  We explore the phenomenology of quantum entanglement at collider experiments
by computing the polarization density matrix of processes yielding two massive
gauge bosons. After reviewing the formalism, we detail observables suitable to
test the presence of entanglement and quantum correlations in the di-boson
system. The implied violation of Bell inequalities can be observed with future
data at the LHC in the decays of the Higgs boson to $Z$ boson pairs.
