States insensitive to the Unruh effect for multi-level detectors

Phys. Rev. D 57, p. 1045 (1998)     gr-qc/9707058    

If an atom or detector is in inertial motion its ground state is stable. Only excited states decay through spontaneous emission of a photon. This is depicted for a two-level detector in the following diagram:

stable ground state        spontaneous emission
Quantum field theory predicts that if the detector is accelerated its ground state can be spontaneously excited even when moving through the vacuum. This is the Unruh effect:
We examined how a multi-level detector behaves if it is accelerated through the vacuum. As a model for such a detector we considered a kind of multi-level atom. We found that for certain detectors there are stable superpositions of two ground states which are not sensitive to the Unruh effect, i.e. they are not spontaneously excited:

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