Classical fields in the one-dimensional Bose gas: applicability and determination of the optimal cutoff

To finalize information about the accuracy of the classical field approach for the 1d Bose gas, the lowest temperature quasicondensate was studied by comparing the extended Bogoliubov model of Mora and Castin, to its classical field analogue. The parameters for which the physics is well described by matter waves are now presented for all 1d regimes, and concurrently, the optimal cutoff that best matches all observables together is also provided. This cutoff rises strongly with density when the chemical potential is higher than the thermal energy to account for kinetic energy. As a consequence, clouds that reach this coldest quantum fluctuating regime are better described using a trap basis than plane waves. This contrasts with higher temperature clouds for which the basis choice is less important. In passing, estimates for chemical potential, density fluctuations, kinetic and interaction energy in the low temperature quasicondensate are obtained up to several leading terms.