We compare the vacuum measured by two portable cold-atom vacuum standards (pCAVSs) based on ultracold 7Li atoms. pCAVSs are quantum-based standards that use a priori scattering calculations to convert a measured loss rate of cold atoms from a conservative trap into a background gas pressure. Our pCAVS devices share the same laser system and measure the vacuum concurrently. The two pCAVSs together detected a leak with a rate on the order of 10−6 Pa l/s. After fixing the leak, the pCAVS measured pressure of about 40 nPa with 2.6% uncertainty. The two pCAVSs agree within their uncertainties, even when swapping some of their component parts. Operation of the pCAVS was found to cause some additional outgassing, on the order of 10−8 Pa l/s, which can be mitigated in the future by better thermal management.
We compare the vacuum measured by two portable cold-atom vacuum standards (pCAVSs) based on ultracold 7Li atoms. pCAVSs are quantum-based standards that use a priori scattering calculations to convert a measured loss rate of cold atoms from a conservative trap into a background gas pressure. Our pCAVS devices share the same laser system and measure the vacuum concurrently. The two pCAVSs together detected a leak with a rate on the order of 10−6 Pa l/s. After fixing the leak, the pCAVS measured pressure of about 40 nPa with 2.6% uncertainty. The two pCAVSs agree within their uncertainties, even when swapping some of their component parts. Operation of the pCAVS was found to cause some additional outgassing, on the order of 10−8 Pa l/s, which can be mitigated in the future by better thermal management.
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.
We describe the cold-atom vacuum standards (CAVS) under development at the National Institute of Standards and Technology (NIST). The CAVS measures pressure in the ultra-high and extreme-high vacuum regimes by measuring the loss rate of sub-millikelvin sensor atoms from a magnetic trap. Ab initio quantum scattering calculations of cross sections and rate coefficients relate the density of background gas molecules or atoms to the loss rate of ultra-cold sensor atoms. The resulting measurement of pressure through the ideal gas law is traceable to the second and the kelvin, making it a primary realization of the pascal. At NIST, two versions of the CAVS have been constructed: a laboratory standard used to achieve the lowest possible uncertainties and pressures, and a portable version that is a potential replacement for the Bayard-Alpert ionization gauge. Both types of CAVSs are connected to a combined extreme-high vacuum flowmeter and dynamic expansion system to enable sensing of a known pressure of gas. In the near future, we anticipate being able to compare the laboratory scale CAVS, the portable CAVS, and the flowmeter/dynamic expansion system to validate the operation of the CAVS as both a standard and vacuum gauge.
We present our progress towards a comparison of NIST's cold atom primary vacuum standard and a dynamic expansion vacuum standard. The cold atom vacuum standard (CAVS) converts the loss rate of atoms from a magnetic trap to a vacuum pressure using ab initio calculations of the quantum atom-molecule collision cross-section. To validate the CAVS, we have constructed a new flowmeter and dynamic expansion system that can produce low-uncertainty pressures in the ultra-high-vacuum range that is required for atom trapping. We discuss the operation and systematics of both the CAVS and flowmeter.
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.
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.