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Physical Review A

Observation of orthopositronium thermalization in silica aerogel at cryogenic temperatures

Kenji Shu, Akira Ishida, Toshio Namba, S. Asai, Nagayasu Oshima, Brian E. O’Rourke, Kenji Ito

Published Nov 24, 2021• 9 citations
Publisher

Abstract

We report orthopositronium $(o\text{\ensuremath{-}}\mathrm{Ps})$ thermalization observed in cryogenic silica aerogel cooled from room temperature to 25 K. It was observed that the $o\text{\ensuremath{-}}\mathrm{Ps}$ thermalization rate gets slower as $o\text{\ensuremath{-}}\mathrm{Ps}$ itself is colder than room temperature. This energy dependence can be explained based on a model where the efficiency of kinetic energy exchange between $o\text{\ensuremath{-}}\mathrm{Ps}$ and silica is determined by the de Broglie wavelength of $o\text{\ensuremath{-}}\mathrm{Ps}$. The present measurement confirmed that a cooling method which utilizes the thermalization process at cryogenic temperatures followed by laser cooling is applicable for the efficient production of an $o\text{\ensuremath{-}}\mathrm{Ps}$ Bose-Einstein condensate.

DOI
10.1103/physreva.104.l050801

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Abstract We report recent progress toward achieving positronium (Ps) Bose–Einstein condensation (BEC) by combining thermalization in nanoporous materials with ultraviolet (UV) laser Doppler cooling. Experiments were performed using silica aerogels held at two distinct thermal conditions—approximately room temperature and cryogenic temperature down to 8 K (measured at the sample-holder)—to study the laser-induced annihilation of Ps atoms excited to the 2P state. Resonant excitation at 243 nm significantly enhanced gamma-ray emission, with the annihilation signal reduced by approximately 50 % at low temperature compared to room temperature. Increasing the delay between Ps formation and laser excitation further reduced the annihilation probability. These observations confirm that Ps thermalization affects 2P-Ps annihilation. The results support a model of surface-induced 2P-state decay and suggest that excited-state Ps serves as a sensitive probe of surface interactions in nanoporous media. This constitutes an important step toward implementing laser cooling of Ps confined in solids—an essential prerequisite for realizing Ps-BEC. Ongoing efforts focus on systematic evaluation of pore structure, surface chemistry, and temperature effects to optimize Ps dynamics toward condensation.
Abstract A proposed new method of cooling positronium is to realize the Bose–Einstein condensation (BEC) of positronium. We perform detailed studies of three processes: (1) thermalization processes between positronium and the silica walls of a cavity, (2) Ps–Ps scattering and (3) laser cooling. The thermalization process is shown to be not sufficient for BEC. Ps–Ps collision is shown to have a big effect on the cooling performance. We combine both methods and establish an efficient cooling process for BEC. We also propose a new optical laser system for the cooling.
We have measured the kinetic energy of positronium (Ps) atoms emitted into a vacuum from a porous silica film subsequent to positron bombardment, via the Doppler spread of the linewidth of the Ps 13S{1}^{3}S13S-23P{2}^{3}P23P transition. We find that the deeper in the target film that positrons are implanted the colder is the emitted Ps, an effect we attribute to cooling via collisions in the pores as the atoms diffuse back to the film surface. We observed a lower limit to the mean Ps kinetic energy associated with motion in the direction of the laser, ${E}_{x}=42\ifmmode\pm\else\textpm\fi{}3$ meV, that is consistent with conversion of the confinement energy of Ps in the 2.7-nm-diameter pores to kinetic energy in vacuum. An implication is that a porous sample would need to be composed of pores greater than around 10 nm in diameter in order to produce thermal Ps in vacuum with temperatures of less than 100 K. By performing Doppler spectroscopy on intense pulses of Ps we have experimentally demonstrated the production of many excited-state Ps atoms simultaneously, which could have numerous applications, including laser cooling and fundamental spectroscopic studies of Ps and the production of antihydrogen.
Realization of the Bose–Einstein condensation (BEC) of positronium is a long-standing challenge of positron physics. Since the positron is the antimatter of the electron, the positronium is the antimatter of itself, and its gravity interaction is a sum of matter and antimatter components. In this sense, it can be used to study antimatter gravity. It can also be used as a source of a γ-ray laser. We have proposed a new method to realize a positronium BEC: a combination of thermalization in a cold silica target and laser cooling using 1S-2P transitions. We have started some basic studies based on our new idea. Here we report a preliminary result of our positronium thermalization measurement in cryogenic environment and development status of a new laser system for positronium cooling.
Abstract The quest for making a triplet positronium (Ps) Bose–Einstein condensate confined in a micron-sized cavity in a material such as porous silica faces at least three interrelated problems: (1) About 10710^7107 10 7 spin polarized Ps atoms must be injected into a small cavity within a porous solid material without vaporizing it. (2) It is known that Ps atoms confined in 30–100 nm diameter cavities in porous silica do not remain in the gas phase, but become stuck to the cavity walls at room temperature (Cooper et al., Phys. Rev. B 97:205302, 2018). (3) Cooling a gas of Ps atoms to cryogenic temperatures by energy exchange with the walls would be a very slow process (Saito and Hyodo, in: Surko, Gianturco (eds) New Directions in Antimatter Chemistry and Physics, Springer Dordrecht, Netherlands, 2001) because of the relatively low collision rate with the walls and the large mismatch between the masses of the Ps and the wall atoms. A possible solution of these difficulties is presented, based on cooling the implanted positrons in an isotopically pure diamond single crystal target, subsequent saturating of the wall Ps coverage so that a substantial portion of the Ps will be in the gaseous state, and thermalizing the gas-phase Ps via collisions with the low effective mass wall Ps. A design process for the target material is outlined as well, including preliminary results in porous cavity fabrication using focused ion beam milling. Graphical abstract
We report on the experimental observation of stimulated cooling in the non-equilibrium Bose-Einstein condensate (BEC) of weakly interacting exciton-polaritons from approximately room temperature down to 20K. By resolving the condensate in energy-momentum space and performing interferometric measurements, we distinguish the condensate from thermalized particles yet occupying excited states macroscopically. In contrast to the analytical quantum theories of non-equilibrium BEC [Shishkov et al., Phys. Rev. Lett. 128, 065301 (2022)], we observe segmentation of the particle density along the excited states into two fractions both following Bose-Einstein distribution, albeit with different effective temperatures and chemical potentials. Our results indicate that the temperature of the weakly interacting Bose gas is universally set by the density-dependent chemical potential, revealing a defining property of non-equilibrium BECs. Finally, we demonstrate that the stimulated nature of the cooling process directly governs the emergence of quantum coherence of the condensate and shapes the dissipative properties of the excited states.