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

TL;DR

This paper reports the direct observation of the quantum structure of Bogoliubov excitations in a polaritonic system, revealing large photon pair correlations and time-ordering as signatures of particle-hole superpositions.

Contribution

It provides the first direct measurement of the microscopic quantum structure of elementary excitations in a weakly interacting bosonic system.

Findings

Observation of large pair correlations at low excitation levels

Detection of strong time-ordering in emitted photons

Evidence of particle-hole quantum superpositions in Bogoliubov excitations

Abstract

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.