  Future hadron collider experiments are expected to record large to huge
samples of $t\bar t$ events. The analysis of these data with respect to $t\bar
t$ spin-spin correlations requires precise predictions of the production of
these quark pairs in a general spin configuration. Therefore we have computed,
at next-to-leading order (NLO) in the QCD coupling, the spin density matrices
describing $t\bar t$ production by quark antiquark annihilation, which is the
dominant production process at the Tevatron. Moreover we have computed the
strength of the $t\bar t$ spin correlation at NLO, using various spin
quantization axes.
