  The quantum theory of the electromagnetic field uncovered that classical
forms of light were indeed produced by distinct superpositions of nonclassical
multiphoton wavepackets. Specifically, partially coherent light represents the
most common kind of classical light. Here, for the first time, we demonstrate
the extraction of the constituent multiphoton quantum systems of a partially
coherent light field. We shift from the realm of classical optics to the domain
of quantum optics via a quantum representation of partially coherent light
using its complex-Gaussian statistical properties. Our formulation of the
quantum Gaussian-Schell model unveils the possibility of performing
photon-number-resolving detection to isolate the constituent quantum
multiphoton wavepackets of a classical light field. We experimentally verified
the coherence properties of isolated vacuum systems and wavepackets with up to
sixteen photons. Our findings not only demonstrate the possibility of observing
quantum properties of classical macroscopic objects, but also establish a
fundamental bridge between the classical and quantum worlds.
