  We demonstrate that local application of a weak external field increases
distinguishability between states with and without initial correlations. We
consider the case where a two-level system linearly and adiabatically interacts
with an infinite number of bosons. We evaluate the trace distance (or,
equivalently, the Hilbert-Schmidt distance) between the quantum states which
evolve from two kinds of initial states; the correlated Gibbs state and its
uncorrelated marginal state. We find that the trace distance increases above
its initial value for any and all parameter settings. This indicates that we
can explore the existence of initial system-environment correlations with
applying a local external field, which causes the breakdown of the
contractivity.
