Cold atoms play a very important role in metrology. The ultra-high or extreme-high vacuum pressure can be accurately measured by investigating the collision of cold atoms and residual atoms in the environment. We report a vacuum pressure metrometer using cold 7Li atoms as sensor atoms in a magneto-optical trap and a magnetic trap. The accuracy, uncertainty, and feasibility of the measurements in such two different traps are studied and compared in the pressure range from 10−5 to 10−8 Pa. It is found that the vacuum pressure measurement in a magneto-optical trap is more advantageous with a higher accuracy, higher signal-to-noise ratio, and shorter measurement time under current conditions. The results may be used to realize an applicable cold atom quantum vacuum standard.
Cold atoms play a very important role in metrology. The ultra-high or extreme-high vacuum pressure can be accurately measured by investigating the collision of cold atoms and residual atoms in the environment. We report a vacuum pressure metrometer using cold 7Li atoms as sensor atoms in a magneto-optical trap and a magnetic trap. The accuracy, uncertainty, and feasibility of the measurements in such two different traps are studied and compared in the pressure range from 10−5 to 10−8 Pa. It is found that the vacuum pressure measurement in a magneto-optical trap is more advantageous with a higher accuracy, higher signal-to-noise ratio, and shorter measurement time under current conditions. The results may be used to realize an applicable cold atom quantum vacuum standard.
The redefinition of the International System of Units (SI) promotes the transformation of the vacuum measurement system toward quantization, and the quantization of vacuum parameters is one of the most leading, prospective and subversive research directions in the field of international vacuum metrology, and the quantum vacuum measurement is based on the quantum effect of the microscopic particle system, and the use of optical means and the theory of quantum mechanics to realize the precision measurement of the vacuum parameters. We develop a lithium-cooled atom vacuum measurement apparatus, which mainly consists of a 7Li atom trap system and a continuous expansion vacuum system. In this work, an experimental study of ultrahigh vacuum measurement is carried out by manipulating 7Li atoms and utilizing the loss characteristics of lithium cold atoms in magneto-optical and magnetic traps, and the results show that for the four commonly used gas molecules in vacuum, namely N2, Ar, He, and H2, in the vacuum range of (3×10–8–4×10–5) Pa, the maximum measurement uncertainty is 7.6%–6.0% (k = 2) based on 7Li cold atoms, and the cold atom vacuum measurement results are in good agreement with those of the traditional ionization vacuum gauges, and their relative sensitivities are in good agreement with those of the ionization vacuum gauges, and the maximal deviation of the relative sensitivity factor is less than 8%, which verifies the accuracy and reliability of the cold-atom quantum vacuum measurements. The research results are of great significance in promoting the development of new cross-generation vacuum measurement technology and meeting the needs of space science exploration, ultra-precision measurement and high-end equipment manufacturing.
Ultra-high vacuum measurement and extremely high vacuum (UHV/XHV) measurement play an important role in high-tech fields such as deep space exploration, particle accelerators, and nanoscience; with the continuous extension of the lower limit of measurement, especially when it reaches the order of 10–10 Pa, higher requirements are placed on the accuracy of the measurement. At present, in the field of UHV/XHV measurement, ionization gauges based on the principle of neutral gas ionization are commonly applied to the vacuum measurement. However, traditional ionization vacuum gauges during use can create electronic excitation desorption effects, soft X-rays, and the effect of hot cathode outgassing, thereby affecting the accuracy of measurement and limiting the lower limit of measurement. Compared with the traditional measurement technology, this method uses the relationship between the loss rate and pressure caused by the collision of cold atoms trapped in the trap depth with the background gas to calculate the gas density and inversely calculate the vacuum pressure. Based on the intrinsic quantum mechanical properties of cold atom collisions, this method is expected to be developed into a new vacuum traceability standard. In this paper, based on the small-angle approximation and impulse approximation under the quantum scattering theory, the loss rate coefficient of the collision of 6Li cold atoms with background gas molecules is calculated. According to the ideal gas equation, the pressure inversion formula is obtained. The collision loss rate is extracted by accurately fitting the loss curve of the cold atom. In order to improve the accuracy of vacuum inversion and reduce the influence of quantum diffractive collision on loss rate measurement, the trap depth under the conditions of a certain cooling laser intensity, detuning, and magnetic field gradient is determined by the photoassociation method. Finally, in a range of 1 × 10–8–5 × 10–6 Pa, the inverted pressure value is compared with the measured value of the ionization meter, proving that this method has good accuracy and reliability in the inversion of vacuum pressure. At present, the main factor restricting the improvement of accuracy is the influence of the collision between the excited atoms in the magneto-optical trap and the background gas on the loss rate measurement. In the future, with the proportion of excited atoms and the excited state C6 coefficient to be precisely determined, the uncertainty of vacuum pressure measurement can be further reduced.
We describe how laser-cooled and trapped alkali-metal atoms have been used as pressure sensors in the high- and ultra-high-vacuum regimes operating with relative uncertainties of just over a percent at pressures from a few nPa and up to a few tens of μPa. These sensors can serve as primary standards of pressure as the determination of pressure requires only knowledge of collision cross sections between atoms and molecules, eliminating the need for calibration with other devices. The collision cross sections can be measured independently or computed with rigorous quantum methods at percent relative accuracies. We argue that this technology has advanced sufficiently to provide an alternative to the existing, expensive pressure standard in the ultra-high vacuum regime, the orifice flow standard. Experiments with trapped atoms can be used to calibrate cheaper pressure sensors (such as devices based on ionization gauges). However, the technology has not yet matured enough to be of use in other research fields because of the complexity and cost of the experiments. We propose a roadmap for this new pressure standard for vacuum metrology.
Quantum metrology technology is an advanced measurement method, which can achieve the accuracy that classical metrology is difficult to measure, and supplement the accuracy and stability defects caused by the standard quantum limit. In addition, reproducing quantum effects and quantum phenomena into units of metrology, which is obviously superior to the traditional measurement technology in terms of measurement accuracy, sensitivity and stability that could realize the quantum value transfer and traceability activities. With the development of laser cooling and trapping atom technology, the pressure measurement range of cold atom quantum-based vacuum metrology(CAVM) may reach 10-4~10-10 Pa or even lower . Benefiting from the loss rate of cold atoms in CAVM is only controlled by the collision cross section between cold atoms and gases, CAVM has the potential to develop into a emerging generation of vacuum metrology technology27. In this paper, the basic concept, structure and working principle of magneto-optical traps(MOT) for laser cooling atoms and trapping atoms are briefly described; Then, the basic principle of the CAVM is introduced. Furthermore, through collecting and sorting out a large number of literature about this technology, the advantages and key technologies of this technique in recent years are comprehensively analyzed and summarized; Finally, the developing trend of the CAVM is prospected.
We present the measurements of thermalized collisional rate coefficients for ultra-cold 7Li and 87Rb colliding with room-temperature He, Ne, N2, Ar, Kr, and Xe. In our experiments, a combined flowmeter and dynamic expansion system, a vacuum metrology standard, is used to set a known number density for the room-temperature background gas in the vicinity of the magnetically trapped 7Li or 87Rb clouds. Each collision with a background atom or molecule removes a 7Li or 87Rb atom from its trap, and the change in the atom loss rate with background gas density is used to determine the thermalized loss rate coefficients with fractional standard uncertainties better than 1.6% for 7Li and 2.7% for 87Rb. We find consistency—a degree of equivalence of less than one—between the measurements and recent quantum-scattering calculations of the loss rate coefficients [Kłos and Tiesinga, J. Chem. Phys. 158, 014308 (2023)], with the exception of the loss rate coefficient for both 7Li and 87Rb colliding with Ar. Nevertheless, the agreement between theory and experiment for all other studied systems provides validation that a quantum-based measurement of vacuum pressure using cold atoms also serves as a primary standard for vacuum pressure, which we refer to as the cold-atom vacuum standard.