Since the pioneering work of Lüscher in the 1980s it is well known that considering quantum systems in finite volume, specifically, finite periodic boxes, can be used as a powerful computational tool to extract physical observables. While this formalism has been worked out in great detail in the two-body sector, much effort is currently being invested into deriving analogous relations for systems with more constituents. This work is relevant not only for nuclear physics, where lattice methods are now able to calculate few- and many-nucleon states, but also for other fields such as simulations of cold atoms. This article discusses recent progress regarding the extraction of few-body bound-state and resonance properties from finite-volume calculations of systems with an arbitrary number of constituents.

We review recent experiments carried out by the PiHe collaboration of the Paul Scherrer Institute (PSI) that observed an infrared transition of three-body pionic helium atoms by laser spectroscopy. These measurements may lead to a precise determination of the charged pion mass, and complement experiments of antiprotonic helium atoms carried out at the new ELENA facility of CERN.

The variational Monte Carlo method is employed to conduct a comprehensive investigation of the compressed ground and excited states of plasma-embedded lithium atom within impenetrable spherical boxes of varying radii. The study focuses on the low-lying excited doublet states 1 , 1 p, and 1 d, utilizing plasma potentials such as the screened Coulomb (SCP), exponential cosine screened Coulomb (ECSCP), and Hulthén potentials. Energy eigenvalues are determined using appropriate trial wave functions, which account for electron-electron repulsion and spin parts to adhere to the Pauli Exclusion Principle. Moreover, two factors related to the wave function of the compressed system and ECSCP model are considered. The results reveal an intriguing relative ordering for the lithium atom using the three plasma models, with many of the findings being significant contributions yet to be explored.