Spectroscopy of Dense Positronium

玻色-爱因斯坦凝聚态(BEC)是一种由许多相同的原子组成的冷气体，这些原子聚集在容器的最低能量状态。BEC可以表现出无摩擦流动和独特的干涉特性，使其成为原子模拟的激光器。激光中的光的相干性在光通信、医学成像和各种工业应用中得到了非凡的应用。原子的相干性可能会为基于原子的量子计算、物质的新状态模拟以及高精度计量学的进步开辟新的途径。有人提出，由正电子(Ps)原子(电子及其正电子反粒子的束缚态)形成的BEC将具有独特的兴趣，因为它可以表现出双重相干，同时像BEC一样凝聚，并且在足够高的密度下，由于受激发湮灭而表现出类似激光的效应。因为正电子比普通原子轻几千倍，所以正电子BEC不需要在超低温下保持，甚至可以在室温下存在，这使得最终制造实用的伽马射线激光器变得很方便。正电子BEC的一个应用是能够诱导相干的Ps原子发射，以产生用于超高精度光学光谱测量的极单能原子。据预测，正电子BEC形成的正电子原子密度将是以前实验室生产的正电子原子密度的100倍。来自加州大学河滨分校(UCR)的两名科学家提议在100倍的密度下研究正电子，并观察第一个正电子BEC。这是加州大学河滨分校(UCR)正在进行的一个项目，目标是研究冷密的正电子气体(Ps)的物理，这是一种电子及其正电子反粒子的超轻氢束缚态。尽管Ps的三重基态的平均寿命只有142 ns，但精确测量Ps的能级以及观察Ps与原子、分子和凝聚态物质的相互作用是可能的。到目前为止，UCR的科学家们已经能够创造出足以产生双电子分子(PS2)的Ps密度。在该项目的这个新阶段，他们将创造一种密度更高的Ps气体，并研究玻色-爱因斯坦统计数据对单位自旋玻色粒子原子的影响。具体目标是：(1)在表面密度比以前大100倍的靶中产生Ps脉冲；(2)测量低温下腔中Ps原子的冷却速率；(3)由于玻色统计的影响，在2D腔中观测到低动量冷Ps原子的过剩；以及(4)在3D腔中寻找低温下的ps玻色-爱因斯坦凝聚体(BEC)。用于识别BEC的PS动量分布和温度将使用美国国家科学基金会专门为此设计的新仪器，直接实时测量。

A Bose-Einstein condensate (BEC) is a cold gas of many identical atoms that have collected into the lowest energy state of a container. A BEC can exhibit frictionless flow and unique interference properties that make it an atomic analog of a laser. The coherence of light in lasers has found extraordinary applications in optical communication, medical imaging, and various industrial applications. The coherence of atoms may open new paths to atom-based quantum computation, simulation of new states of matter, and advances in high precision metrology. It has been proposed that a BEC formed from positronium (Ps) atoms (bound states of electrons and their positron antiparticles) would be of unique interest because it could exhibit double coherence, simultaneously condensing like a BEC and, at sufficiently high densities, exhibiting laser-like effects due to stimulated annihilation. Because positronium is thousands of times lighter than ordinary atoms, a positronium BEC does not need to be held at ultra-low temperatures and could even exist at room temperature, making it convenient for eventually making a practical gamma ray laser. An application of the positronium BEC would be the ability to induce coherent Ps atom emission to produce extremely monoenergetic atoms for ultrahigh precision optical spectroscopy measurements. The positronium BEC is predicted to form at 100 times higher density of positronium atoms than has ever been produced in a laboratory before. Two scientists from the University of California, Riverside (UCR) propose to study positronium at 100 times higher density and to observe the first positronium BEC. This is an ongoing project at the University of California Riverside (UCR) with the goal of studying the physics of a cold, dense gas of positronium (Ps), an ultra-light hydrogenic bound state of an electron and its positron antiparticle. Although the triplet ground state of Ps has a mean lifetime of only 142 ns, it is possible to make precision measurements of the energy levels of Ps and to observe Ps interactions with atoms, molecules and condensed mater. Scientists at UCR have thus far been able to create Ps densities sufficient to produce the dipositronium molecule (Ps2) for the first time. In this new phase of the project they will create a much denser Ps gas and study the effect that Bose-Einstein statistics is expected to have on atoms that are unit spin Bose particles. The specific objectives are: (1) Produce Ps pulses in a target with surface densities more than one hundred times greater than before; (2) Measure cooling rates of Ps atoms in cavities at cryogenic temperatures; (3) Observe an excess of low momentum cold Ps atoms in 2D cavities due to the effect of Bose statistics; and (4) Search for a Ps Bose-Einstein condensate (BEC) in 3D cavities at low temperature. Ps momentum distributions and temperatures for identifying a BEC will be measured directly in real time with a newly NSF-funded instrument specifically designed for this purpose.