant No. 2019SHDZX01), the National Natural Science Foundation of China (Grant No. 11134003), and the Excellent Academic Leaders Program of Shanghai, China (Grant

The optical atomic clocks have the potential to transform global timekeeping, relying on the state-of-the-art accuracy and stability, and greatly improve the measurement precision for a wide range of scientific and technological applications. Herein we report on the development of the optical clock based on 171Yb atoms confined in an optical lattice. A minimum width of 1.92-Hz Rabi spectra has been obtained with a new 578-nm clock interrogation laser. The in-loop fractional instability of the 171Yb clock reaches 9.1 × 10−18 after an averaging over a time of 2.0 × 104 s. By synchronous comparison between two clocks, we demonstrate that our 171Yb optical lattice clock achieves a fractional instability of .

Direct measurement of the frequency ratio for Hg and Yb optical lattice clocks and closure of the Hg/Yb/Sr loop.

We performed the first direct measurement of the frequency ratio between a mercury (199Hg) and an ytterbium (171Yb) optical lattice clock to find νHg/νYb = 2.177 473 194 134 565 07(19) with the fractional uncertainty of 8.8 × 10-17. The ratio is in excellent agreement with expectations from the ratios νHg/νSr and νYb/νSr obtained previously in comparisons against a strontium (87Sr) optical lattice clock. The completed closure (νHg/νYb)(νYb/νSr)(νSr/νHg) - 1 = 0.4(1.3) × 10-16 tests the frequency reproducibility of the optical lattice clocks beyond what is achievable in comparison against the current realization of the second in the International System of Units (SI).

Modeling light shifts in optical lattice clocks

We present an extended model for the lattice-induced light shifts of the clock frequency in optical lattice clocks, applicable to a wide range of operating conditions. The model extensions cover radial motional states with sufficient energies to invalidate the harmonic approximation of the confining potential. We reevaluate lattice-induced light shifts in our Yb optical lattice clock with an uncertainty of 6.1E-18 under typical clock operating conditions.

Development of 8-branch Er:fiber frequency comb for Sr and Yb optical lattice clocks.

We demonstrate an 8-branch Er:fiber frequency comb with seven application ports, which can be individually optimized for applications with different wavelengths. The beat between the comb and a cw laser has a signal-to-noise ratio exceeding 30 dB at a resolution bandwidth of 300 kHz. The 8-branch frequency comb is used to perform frequency locking for four repumping and lattice lasers, and the frequency measurement of two clock lasers of strontium and ytterbium optical lattice clocks. We have achieved reliable optical lattice clock operation, thanks to the stable frequency locking and measurement obtained by using the 8-branch frequency comb. The developed frequency comb is a powerful experimental tool for various applications, including not only optical lattice clocks, but also research on quantum optics that use many frequency-stabilized lasers.

Uncertainty Evaluation of an 171Yb Optical Lattice Clock at NMIJ.

We report an uncertainty evaluation of an 171Yb 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 6s2 1S0-6s6p 3P0 clock 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 aZ are 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) μ Hz and aZ=-6.6(3) Hz/mT2. These improved coefficients should reduce the total systematic uncertainties of Yb lattice clocks at other institutes.

Decomposed description of Ramsey spectra under atomic interactions

We introduce a description of Ramsey spectra under atomic interactions as a sum of decomposed components with differing dependence on interaction parameters. This description enables intuitive understanding of the loss of contrast and asymmetry of Ramsey spectra. We derive a quantitative relationship between the asymmetry and atomic interaction parameters, which enables their characterization without changing atom density. The model is confirmed through experiments with a Yb optical lattice clock.