Lee-Yang theory of Bose-Einstein condensation

Bose-Einstein condensation happens as a gas of bosons is cooled below its transition temperature, and the ground state becomes macroscopically occupied. The phase transition occurs in the thermodynamic limit of many particles. However, recent experimental progress has made it possible to assemble quantum many-body systems from the bottom up, for example, by adding single atoms to an optical lattice one at a time. Here, we show how one can predict the condensation temperature of a Bose gas from the energy fluctuations of a small number of bosons. To this end, we make use of recent advances in Lee-Yang theories of phase transitions, which allow us to determine the zeros and the poles of the partition function in the complex plane of the inverse temperature from the high cumulants of the energy fluctuations. By increasing the number of bosons in the trapping potential, we can predict the convergence point of the partition-function zeros in the thermodynamic limit, where they reach the inverse critical temperature on the real axis. Using fewer than 100 bosons, we can estimate the condensation temperature for a Bose gas in a harmonic potential in two and three dimensions, and we also find that there is no phase transition in one dimension as one would expect.

Automated summary

Bose-Einstein condensation occurs when a gas of bosons is cooled below its transition temperature, leading to macroscopic occupation of the ground state. Recent progress in experimental techniques allows for the assembly of quantum many-body systems from single atoms, enabling the prediction of the condensation temperature based on energy fluctuations of a small number of bosons. By utilizing Lee-Yang theories of phase transitions, it is possible to determine the behavior of the partition function and predict the convergence point of the zeros in the complex plane of the inverse temperature. This research provides insights into the condensation temperature of a Bose gas in different dimensions and confirms the absence of phase transition in one dimension.