  If the boundary conditions of the quantum vacuum are changed in time, quantum
field theory predicts that real, observable particles can be created in the
initially empty modes. Here, we realize this effect by changing the boundary
conditions of a spinor Bose-Einstein condensate, which yields a population of
initially unoccupied spatial and spin excitations. We prove that the
excitations are created as entangled excitation pairs by certifying
continuous-variable entanglement within the many-particle output state.
