Supplementary videos of positron cooling in He, Ne, Ar, Kr and Xe at http://www.am.qub.ac.uk/users/dgreen09/coolingvideos.html ; This version contains additional References, is significantly reduced in size and has improved text
Understanding the dynamics of positron cooling in gases, including the fraction of positrons surviving to thermalisation, is critical for accurate interpretation of positron lifetime spectra, for the development of efficient positron cooling in traps and accumulators, and for a cryogenically cooled, ultra-high-energy-resolution, trap-based positron beam. Here, positron cooling and annihilation in noble gases is simulated using accurate scattering and annihilation cross sections calculated \emph{ab initio} with many-body theory. It is shown that a strikingly small fraction of positrons survive to thermalisation: $\sim$0.1 in He, $\sim$0 in Ne (due to cooling effectively stalling in the relatively deep momentum-transfer cross-section minimum), $\sim$0.15 in Ar, $\sim$0.05 in Kr and $\sim$0.01 in Xe. For Xe, the time-varying annihilation rate $\bar{Z}_{\rm eff}(τ)$ is shown to be highly sensitive to the depletion of the distribution due to annihilation, conclusively explaining the long-standing discrepancy between gas-cell and trap-based measurements in Xe. The \emph{ab initio} calculations enable the first simultaneous probing of the energy dependence of the the scattering cross section and annihilation rate. Overall, the use of the accurate atomic data gives $\bar{Z}_{\rm eff}(τ)$ in close agreement with experiment for all noble gases except Ne, the experiment for which is proffered to have suffered from incomplete knowledge of the fraction of positrons surviving to thermalisation and/or the presence of impurities.
Optimal Strategies for Optical Quantum Memories Using Long-Lived Noble-Gas Spins
Cornell University（康奈尔大学）；Weizmann Institute of Science（魏茨曼科学研究所）；Max-Planck-Institut für Quantenoptik（马克斯·普朗克量子光学研究所）；Ludwig-Maximilians-Universität München（慕尼黑路德维希-马克西米利安大学）；Harvard-MIT Center for Ultracold Atoms（哈佛-麻省理工学院超冷原子中心）；Harvard University（哈佛大学）；Joint Quantum Institute and Joint Center for Quantum Information and Computer Science, NIST/University of Maryland（联合量子研究所和联合量子信息与计算机科学中心，美国国家标准与技术研究院/马里兰大学）
