  Explaining how microscopic entities collectively produce macroscopic
phenomena is a fundamental goal of many-body physics. Theory predicts that
large-scale entanglement is responsible for exotic macroscopic phenomena, but
observation of entangled particles in naturally occurring systems is extremely
challenging. Synthetic quantum systems made of atoms in optical lattices have
been con- structed with the goal of observing macroscopic quantum phenomena
with single-atom resolution. Serious challenges remain in producing and
detecting long-range quantum correlations in these systems, however. Here we
exploit the strengths of photonic technology, including high coherence and
efficient single-particle detection, to study the predicted large-scale
entanglement underlying the macroscopic quantum phenomenon of polarization
squeezing. We generate a polarization-squeezed beam, extract photon pairs at
random, and make a tomographic reconstruction of their joint quantum state. We
present experimental evidence showing that all photons arriving within the
squeezing coherence time are entangled, that entanglement monogamy dilutes
entanglement with increasing photon density and that, counterintuitively,
increased squeezing can reduce bipartite entanglement. The results provide
direct evidence for entanglement of macroscopic numbers of particles and
introduce micro-analysis to the study of macroscopic quantum phenomena.
