Precision spectroscopy with a single ^(40)Ca^+ ion in a Paul trap

Precision measurement of the 4s2 S1/2-3d2 D5/2 clock transition based on 40Ca＋ ion at 729 nm is reported. A single 40Ca＋ ion is trapped and laser-cooled in a ring Paul trap, and the storage time for the ion is more than one month. The linewidth of a 729 nm laser is reduced to about 1 Hz by locking to a super cavity for longer than one month uninterruptedly. The overall systematic uncertainty of the clock transition is evaluated to be better than 6.5 ×10^-16. The absolute frequency of the clock transition is measured at the 10^-15 level by using an optical frequency comb referenced to a hydrogen maser which is calibrated to the SI second through the global positioning system （GPS）, The frequency value is 411 042 129 776 393.0（1.6） Hz with the correction of the systematic shifts. In order to carry out the comparison of two 40Ca＋ optical frequency standards, another similar 40Ca＋ optical frequency standard is constructed. Two optical frequency standards exhibit stabilities of 1 × 10^-14 T-1/2 with 3 days of averaging. Moreover, two additional precision measurements based on the single trapped 40Ca＋ ion are carried out. One is the 3d2Ds/2 state lifetime measurement, and our result of 1174（10） ms agrees well with the results reported in [Phys. Rev. A 62 032503 （2000）] and [Phys. Rev. A 71 032504 （2005）]. The other one is magic wavelengths for the 4s2S1/2-3d2Ds/2 clock transition; λ ｜mj｜=1/2= 395.7992（7） nm and λ｜m｜=3/2 = 395.7990（7） nm are reported, and it is the first time that two magic wavelengths for the 40Ca＋ clock-transition have been reported.

Cavity-Enhanced Saturation Spectroscopy of Molecules with sub-k Hz Accuracy

Saturation spectroscopy is frequently used to obtain sub-Doppler measurement of atomic and molecular transitions. Optical resonant cavities can be used to enhance the effective absorption path length, and the laser power inside the cavity as well to saturate very weak ro-vibrational transitions of molecules. Three different cavity-enhanced methods, cavity enhanced absorption spectroscopy, cavity ring-down spectroscopy, and noise-immune cavity enhanced optical heterodyne molecular spectroscopy (NICE-OHMS), were compared by measuring the Lamb dip of a C2H2 line at 1.4 μm using a cavity with a finesse of 120000. The center of the line was determined by different cavity-enhanced methods, each giving a sub-kHz (δv/v≈10-12) statistical uncertainty. The sensitivity and precision of different methods were analyzed and compared. As demonstrated in this study, the NICE-OHMS method is the most sensitive one, but more investigation on the systematic uncertainty is necessary before its application in metrology studies toward a sub-kHz accuracy.

Absolute dielectronic recombination rate coefficients of highly charged ions at the storage ring CSRm and CSRe

Dielectronic recombination(DR)is one of the dominant electron-ion recombination mechanisms for most highly charged ions(HCIs)in cosmic plasmas,and thus,it determines the charge state distribution and ionization balance therein.To reliably interpret spectra from cosmic sources and model the astrophysical plasmas,precise DR rate coefficients are required to build up an accurate understanding of the ionization balance of the sources.The main cooler storage ring(CSRm)and the experimental cooler storage ring(CSRe)at the Heavy-Ion Research Facility in Lanzhou(HIRFL)are both equipped with electron cooling devices,which provide an excellent experimental platform for electron-ion collision studies for HCIs.Here,the status of the DR experiments at the HIRFL-CSR is outlined,and the DR measurements with Na-like Kr25^(+)ions at the CSRm and CSRe are taken as examples.In addition,the plasma recombination rate coefficients for Ar12^(+),14^(+),Ca14^(+),16^(+),17^(+),Ni19^(+),and Kr25^(+)ions obtained at the HIRFL-CSR are provided.All the data presented in this paper are openly available at https://doi.org/10.57760/sciencedb.j00113.00092.

Laser stabilizing to ytterbium clock transition with Rabi and Ramsey spectroscopy

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/pτ.