  The Higgs amplitude mode is a collective excitation studied and observed in a
broad class of matter, including superconductors, charge density waves,
antiferromagnets, 3He p-wave superfluid, and ultracold atomic condensates. In
all the observations reported thus far, the amplitude mode was excited by
perturbing the condensate out of equilibrium. Studying an exciton-polariton
condensate, here we report the first observation of this mode purely driven by
intrinsic quantum fluctuations without such perturbations. By using an
ultrahigh quality microcavity and a Raman spectrometer to maximally reject
photoluminescence from the condensate, we observe weak but distinct
photoluminescence at energies below the condensate emission. We identify this
as the so-called ghost branches of the amplitude mode arising from quantum
depletion of the condensate into this mode. These energies, as well as the
overall structure of the photoluminescence spectra, are in good agreement with
our theoretical analysis.
