Spectroscopy of Dense Positronium

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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.