Emission of time-ordered photon pairs from a single polaritonic Bogoliubov mode

In many-body quantum systems, interactions drive the emergence of correlations that are at the heart of the most intriguing states of matter. A remarkable example is the case of weakly interacting bosonic systems, whose ground state is a squeezed vacuum state, and whose elementary excitations have a collective nature. In this work, we report on the direct observation of the peculiar microscopic quantum structure of these elementary excitations. We perform time- and frequency-resolved two-photon correlation measurements on the fluctuations of weakly interacting polaritons in a resonantly-driven microcavity, and observe that upon decreasing the average number of fluctuation quanta below unity, large pair correlations build up together with strong time-ordering of the emitted photons. This behavior is a direct signature of the particle-hole quantum superposition which is at the heart of Bogoliubov excitations.