A system of particles moving along one dimension, as is the case with Bose gases, looks deceivingly simple, but it is actually an extremely sophisticated system due to the intricate interplay of thermal motion, collisions and quantum statistics. Understanding the properties of ensembles of bosons with pairwise contact repulsive interactions is crucial for both fundamental research and the development of future quantum technologies, high-critical-temperature superconductors and quantum computers. Similar systems have been experimentally realised since 2004 with ultracold atomic gases.

At very low temperatures, the Luttinger liquid theory provides a unified description of one-dimensional bosonic ensembles in terms of phononic excitations with a linear dispersion. As temperature increases, however, such unified picture breaks down. Higher momenta get explored and the deviation of the spectrum from the simple linear behaviour becomes important. A problem which remained open until now was understanding the microscopic mechanisms ruling the different thermodynamic behaviours for weak and strong interactions.

A study published in Physical Review Research, which is the result of a tight collaboration between the Universitat Politècnica de Catalunya · BarcelonaTech (UPC) and the Institute of Photonic Sciences (ICFO), a university research institute affiliated to the UPC, demonstrated that for weak interactions the finite temperature contributions to the thermodynamic variables can be used to describe gas in terms of a set of non-interacting fictitious particles. In the strongly-interacting regime, their thermal properties instead naturally emerge from the approximation of interactions between bosons as if they were hard spheres.

This study, conducted by the researchers from the UPC’s Barcelona Quantum Monte Carlo research group and the Department of Physics Giulia De Rosi, Pietro Massignan and Grigori Astrakharchik and the ICREA researcher at ICFO Maciej Lewenstein, will stimulate further investigations aimed to characterise the microscopic nature of one-dimensional Bose gases. These predictions apply to different quantum systems, such as helium fluids, quantum mixtures and impurities immersed in a quantum bath.