  The top quark decays before it hadronises. Before its spin state can be
changed in a process of strong interaction, it is directly transferred to the
top quark decay products. The top quark spin can be deduced by studying angular
distributions of the decay products. The Standard Model predicts the
top/anti-top quark ($t\bar{t}$) pairs to have correlated spins. The degree is
sensitive to the spin and the production mechanisms of the top quark. Measuring
the spin correlation allows to test the predictions. New physics effects can be
reflected in deviations from the prediction. In this thesis the spin
correlation of $t\bar{t}$ pairs, produced at a centre-of-mass energy of
$\sqrt{s} = 7\,$TeV and reconstructed with the ATLAS detector, is measured. The
dataset corresponds to an integrated luminosity of $4.6~\text{fb}^{-1}$.
$t\bar{t}$ pairs are reconstructed in the $\ell$+jets channel using a kinematic
likelihood fit offering the identification of light up- and down-type quarks
from the $t \rightarrow bW \rightarrow bq\bar{q}'$ decay. The spin correlation
is measured via the distribution of the azimuthal angle $\Delta\phi$ between
two top quark spin analyzers in the laboratory frame. It is expressed as the
degree of $t\bar{t}$ spin correlation predicted by the Standard Model,
$f_{\text{SM}}$. The results of \begin{align*} &f_{\text{SM}}(
\Delta\phi(\text{charged lepton, down-type quark} )) &= 1.53 \pm
0.14\,\text{(stat.)} \pm 0.32\,\text{(syst.)}, \\ &f_{\text{SM}} (
\Delta\phi(\text{charged lepton, b-quark} )) &= 0.53 \pm 0.18\,\text{(stat.)}
\pm 0.49\,\text{(syst.)}, \\ &f_{\text{SM}} ( \Delta\phi(\text{combined})) &=
1.12 \pm 0.11\,\text{(stat.)} \pm 0.22\,\text{(syst.)}, \end{align*} are
consistent with the Standard Model prediction of $f_{\text{SM}}= 1.0$.
