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arXiv:2310.08760 · Physical Review Letters

Positronium Laser Cooling via the 13S−23P Transition with a Broadband Laser Pulse

L. T. Glöggler, Natali Gusakova, B. Rienäcker, A. Camper, R. Caravita, S. Ḣuck, Marco Volponi, T. Wolz, Luca Penasa, V. Krumins, F. P. Gustafsson, D. Comparat, M. Auzins, Benedikt Ludwig Bergmann, Petr Burian, Roberto Sennen Brusa, F. Castelli, G. Cerchiari, R. Ciuryło, Giovanni Consolati, M. Doser, L. K. Graczykowski, Małgorzata Grosbart, F. Guatieri, S. Haider, M. A. Janik, Grzegorz H. Kasprowicz, G. Khatri, Łukasz Kłosowski, G. Kornakov, Lidia Lappo, A. Linek, J. Malamant, S. Mariazzi, V. Petráček, Mariusz Piwiński, S. Pospı́šil, Luca Povolo, F. Prelz, S. A. Rangwala, T. Rauschendorfer, Bharat Singh Rawat, Volodymyr Rodin, Ole Myren Rohne, H. Sandaker, Petr Smolyanskiy, Tomasz Sowiński, D. Tefelski, Theodoros Vafeiadis, Carsten Peter Welsch, M. Zawada, J. Zieliński, Nicola Zurlo

Published Feb 22, 2024• 21 citations
arXivHTMLPDF

Abstract

We report on laser cooling of a large fraction of positronium (Ps) in free flight by strongly saturating the 1^{3}S-2^{3}P transition with a broadband, long-pulsed 243 nm alexandrite laser. The ground state Ps cloud is produced in a magnetic and electric field-free environment. We observe two different laser-induced effects. The first effect is an increase in the number of atoms in the ground state after the time Ps has spent in the long-lived 2^{3}P states. The second effect is one-dimensional Doppler cooling of Ps, reducing the cloud's temperature from 380(20) to 170(20) K. We demonstrate a 58(9)% increase in the fraction of Ps atoms with v_{1D}<3.7×10^{4} ms^{-1}.

DOI
10.1103/physrevlett.132.083402

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We report on laser cooling of a large fraction of positronium (Ps) in free flight by strongly saturating the 1^{3}S-2^{3}P transition with a broadband, long-pulsed 243 nm alexandrite laser. The ground state Ps cloud is produced in a magnetic and electric field-free environment. We observe two different laser-induced effects. The first effect is an increase in the number of atoms in the ground state after the time Ps has spent in the long-lived 2^{3}P states. The second effect is one-dimensional Doppler cooling of Ps, reducing the cloud's temperature from 380(20) to 170(20) K. We demonstrate a 58(9)% increase in the fraction of Ps atoms with v_{1D}<3.7×10^{4} ms^{-1}.
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On the basis of three dimensional (3-D) Monte Carlo simulation, we studied details of laser cooling (Doppler cooling) of ortho-Positronium ( o -Ps) which would be applied to the achievement of Bose–Einstein condensation of positronium. The cooling is carried out by an ultraviolet laser with a wavelength of 243 nm being equivalent to the 1 S –2 P transition energy of o -Ps. For this purpose, we have constructed a laser cooling apparatus which consists of a slow positron generator and a long pulse, wide linewidth laser. A 3-D Monte Carlo calculation, which incorporated various parameters of laser beam and positronium distributions expected in the current laser cooling apparatus, revealed that 7% of o -Ps was able to be cool down to 1 K and to be confined in a relatively small volume without employing a special trap.
Abstract When laser radiation is skilfully applied, atoms and molecules can be cooled 1–3 , allowing the precise measurements and control of quantum systems. This is essential for the fundamental studies of physics as well as practical applications such as precision spectroscopy 4–7 , ultracold gases with quantum statistical properties 8–10 and quantum computing. In laser cooling, atoms are slowed to otherwise unattainable velocities through repeated cycles of laser photon absorption and spontaneous emission in random directions. Simple systems can serve as rigorous testing grounds for fundamental physics—one such case is the purely leptonic positronium 11,12 , an exotic atom comprising an electron and its antiparticle, the positron. Laser cooling of positronium, however, has hitherto remained unrealized. Here we demonstrate the one-dimensional laser cooling of positronium. An innovative laser system emitting a train of broadband pulses with successively increasing central frequencies was used to overcome major challenges posed by the short positronium lifetime and the effects of Doppler broadening and recoil. One-dimensional chirp cooling was used to cool a portion of the dilute positronium gas to a velocity distribution of approximately 1 K in 100 ns. A major advancement in the field of low-temperature fundamental physics of antimatter, this study on a purely leptonic system complements work on antihydrogen 13 , a hadron-containing exotic atom. The successful application of laser cooling to positronium affords unique opportunities to rigorously test bound-state quantum electrodynamics and to potentially realize Bose–Einstein condensation 14–18 in this matter–antimatter system.
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.