Is Calcium A Molecule?

Have you ever considered that is calcium amolecule? The world's most accurate physical quantity currently measured isoptical frequency, which has reached an accuracy of 10-18 orders of magnitude.The optical frequency scale is a set of high-precision measurement sensors usedto achieve high-precision time/frequency measurements. Atomic optical frequencyscale research is a typical representative of the pursuit of extreme precisionmeasurement, and it is also the most accurate atomic frequency scale available.Scientists study is calcium a molecule. High-precision optical frequency scalehelps to improve the accuracy of the definition of fundamental physicalquantities, the measurement of whether fundamental physical constants changewith time, and the testing of fundamental physical laws, thus advancing fundamentalphysics research and exploring new physics.

Scientists explore is calcium a molecule.Recently, a research team from the Institute of Precision Measurement Scienceand Technological Innovation of the Chinese Academy of Sciences, Gao Kling, hasdeveloped a calcium ion optical frequency scale with an uncertainty of 3x10-18(equivalent to 10.5 billion years without a second difference), becoming thefifth international optical frequency scale with an uncertainty index at thatlevel. The related research results were published in Physical ReviewApplications.

So, is calcium a molecule? The Institute ofPrecision Measurement has been conducting experimental research on calcium ionoptical frequency scale since 2000, exploring a series of key scientific andtechnical problems such as suppressing ion motion effects, solving ultra-narrowlinewidth lasers, and precise control of experimental environment effects, andrealized the first domestic optical frequency scale with an uncertainty of7.8X10-16 in 2011 [PRA 84, 053841 (2011)]; improving the performance of clockleap laser The uncertainty of the optical frequency scale was improved to5.1x10-17 in 2016 by using a magic confinement field to suppress themicro-motion effect [PRL 116, 013001 (2016)]; the uncertainty of the calciumion optical frequency scale was improved to 1.3x10-17 in 2019 by experimentallyaccurate measurement of the magic confinement field frequency and thedifference of the static polarization rate of the clock leap obtained [Phys.Rev. A 99, 011401(R) (2019)].

The optical frequency scale system iscomplex with a reference atom/ion system, and its measurement accuracy isaffected by various environmental factors and conditions such as electric,magnetic, and blackbody radiation fields, and lasers for optical frequencymeasurements. The main factor limiting the uncertainty of the calcium ionoptical frequency scale into 10-18 is the blackbody radiation frequency shift(BBR shift). Blackbody radiation is correlated with the chosen optical frequencyscale system and is proportional to the fourth power of the ambienttemperature, which is very sensitive to temperature. In the current field ofoptical frequency scale research, except for a few reference systems such asAl+ that are insensitive to BBR shift, the optical clock is not less than 10-15magnitudes of frequency shift at room temperature environment; for most currentatomic frequency scales, including cesium fountain clocks, single ion opticalfrequency scales and optical lattice atomic optical frequency scales, the totalsystem uncertainty is limited by the BBR frequency shift uncertainty.Previously, some studies have accurately controlled and evaluated the ambienttemperature of strontium atomic optical frequency scale by an active temperaturecontrol system consisting of 16 servo loops and more than 30 sensors; somestudies have designed a vacuum heat shield and accurately evaluated the ambienttemperature of ytterbium atomic optical frequency scale; some studies have useda complex liquid helium cooling technique to reduce the blackbody radiationeffect of strontium atomic optical frequency scale. In contrast, the vacuumsystem of the ion optical frequency scale has ion traps and heating effectsthat make the evaluation of the blackbody radiation frequency shift moredifficult.

In order to solve the blackbody radiationfrequency shift problem of calcium ion light spectrometer, Gauquelin's teamproposes to place the ion trap in liquid nitrogen cryogenic environment (78Kelvin), which reduces the sensitivity of blackbody radiation frequency shiftto temperature by 64 times compared with room temperature (300 Kelvin), and thecryogenic solution can greatly reduce the blackbody radiation frequency shiftand its uncertainty. Compared with liquid helium systems, liquid nitrogensystems are relatively inexpensive and relatively simple systems. However, theimplementation of ion light spectroscopy in the cryogenic environment of liquidnitrogen is challenging. Liquid nitrogen evaporates and needs to be replenishedcontinuously during use, and changes in liquid nitrogen volume during systemoperation tend to cause ion position shifts, resulting in loss of fluorescencesignal, large first-order Doppler shifts, and large ion micromotion. How to ensurehigh accuracy for other physical and environmental effects at low temperatures.After several iterations of design and error correction, the research teamreduced the ion trap movement in vertical and horizontal directions by settingthe liquid nitrogen container to a better thermal conductivity oxygen-freecopper material and adding a thimble between the liquid nitrogen container andthe vacuum cavity, among other designs. The first-order Doppler frequency shiftcaused by the small movement of the ion trap was also accurately evaluated bythe frequency ratio between the two phase directional clock leap detectionlasers, and the effect of liquid nitrogen volume change on the residualelectric field in the ion trap was reduced by daily optimization and evaluationof the micro-motion three-dimensional sideband spectra using the micro-motionthree-dimensional sideband spectra, and a specific main magnetic fielddirection was set to reduce the effect of liquid nitrogen volume change on theclock leap electric quadrupole The method is the first in the world to achieveliquid nitrogen cryogenic calcium ion optical frequency labeling with anuncertainty of 3x10-18.