  The measurement of quantum entanglement can provide a new and most sensitive
probe to physics beyond the Standard Model. We use the concurrence of the
top-quark pair spin states produced at colliders to constrain the magnetic
dipole term in the coupling between top quark and gluons, that of $\tau$-lepton
pairs spin states to bound contact interactions and that of $\tau$-lepton pairs
or two-photons spin states from the decay of the Higgs boson in trying to
distinguish between CP-even and odd couplings. These four examples show the
power of the new approach as well as its limitations. We show that differences
in the entanglement in the top-quark and $\tau$-lepton pair production cross
sections can provide constraints better than those previously estimated from
total cross sections or classical correlations. Instead, the final states in
the decays of the Higgs boson remain maximally entangled even in the presence
of CP odd couplings and cannot be used to set bounds on new physics. We discuss
the violation of Bell inequalities featured in all four processes.
