Acta Physica Sinica

40Ca+ optical frequency standards with high accuracy

Guan Hua, Yao Huang, Chengbin Li, Kelin Gao

Published Jan 1, 2018• 4 citations

Abstract

With the development of the technologies in the lasers and the manipulation of cold atoms, the high precision optical frequency standards have been extensively studied and built in recent years. These high precision frequency standards may play an important role in establishing the new time reference, promoting the researches in the fundamental fields, fulfilling the national strategic needs, etc. In this paper, the research progress of high accuracy 40Ca+ optical frequency standard in Wuhan Institute of Physics and Mathematics (WIPM) of Chinese Academy of Sciences is presented. A new ULE super cavity is adopted for stabilizing the frequency of 729 nm clock laser, and the stability of the laser is improved now to 2×10-15 in a duration of 1-100 s. By controlling the external fields and other environmental influences, especially suppressing the micromotion effects of the trapped ion, the uncertainty of the optical frequency standard based on a single 40Ca+ is reduced to 5.5×10-17. The stability of 5×10-17 in a duration of 20000 s is achieved via the comparison between two 40Ca+ optical frequency standards. Several precision measurement experiments are performed, based on the high precision 40Ca+ optical frequency standard. The absolute value of the clock transition frequency of the 40Ca+ optical frequency standard is measured second time, using an optical comb referenced to a hydrogen maser which is calibrated via GPS referenced to UTC (NIM)) using the precise point positioning data-processing technique. The frequency offset of UTC (NIM) relative to the SI second can be evaluated through BIPM circular-T reports, and the newly measured value of m 4s 2S1/2-3m d 2D5/2 transition is adopted by CCTF-20, thus updating the recommended value of 40Ca+ optical clock transition. Besides the absolute frequency measurement, the magic wavelengths of 40Ca+ optical clock transition are measured precisely, and this work is a milestone for establishing all-optical trapped-ion clocks. The lifetime of the m 3 d 2D3/2 and m 3 d 2D5/2 state in 40Ca+ are precisely measured, too. The work mentioned above contributes to the researches of the precision measurements based on cold atomic systems.

DOI: 10.7498/aps.67.20180876

Related papers

Optical frequency standard based on a single 40Ca+

Kelin Gao

Research on the development of the optical frequency standard based on trapped and cold 40 Ca + with the 4s 2 S 1/2 -3d 2 D 5/2 clock transition at 729 nm is reported. A single calcium ion was trapped and laser cooled in the Paul trap and stay in trap for more than 15 days. The linewidth of a 729 nm laser was reduced to less than 10 Hz by locking to a cavity for longer than 50 hours uninterruptedly. The overall systematic uncertainty of the clock transition has been characterized to be better than 6.5×10 −16 . The absolute frequency of the clock transition was measured at 10 −15 level using an optical frequency comb referenced to a Hydrogen maser, which was calibrated to the SI second through the global positioning system (GPS). The frequency value was 411042129776393.0(1.6) Hz after the correction of the systematic shifts.

Chinese Science BulletinMar 1, 20139 citations

The progress of the precision spectroscopy with single trapped and laser cooled 40Ca+ ion

Hua Guan, 高克林, Yao HUANG

The experimental study at Wuhan Institute of Physics and Mathematics (WIPM), Chinese Academy of Scienses (CAS) on the precision spetroscopy based on single trapped Ca+ ion is described in this paper. A single Ca+ ion is trapped in a miniature Paul trap and laser cooled to Lamb-Dicke regime, and the ion can be trapped more than 2 months. The linewidth of the 729 nm laser for probing the Ca+ 4s2S1/2-3d2D5/2 clock transition is reduced to lessthan 1 Hz by locked to a super cavity, and the 729 nm laser can be locked more than 1 month continuously. Two similar Ca + ion optical frequency standards are set up for frequency comparison. The systematic uncertainty is evaluated to be <6 ´ 10−17 for both ftrquency standards. The frequency difference of two frequency standards is measured to be 3.0 (5.5) ´ 10−17 within 42 d. The Allan deviation for a single clock is measured to be 1×10−14τ−1/2 with a 3-d-continuous comparison, the stability for a reaches 7 ´ 10−17 at an averaging time of 20000 s. Meanwhile, the absolute frequency of the Ca+ 4s2S1/2-3d2D5/2 clock transition is measured to be 411042129776401.7(1.1) Hz with respect to the SI second through the Global Positioning System (GPSx). The precision spectroscopy experiments are carried out based on the single Ca+ ion system. The magic wavelengths for the Ca+ optical frequency standardare measured to be λ|mj|=1/2=395.7992(7) nm and λ|mj|=3/2=395.7990(7) nm, which agrees with theoretical calculation made by Tang et al. The lifetime of the 3d2D5/2 is measured to be 1174(10) ms, which is agree with the experimental measurement made by University of Oxford and University of Innsbruck and the recent theoretical calculation made by Sahoo.

Zhongguo kexue. Wulixue Lixue TianwenxueJun 17, 20160 citations

40Ca+ optical clocks in China

Hua Guan, Baolin Zhang, Huaqing Zhang +4 more

In this review, the improvements made in the past two decades for 40Ca+ optical clocks in China are presented. Uncertainty of below 1.3×10−17 was achieved by applying the “magic” RF trapping frequency and introducing a generalized Ramsey excitation scheme. The stability of 40Ca+ optical clocks has been improved to 2.5×10−15/τ with an uptime rate of 93.8% by implementing two ameliorated lock algorithms in the Ramsey excitation scheme. A long-term clock frequency comparison shows a clock stability of 6.3 × 10−18 in an averaging time of 524 000 s. A robust and transportable clock installed in an air-conditioned car trailer can achieve almost the same performance of laboratory clocks. A height difference between transportable and laboratory clocks was measured with an uncertainty of 0.33 m and the absolute frequency of 40Ca+ optical clock transitions was remeasured as 411 042 129 776 400.41(23) Hz, with a fractional uncertainty of 5.6×10−16 based on a Cs fountain clock in the National Institute of Metrology after the transportable clock was transported from Wuhan to Beijing. The author predicts that transportable single-ion optical clocks especially based on the 40Ca+ will make a significant contribution to the construction of a world-wide optical clock network and the redefinition of the unit of time in the future.

AVS Quantum ScienceOct 11, 20214 citations

Frequency measurement of the40Ca+clock transition using a LD-based clock laser and UTC(NICT)

K. Matsubara, Y. Li, Shigeo Nagano +6 more

We developed an optical frequency standard with the 4 2S1/2-3 2D5/2 electric quadrupole transition of 40Ca+ ions. Its absolute transition frequency is 411 042 129 776 390(±7) Hz. The accuracy is limited by the electric quadrupole shift and the ambient magnetic field fluctuation. To determine the absolute transition frequency with a better accuracy, we have observed two pairs of the symmetrically-splitting Zeeman components and measured the transition frequency corrected for the electric quadrupole shift. In addition, we are developing a magnetic-shielded ion-trap chamber to suppress the transition-line broadening caused by the magnetic field fluctuation.

Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIEAug 20, 20090 citations

Progress on the 40Ca+ ion optical clock*

Baolin Zhang, Yao Huang, Huaqing Zhang +4 more

Progress of the 40Ca+ ion optical clock based on the 42S1/2 – 3d 2D5/2 electric quadrupole transition is reported. By setting the drive frequency to the “magic” frequency Ω 0, the frequency uncertainty caused by the scalar Stark shift and second-order Doppler shift induced by micromotion is reduced to the 10−19 level. By precisely measuring the differential static scalar polarizability Δα 0, the uncertainty due to the blackbody radiation (BBR) shift (coefficient) is reduced to the 10−19 level. With the help of a second-order integrating servo algorithm, the uncertainty due to the servo error is reduced to the 10−18 level. The total fractional uncertainty of the 40Ca+ ion optical clock is then improved to 2.2× 10−17, whereas this value is mainly restricted by the uncertainty of the BBR shift due to temperature fluctuations. The state preparation is introduced together with improvements in the pulse sequence, and furthermore, a better signal to noise ratio (SNR) and less dead time are achieved. The clock stability of a single clock is improved to 4.8 × 10 − 15 / τ ( in seconds ) .

Chinese Physics BMay 19, 202018 citations

Realization of advanced 171Yb optical lattice frequency standard

Gianmaria Milani

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.

May 2, 20181 citation