High-precision frequency measurement of the 423-nm Ca i line

Abstract

We have performed an accurate frequency calibration of the 4s 2 1 S 0 → 4s4p 1 P 1 principal resonance line of the neutral calcium atom at 423 nm. Doppler-free cw excitation on a Ca atomic beam was performed by utilizing a Sagnac geometry in the alignment of the excitation beams. From frequency calibrations against a frequency comb, stabilized to a global positioning system (GPS) disciplined Rb standard, the transition frequency is determined at 709 078 373.01(35) MHz for the main 40 Ca isotope. Slightly lower accuracies are obtained for the transition frequencies of the less abundant isotopes. The achieved fractional uncertainty of 5 × 10 −10 exceeds the requirements for including this transition in investigations that aim to probe a possible variation in the fine-structure constant α on cosmological time scales.

Precision spectroscopy using a partially stabilized frequency comb

“…Comparing our measured value of the 40 Ca 4s 21 S 0 → 4s4p 1 P°1 transition frequency to the value of 709 078 373.01(35) MHz from [30] shows good agreement, with our value 0.82 MHz higher. Our measurement cannot be considered definitive because the laboratory magnetic fields are not canceled.…”

Comparison of resonance frequencies of major atomic lines in 398–423 nm

“…The light shift is expected to be smaller than 0.3 MHz for all the measured transitions. This is estimated from the calculation for the shift due to neighboring transitions and from the measurements by other groups [4,5]. We have also experimentally confirmed that the light shift is negligibly small by measuring the resonance frequencies for different probe laser powers for the saturated absorption spectroscopy.…”

“…40 Ca has the abundance of 96.9 % and the nuclear spin of 0. The absolute frequency of this transition is 709 078 373.01(35) MHz [4]. The excitation spectrum is observed by using the cold atomic beam (Fig.…”

“…In this wavelength range the use of the etalon is especially efficient, since it is very difficult to directly measure laser frequencies with optical frequency combs for this range without using a frequency doubling system. The etalon resonance frequencies are calibrated to the absolute frequencies of Ca at 423 nm and Rb at 422 nm, whose uncertainties are less than 0.4 MHz [4,5]. Except these two atomic lines, the absolute frequencies of the atomic lines observed in this paper have not been determined precisely so far.…”

Hanle Detection for Optical Clocks

“…The frequency range is calibrated by the Ca atom isotope. The 423 nm transition frequency of 40 Ca and 44 Ca isotope is apart from 773.8 MHz [27]. …”