  The Unruh effect is the phenomenon that accelerated observers detect
particles even when inertial observers experience the vacuum state. In
particular, uniformly accelerated observers are predicted to measure thermal
radiation that is proportional to the acceleration. Here we consider the Unruh
effect for a detector that follows a quantum superposition of different
accelerated trajectories in Minkowski spacetime. More precisely, we analyse the
excitations of a pointlike multilevel particle detector coupled to a massless
real scalar field and moving in the superposition of accelerated trajectories.
We find that the state of the detector excitations is, in general, not a mere
(convex) mixture of the thermal spectrum characteristics of the Unruh effect
for each trajectory with well-defined acceleration separately. Rather, for
certain trajectories and excitation levels, and upon the measurement of the
trajectory state, the state of the detector excitations features in addition
off-diagonal terms. The off-diagonal terms of these "superpositions of thermal
states" are related to the distinguishability of the different possible states
in which the field is left after its interaction with detector's internal
degrees of the freedom.

Quantum Physics;General Relativity and Quantum Cosmology