Spectral analysis for compressible quantum fluids

Turbulent fluid dynamics typically involves excitations on many different length scales. Classical incompress-ible fluids can be cleanly represented in Fourier space enabling spectral analysis of energy cascades and other turbulence phenomena. In quantum fluids, additional phase information and singular behavior near vortex cores thwarts the direct extension of standard spectral techniques. We develop a formal and numerical spectral analysis for U(1) symmetry-breaking quantum fluids suitable for analyzing turbulent flows, with specific application to the Gross-Pitaevskii fluid. Our analysis builds naturally on the canonical approach to spectral analysis of velocity fields in compressible quantum fluids and establishes a clear correspondence between energy spectral densities, power spectral densities, and autocorrelation functions, applicable to energy residing in velocity, quantum pressure, interaction, and potential energy of the fluid. Our formulation includes all quantum phase information and also enables arbitrary resolution spectral analysis, a valuable feature for numerical analysis. A central vortex in a trapped planar Bose-Einstein condensate provides an analytically tractable example with spectral features of interest in both the infrared and ultraviolet regimes. Sampled distributions modeling the dipole gas, plasma, and clustered regimes exhibit velocity correlation length increasing with vortex energy, consistent with known qualitative behavior across the vortex clustering transition. The spectral analysis of compressible quantum fluids presented here offers a rigorous tool for analyzing quantum features of superfluid turbulence in atomic or polariton condensates.

Automated summary

The study of turbulent flows requires the consideration of excitations on different length scales. In this paper, we develop a spectral analysis method for quantum fluids with U(1) symmetry-breaking, which can be used to analyze turbulent flows. Our method takes into account the quantum phase information and allows for arbitrary resolution spectral analysis, providing a rigorous tool for analyzing quantum features of superfluid turbulence.