  We study the quantum properties of top quark pairs in lepton colliders with
unpolarised beams, including spin correlations, entanglement, and violation of
Bell inequalities. We present analytical results in the SM and in the SMEFT and
discuss several practical aspects, like the choice of quantization axes and $t
\bar t$ threshold effects. We also note a correspondence between parity
symmetry and entanglement. We find that quantum observables exhibit a rich
phenomenology in the SM, and can also provide additional leverage in detecting
new physics residing at higher scales.
