  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.
