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.
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.
The cold atomic gravimeter (CAG) has the advantage of high measurement accuracy and does not need to be calibrated on a regular basis. To achieve cold atom interference, it is first necessary to cool and trap the atoms by magneto-optical trap (MOT). However, there are many types of MOTs, and their trapping and cooling results directly affect the atomic interference, and thus, the measurement accuracy of a CAG. MOTs should be designed or selected correctly for different application needs. This paper reviews the research history of MOTs and analyzes their structure and principles. The current status of applications of different types of MOTs is highlighted. Their advantages and disadvantages are summarized, and perspectives for the development of MOTs for cold atomic gravimetry are presented.
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.
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.
The development of quantum technologies has been an important topic of research and technological development over the last few years, with numerous projects aiming to exploit cold atoms and Bose-Einstein condensates for sensing applications and probes of fundamental physics. To further develop the emerging applications of new quantum mechanical phenomena, the UK established the a national quantum tech program in 2014. The University of Nottingham, alongside university partners across the country, is currently involved in advancing quantum-based technologies for sensors and metrology applications. A key part of many experiments in this area is the Magneto-Optical Trap, or MOT, which plays a pivotal role in the initial stages of the atom cooling required to reach the nK temperatures necessary to form a condensate. Traditionally, MOTs are typically comprised of multiple pieces of specialised equipment including an ultra-high vacuum chamber, several lasers and magnetic field generating devices, all of which occupy 10's cm in linear dimensions on a laboratory bench and require mains power. As in previous quantum-enabled technologies, including MRI scanners, simulation and optimisation may prove crucial in reducing the size, weight and power requirements of such systems, and for improving their performance. Consequently, it is natural to apply such optimisation methods to emerging quantum technologies, in order to meet the restrictions on experimental size, weight and power necessary the practical cold atom-based sensing systems. As it forms a crucial part of many cold atom quantum technologies, the MOT is a natural initial target for simulation and subsequent optimisation. To develop system design capabilities, this thesis firstly presents three optimisation methods for permanent magnets. One of these builds on previous work to optimise ferromagnetic films for Ioffe-Pritchard arrays (chapter \ref{Magnetic Plate Optimisation}), the others are novel optimisation methods, drawing on more general approaches and on shimming methods frequently used in MRI. Permanent magnets were chosen for use in MOTs in order to reduce the required power and volume of current experimental equipment. While each of the three optimisation methods have different capabilities, two were particularly powerful for designing high field fidelity MOTs. One of these methods was developed specifically to design magnet systems for use with the 3D printed vacuum chamber of the Innovate UK-funded OptaMOT project, illustrating its potential to develop experimental systems. The magnetic field was generated to high accuracy by using the new optimisation algorithm, presented in chapter \ref{Spherical Harmonic Optimisation}, by using mass produced magnets, thereby offering improved practicality over designs that require, expensive, custom-made magnets. With the aim of improving the holistic design capabilities required to advance MOTs, simulation software for the dynamics of atoms in MOTs was also developed. The simulations were capable of modelling the dynamics of atom clouds in both a typical 6 beam MOT and an optical molasses arrangement. It was also used to elucidate the action of a significantly more complicated beam arrangement exploiting more advanced physical phenomena. Remarkably, the arrangement, developed by Hub partners at The University of Southampton, experimentally shows a high atom number and the potential presence of a restoring force without an applied magnetic field. The laser beam geometry also exhibits an unusual dependence of the final atom number on laser beam detuning and power. The simulations provided insights into the potential origins of this dependence (suggesting a combination of beam interference and an additional detuning from the laser fields), whilst confirming that the laser beam configuration alone creates a damping force, as with an optical molasses arrangement. However, the origins and presence of a restoring force still, unfortunately, remain unknown.