  We consider the non-equilibrium evolution in the spin-1/2 XXZ Heisenberg
chain for fixed magnetization after a local quantum quench. This model is
equivalent to interacting spinless fermions. Initially an infinite magnetic
field is applied to n consecutive sites and the ground state is calculated. At
time t=0 the field is switched off and the time evolution of observables such
as the z-component of spin is computed using the Time Evolving Block Decimation
(TEBD) algorithm. We find that the observables exhibit strong signatures of
linearly propagating spinon and bound state excitations. These persist even
when integrability-breaking perturbations are included. Since bound states
("strings") are notoriously difficult to observe using conventional probes such
as inelastic neutron scattering, we conclude that local quantum quenches are an
ideal setting for studying their properties. We comment on implications of our
results for cold atom experiments.
