Observation of the amplitude mode in a microcavity polariton condensate driven by quantum fluctuations*

We directly observe emission into the amplitude mode from an exciton-polariton condensate: the signature of this mode being the ghost-branch photoluminescence at energies below the condensate. This amplitude mode, the number-fluctuation variant of the Goldstone mode, appears in many forms of condensed matter; however, the polariton system gives a unique way to passively monitor this mode in the steady-state system without driving the population far out of equilibrium. This mode is predicted to manifest as an excitation of condensed particles into the upper-polariton state. Here we present experimental results in agreement with our theoretical analysis. Additionally, we present a family of ghost-branch modes that require an expanded theory to capture.