  The vacuum state of a relativistic quantum field contains entanglement
between regions separated by spacelike intervals. Such spatial entanglement can
be revealed using an operational method introduced in Ann. Phys. 351, 112
(2014), Phys. Rev. D 91, 016005 (2014). In this approach, a cavity is
instantaneously divided into halves by an introduction of an extra perfect
mirror. Causal separation of the two regions of the cavity reveals nonlocal
spatial correlations present in the field, which can be quantified by measuring
particles generated in the process. We use this method to study spatial
entanglement properties of nonvacuum Gaussian field states. In particular we
show how to enhance the amount of harvested spatial entanglement by an
appropriate choice of the initial state of the field in the cavity. We find a
counterintuitive influence of the initial entanglement between cavity modes on
the spatial entanglement which is revealed by dividing the cavity in half.
