Speaker
Description
Positronium (Ps), a purely leptonic bound state of an electron and a positron, offers a unique platform for testing fundamental physics, including gravity on antiparticles [1] and a gamma-ray laser [2]. Achieving Bose-Einstein condensation (Ps-BEC) would be a breakthrough, requiring ultracold temperatures (approximately 10 K) and a dense state (around 10¹⁸ cm⁻³) Ps within its short lifetime of 142 ns [3]. As a first step toward Ps-BEC, we recently demonstrated one-dimensional laser cooling of Ps in vacuum, reducing its velocity spread to ~1 K within 100 ns using a chirped laser pulse train [4]. This achievement marks the first successful laser cooling of Ps. To achieve Ps-BEC, laser cooling must be applied to dense Ps confined in nanoporous materials. While silica aerogels allow efficient Ps formation and thermalization [5,6], we found that Ps excited to the 2P state exhibits unexpectedly short annihilation lifetimes in nanopores, hindering cooling cycles [7]. We will present an overview of our Ps-BEC project, including the vacuum laser cooling experiment, the challenges of 2P-Ps annihilation in confinement and the ongoing development of nanoporous media optimized for Ps formation, condensation, and cooling. These advances pave the way toward Ps condensation and, ultimately, a gamma-ray laser.
Acknowledgements: This work was partially supported by JST FOREST Program (Grant Number JPMJFR202L), JSPS KAKENHI Grant Numbers JP16H04526, JP17H02820, JP17H06205, JP17J03691, JP18H03855, JP19H01923, JP23H05462, MATSUO FOUNDATION, Mitutoyo Association for Science and Technology (MAST), Research Foundation for Opto-Science and Technology, The Mitsubishi Foundation, TIA Kakehashi TK17-046 and TK19-016. This work was performed under the approval of the Photon Factory Program Advisory Committee (Proposal No. 2017P009, 2018G100, 2020G101, 2020G631, 2022G087, 2023G157, 2024G143, 2025G151).
[1] D. B. Cassidy and A. P. Mills, Jr., phys. stat. sol. (c) 4, 3419 (2007). [2] H. K. Avetissian et al., Phys. Rev. Lett. 113, 023904 (2014); Phys. Rev. A 92, 023820 (2015). [3] K. Shu, X. Fan, T. Namba et al., J. Phys. B: At. Mol. Opt. Phys. 49, 104001 (2016). [4] K. Shu, A. Ishida, T. Namba, K. Wada, K. Ito, N. Oshima et al., Nature 633, 793 (2024). [5] K. Shu, A. Ishida, T. Namba, K. Ito, K. Wada et al., JJAP Conf. Proc. 9, 011202 (2023). [6] K. Shu, A. Ishida, T. Namba, N. Oshima et al., Phys. Rev. A 104, L050801 (2021). [7] K. Shu, Doctoral dissertation, The University of Tokyo (2020).
