From Quantum Quasiparticles to a Classical Gas

Boltzmann’s kinetic theory excellently describes the behavior of high-temperature gases, whose particles move around randomly and collide frequently. But if the gas is cooled down, its behavior changes dramatically, and Boltzmann’s classical picture must be abandoned for a quantum description. If the gas is made of fermions, it is describable by Fermi liquid theory—a powerful framework that applies to systems ranging from ordinary metals to the interior of neutron stars. Now Zhenjie Yan from the Massachusetts Institute of Technology (MIT), Cambridge, and his colleagues have tracked the crossover from classical to quantum behavior in a homogenous gas made of ultracold lithium atoms [1]. These results will serve as a benchmark for future theory and experiments that explore the complex “boundary” between the quantum and classical regime.