  Decoherence is the process via which quantum superpositions states are
reduced to classical mixtures. Decoherence has been predicted for
relativistically accelerated quantum systems, however examples to date have
involved restricting the detected field modes to particular regions of
space-time. If the global state over all space-time is measured then unitarity
returns and the decoherence is removed. Here we study a decoherence effect
associated with accelerated systems that cannot be explained in this way. In
particular we study a uniformly accelerated source of a quantum field state - a
single-mode squeezer. Even though the initial state of the field is vacuum (a
pure state) and the interaction with the quantum source in the accelerated
frame is unitary, we find that the final state detected by inertial observers
is decohered, i.e. in a mixed state. This unexpected result may indicate new
directions in resolving inconsistencies between relativity and quantum theory.
We extend this result to a two-mode state and find entanglement is also
decohered.
