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International Conference on Exotic Atoms and Related Topics (EXA)

Positronium Doppler laser cooling: results and perspectives

A. Camper, Sara Alfaro Campos, Marcis P. Auzinsh, Michael Berghold, Benedikt Ludwig Bergmann, Petr Burian, Roberto Sennen Brusa, R. Caravita, F. Castelli, G. Cerchiari, R. Ciuryło, Ahmad Chehaimi, Giovanni Consolati, M. Doser, Kamil Eliaszuk, Riley S. Ferguson, Matthias Germann, Anna Giszczak, L. T. Glöggler, L. K. Graczykowski, Małgorzata Grosbart, F. Guatieri, Natali Gusakova, Frederik Parnefjord Gustafsson, Stefan Haider, S. Ḣuck, Christoph Hugenschmidt, M. A. Janik, Timoteusz Januszek, Grzegorz H. Kasprowicz, Kamila Kempny, Gunn Khatri, Łukasz Kłosowski, Georgy Kornakov, V. Krumins, Lidia Lappo, A. Linek, S. Mariazzi, Paweł Moskal, Markus Münster, Piyush Pandey, Daniel Pęcak, Luca Penasa, V. Petráček, Mariusz Piwiński, S. Pospı́šil, F. Prelz, S. A. Rangwala, T. Rauschendorfer, Bharat Singh Rawat, B. Rienäcker, Volodymyr Rodin, Ole Myren Rohne, H. Sandaker, Sushil Kumar Sharma, Petr Smolyanskiy, Theodoros Sowiński, D. Tefelski, Marco Volponi, Carsten Peter Welsch, M. Zawada, J. Zieliński, Nicola Zurlo

Published Mar 31, 2025• 0 citations
Publisher

Abstract

The experimental demonstration of positronium laser cooling with stationary broadband laser pulses with negative detuning is briefly described. Considerations on the limits of the current experiment and possible future developments follow. In particular, the benefit of positron remoderation, use of a magnetic field, positronium polarization, pulse shaping, coherent laser cooling and deceleration are shortly discussed.

DOI
10.22323/1.480.0047

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Experimental progress in positronium laser physics

D. B. Cassidy

The field of experimental positronium physics has advanced significantly in the last few decades, with new areas of research driven by the development of techniques for trapping and manipulating positrons using Surko-type buffer gas traps. Large numbers of positrons (typically ≥106) accumulated in such a device may be ejected all at once, so as to generate an intense pulse. Standard bunching techniques can produce pulses with ns (mm) temporal (spatial) beam profiles. These pulses can be converted into a dilute Ps gas in vacuum with densities on the order of 107 cm−3 which can be probed by standard ns pulsed laser systems. This allows for the efficient production of excited Ps states, including long-lived Rydberg states, which in turn facilitates numerous experimental programs, such as precision optical and microwave spectroscopy of Ps, the application of Stark deceleration methods to guide, decelerate and focus Rydberg Ps beams, and studies of the interactions of such beams with other atomic and molecular species. These methods are also applicable to antihydrogen production and spectroscopic studies of energy levels and resonances in positronium ions and molecules. A summary of recent progress in this area will be given, with the objective of providing an overview of the field as it currently exists, and a brief discussion of some future directions.

The European Physical Journal D•Mar 1, 2018•169 citations

Positronium Laser Deceleration, Cooling and Trapping

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We numerically study laser deceleration, cooling and trapping of host fast ortho-positronium (Ps) bunches as produced in silica microchannel targets using positron bunches from a Surko trap. While Doppler laser cooling routinely allows to cool neutral atom species below milli-Kelvin kinetic temperatures, the fast ground state annihilation and the high recoil shift to line-width ratio in Ps pose new challenging limitations on timing and the required laser power. Earlier theoretical work and first experiments already exhibit viable prospects for radiation pressure cooling to create unprecedented cold and dense Ps ensembles. Our extended numerical studies confirm very good prospects to simultaneously decelerate, cool and optically trap Ps atoms in a standard Doppler cooling geometry on the $1^3\mathrm{S}$--$2^3\mathrm{P}$ transition. Including the full Zeeman state manifolds and adding additional transverse lasers on the $2^3\mathrm{P}$--$3^3\mathrm{D}$ transition improves timing and final temperature to allow for optical trapping at the end. The required laser powers and geometry parameters to implement effective slowing and trapping within a time frame, where the majority of atoms is not annihilated, is in reach of current technology of $\mathrm{mJ}$ energy pulses of approximately $100\ \mathrm{ns}$ duration. Future extensions to collectively enhance cooling and trapping in optical cavities or hollow-core fibres should finally allow fast preparation of ultra-cold Ps systems as future basis of superradiant lasing and Bose--Einstein condensates.

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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.

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We present a formulation and numerical results for positronium (Ps) laser cooling. The formulation is based on the Lindblad master equation and follows the time evolution of the density matrix of Ps atoms. It therefore accounts for atomic coherence, which is necessary to describe the interaction of Ps with the train of short laser pulses generated by the system developed by Shu $\textit{et al.}$ [K. Shu $\textit{et al.}$, Phys. Rev. A $\textbf{109}$, 043520 (2024)]. Using this formulation, we calculate the time evolution of the populations in each internal and momentum state and thereby quantitatively predict the momentum distribution after laser cooling. We present the representative time evolution of the internal-state populations and momentum distribution, together with a comprehensive scan of the laser parameters used to optimize the cooling efficiency. A prominent feature of the simulated distributions is sub-recoil cooling through velocity-selective coherent population trapping, a coherent effect captured by the quantum-mechanical treatment.

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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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Positronium laser cooling in a magnetic field

Christian Zimmer, P. Yzombard, A. Camper +1 more

We study realistic 3D laser cooling of positronium (Ps) in the presence of a magnetic field. Triplet and singlet states mixing due to the magnetic field, and dynamical Stark effect, generally produce higher annihilation rates than in the zero-field case. 3D cooling is efficient only at very low field $B\ensuremath{\lesssim}50\phantom{\rule{0.16em}{0ex}}\mathrm{mT}$ and at high field values $B⪆0.7\phantom{\rule{0.16em}{0ex}}\phantom{\rule{0.16em}{0ex}}\mathrm{T}$. Near $100\phantom{\rule{0.16em}{0ex}}\mathrm{ns}$ long laser pulses, spectrally broad enough to cover most of the Ps Doppler profile and with energy in the $\mathrm{mJ}$ range, are required to cool Ps. Simulations based on full diagonalization of the Stark and Zeeman Hamiltonian and a kinetic Monte Carlo algorithm exactly solving the rate equations indicate that an efficient cooling (typically from $300\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ down to below $50\phantom{\rule{0.16em}{0ex}}\mathrm{K}$) is possible even in a magnetic field. We also propose 3D moving molasses cooling that can produce a well-defined monochromatic Ps beam useful for applications.

Physical Review A•Aug 18, 2021•11 citations
The experimental demonstration of positronium laser cooling with stationary broadband laser pulses with negative detuning is briefly described. Considerations on the limits of the current experiment and possible future developments follow. In particular, the benefit of positron remoderation, use of a magnetic field, positronium polarization, pulse shaping, coherent laser cooling and deceleration are shortly discussed.
The field of experimental positronium physics has advanced significantly in the last few decades, with new areas of research driven by the development of techniques for trapping and manipulating positrons using Surko-type buffer gas traps. Large numbers of positrons (typically ≥106) accumulated in such a device may be ejected all at once, so as to generate an intense pulse. Standard bunching techniques can produce pulses with ns (mm) temporal (spatial) beam profiles. These pulses can be converted into a dilute Ps gas in vacuum with densities on the order of 107 cm−3 which can be probed by standard ns pulsed laser systems. This allows for the efficient production of excited Ps states, including long-lived Rydberg states, which in turn facilitates numerous experimental programs, such as precision optical and microwave spectroscopy of Ps, the application of Stark deceleration methods to guide, decelerate and focus Rydberg Ps beams, and studies of the interactions of such beams with other atomic and molecular species. These methods are also applicable to antihydrogen production and spectroscopic studies of energy levels and resonances in positronium ions and molecules. A summary of recent progress in this area will be given, with the objective of providing an overview of the field as it currently exists, and a brief discussion of some future directions.
We numerically study laser deceleration, cooling and trapping of host fast ortho-positronium (Ps) bunches as produced in silica microchannel targets using positron bunches from a Surko trap. While Doppler laser cooling routinely allows to cool neutral atom species below milli-Kelvin kinetic temperatures, the fast ground state annihilation and the high recoil shift to line-width ratio in Ps pose new challenging limitations on timing and the required laser power. Earlier theoretical work and first experiments already exhibit viable prospects for radiation pressure cooling to create unprecedented cold and dense Ps ensembles. Our extended numerical studies confirm very good prospects to simultaneously decelerate, cool and optically trap Ps atoms in a standard Doppler cooling geometry on the 13S1^3\mathrm{S}13S--23P2^3\mathrm{P}23P transition. Including the full Zeeman state manifolds and adding additional transverse lasers on the 23P2^3\mathrm{P}23P--33D3^3\mathrm{D}33D transition improves timing and final temperature to allow for optical trapping at the end. The required laser powers and geometry parameters to implement effective slowing and trapping within a time frame, where the majority of atoms is not annihilated, is in reach of current technology of mJ\mathrm{mJ}mJ energy pulses of approximately 100 ns100\ \mathrm{ns}100 ns duration. Future extensions to collectively enhance cooling and trapping in optical cavities or hollow-core fibres should finally allow fast preparation of ultra-cold Ps systems as future basis of superradiant lasing and Bose--Einstein condensates.
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 present a formulation and numerical results for positronium (Ps) laser cooling. The formulation is based on the Lindblad master equation and follows the time evolution of the density matrix of Ps atoms. It therefore accounts for atomic coherence, which is necessary to describe the interaction of Ps with the train of short laser pulses generated by the system developed by Shu et al.\textit{et al.}et al. [K. Shu et al.\textit{et al.}et al., Phys. Rev. A 109\textbf{109}109, 043520 (2024)]. Using this formulation, we calculate the time evolution of the populations in each internal and momentum state and thereby quantitatively predict the momentum distribution after laser cooling. We present the representative time evolution of the internal-state populations and momentum distribution, together with a comprehensive scan of the laser parameters used to optimize the cooling efficiency. A prominent feature of the simulated distributions is sub-recoil cooling through velocity-selective coherent population trapping, a coherent effect captured by the quantum-mechanical treatment.
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}.
We study realistic 3D laser cooling of positronium (Ps) in the presence of a magnetic field. Triplet and singlet states mixing due to the magnetic field, and dynamical Stark effect, generally produce higher annihilation rates than in the zero-field case. 3D cooling is efficient only at very low field $B\ensuremath{\lesssim}50\phantom{\rule{0.16em}{0ex}}\mathrm{mT}$ and at high field values B⪆0.7TB⪆0.7\phantom{\rule{0.16em}{0ex}}\phantom{\rule{0.16em}{0ex}}\mathrm{T}B⪆0.7T. Near 100ns100\phantom{\rule{0.16em}{0ex}}\mathrm{ns}100ns long laser pulses, spectrally broad enough to cover most of the Ps Doppler profile and with energy in the mJ\mathrm{mJ}mJ range, are required to cool Ps. Simulations based on full diagonalization of the Stark and Zeeman Hamiltonian and a kinetic Monte Carlo algorithm exactly solving the rate equations indicate that an efficient cooling (typically from 300K300\phantom{\rule{0.16em}{0ex}}\mathrm{K}300K down to below 50K50\phantom{\rule{0.16em}{0ex}}\mathrm{K}50K) is possible even in a magnetic field. We also propose 3D moving molasses cooling that can produce a well-defined monochromatic Ps beam useful for applications.