Observation of a Lee-Huang-Yang Fluid

TL;DR

This study experimentally observes monopole oscillations in a Bose-Einstein condensate mixture where mean-field interactions are canceled, revealing the dominant role of Lee-Huang-Yang quantum fluctuations and confirming their stabilizing effects.

Contribution

First experimental observation of a Lee-Huang-Yang fluid through monopole oscillations in a controlled Bose-Einstein condensate mixture.

Findings

Monopole oscillation frequency matches theoretical predictions for LHY-dominated systems.

The LHY interaction stabilizes the condensate against scattering length variations.

Results demonstrate the potential of LHY interactions for quantum simulation and beyond-LHY regime studies.

Abstract

We observe monopole oscillations in a mixture of Bose-Einstein condensates, where the usually dominant mean-field interactions are canceled. In this case, the system is governed by the next-order Lee-Huang-Yang (LHY) correction to the ground state energy, which describes the effect of quantum fluctuations. Experimentally such a LHY fluid is realized by controlling the atom numbers and interaction strengths in a $^{39}$K spin mixture confined in a spherical trap potential. We measure the monopole oscillation frequency as a function of the LHY interaction strength as proposed recently by J{\o}rgensen et al. [Phys. Rev. Lett. 121, 173403 (2018)] and find excellent agreement with simulations of the complete experiment including the excitation procedure and inelastic losses. This confirms that the system and its collective behavior are initially dominated by LHY interactions. Moreover, the monopole oscillation frequency is found to be stable against variations of the involved scattering lengths in a broad region around the ideal values, confirming the stabilizing effect of the LHY interaction. These results pave the way for using the non-linearity provided by the LHY term in quantum simulation experiments and for investigations beyond the LHY regime.