  The ability to follow the dynamics of a quantum system in a quantitative
manner is of key importance for quantum technology. Despite its central role,
justifiable deduction of the quantum dynamics of a single quantum system in
terms of a macroscopical observable remains a challenge. Here we show that the
relation between the readout signal of a single electron spin and the quantum
dynamics of the single nuclear spin is given by a parameter related to the
measurement strength. We determine this measurement strength in independent
experiments and use this value to compare our analysis of the quantum dynamics
with experimental results. We prove the validity of our approach by measuring
violations of the Leggett-Garg inequality.
