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Physical Review Applied

Suppressing background spectra of lattice lasers in strontium optical clocks

Zhi-Peng Jia, Xing-Yang Cui, Yan-Jun Xie, Xiang Zhang, Guo-Zhen Niu, Xiao-Yong Liu, Qingqing Zhu, Jie Li, Han-Ning Dai

Published Jan 6, 2025• 4 citations
Publisher

Abstract

Recent assessments have enhanced the evaluation of the lattice ac Stark shift in optical clocks to a fractional frequency level below ${10}^{\ensuremath{-}18}$. With this level of accuracy, meticulous attention is required to address shifts from the background spectrum of the lattice laser. Therefore, it is essential for the lattice laser to exhibit a minimal background spectrum. In this investigation, we employ a system based on sum-frequency generation (SFG) of the fiber-amplified lasers as the source for the strontium optical-clock lattice laser. The spectrum purity of this laser system has been confirmed through a heterodyne measurement involving a titanium:sapphire ($\mathrm{Ti}$:$\mathrm{Sa}$) laser. Consequently, we have characterized the background light shift resulting from its background spectrum, which is more than one order of magnitude lower than that of a tapered amplifier (TA). Through the implementation of carefully selected spectral filtering, the background light shift can be suppressed to a level of mid-${10}^{\ensuremath{-}20}$. Additionally, the suitability of the SFG laser is also experimentally verified in a strontium clock.

DOI
10.1103/physrevapplied.23.014014

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Recent assessments have enhanced the evaluation of the lattice ac Stark shift in optical clocks to a fractional frequency level below 10\ensuremath−18{10}^{\ensuremath{-}18}10\ensuremath−18. With this level of accuracy, meticulous attention is required to address shifts from the background spectrum of the lattice laser. Therefore, it is essential for the lattice laser to exhibit a minimal background spectrum. In this investigation, we employ a system based on sum-frequency generation (SFG) of the fiber-amplified lasers as the source for the strontium optical-clock lattice laser. The spectrum purity of this laser system has been confirmed through a heterodyne measurement involving a titanium:sapphire (Ti\mathrm{Ti}Ti:Sa\mathrm{Sa}Sa) laser. Consequently, we have characterized the background light shift resulting from its background spectrum, which is more than one order of magnitude lower than that of a tapered amplifier (TA). Through the implementation of carefully selected spectral filtering, the background light shift can be suppressed to a level of mid-10\ensuremath−20{10}^{\ensuremath{-}20}10\ensuremath−20. Additionally, the suitability of the SFG laser is also experimentally verified in a strontium 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 cancelation 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 cancelation occurs. However, lasers exhibit background spectra from amplified spontaneous emission or other lasing modes that 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 a 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 ${10}^{\ensuremath{-}18}$ fractional clock frequency level. This demonstration is corroborated by direct clock comparisons between a filtered amplified diode laser and a filtered titanium:sapphire laser.
Abstract Results of theoretical evaluations of uncertainties arising from the atom-lattice interaction effects on the clock frequency are presented for Sr atoms. Possibilities for simultaneous tuning of optical lattice frequency and intensity parameters are discussed aiming at minimizing uncertainties induced by interaction of Sr atoms with the field of magic-wavelength optical lattice. The fractional contributions to the clock-frequency shift from the spontaneous emission of the lattice-beam amplifier after having passed the optical fiber are determined at the level below 10 −18 .
In this paper the method for reducing the shift of the "clock" resonance by the example of 87 Sr has been described. This shift arises due to the presence in the optical spectrum of the lattice laser of the incoherent component. This component is a traveling wave. It is shown that the effective decrease of the impact on the shift of the "clock" resonance by introducing the interference filter is achieved. This filter was inserted directly into the optical scheme after the laser source. Examples of the optical lattice spectra with and without filter are considered. The estimates of the efficiency ratio of the use of the interference filters to improve the final uncertainty of the optical reference are made. It is shown that the filter reduces the relative uncertainty from the impact of the incoherent component of the optical radiation of the optical lattice by two orders of magnitude. It should help to reduce the lower bound for the total uncertainty limit of the optical frequency clocks down to 10 −18 .
We report on the magic wavelength measurement of our optical lattice clock based on fermion strontium atoms at the National Institute of Metrology (NIM). A Ti:sapphire solid state laser locked to a reference cavity inside a temperature-stabilized vacuum chamber is employed to generate the optical lattice. The laser frequency is measured by an erbium fiber frequency comb. The trap depth is modulated by varying the lattice laser power via an acousto-optic modulator. We obtain the frequency shift coefficient at this lattice wavelength by measuring the differential frequency shift of the clock transition of the strontium atoms at different trap depths, and the frequency shift coefficient at this lattice wavelength is obtained. We measure the frequency shift coefficients at different lattice frequencies around the magic wavelength and linearly fit the measurement data, and the magic wavelength is calculated to be 368554672(44) MHz.
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
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.