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Chinese Physics B

A Yb optical clock with a lattice power enhancement cavity

Chunyun 春云 Wang 王, Yuan 远 Yao 姚, Haosen 浩森 Shi 师, Hongfu 洪浮 Yu 于, Longsheng 龙生 Ma 马, Yanyi 燕义 Jiang 蒋

Published Dec 29, 2023• 6 citations
Publisher

Abstract

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.

DOI
10.1088/1674-1056/ad1986

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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.
Optical clocks benefit from tight atomic confinement enabling extended interrogation times as well as Doppler- and recoil-free operation. However, these benefits come at the cost of frequency shifts that, if not properly controlled, may degrade clock accuracy. Numerous theoretical studies have predicted optical lattice clock frequency shifts that scale nonlinearly with trap depth. To experimentally observe and constrain these shifts in an ^{171}Yb optical lattice clock, we construct a lattice enhancement cavity that exaggerates the light shifts. We observe an atomic temperature that is proportional to the optical trap depth, fundamentally altering the scaling of trap-induced light shifts and simplifying their parametrization. We identify an "operational" magic wavelength where frequency shifts are insensitive to changes in trap depth. These measurements and scaling analysis constitute an essential systematic characterization for clock operation at the 10^{-18} level and beyond.
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.
A cavity-stabilized 578 nm laser is used to probe the clock transition of ytterbium atoms trapped in optical lattice sites. We obtain a Fourier-limited 4.2-Hz-linewidth Rabi spectrum and a Ramsey spectrum with fringe linewidth of 3.3 Hz. Based on one of the spectra, the 578 nm laser light is frequency-stabilized to the center of the transition to achieve a closed-loop operation of an optical clock. Based on interleaved measurement, the frequency instability of a single optical clock is demonstrated to be 5.4 × 10-16/√τ.
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.
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.
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.