  The particle definition varies across different theories. The quantum field
theory in curved spacetime shows that from the perspective of a linearly
accelerated observer, an inertial empty space may be full of thermal particles.
This effect is known as the Unruh effect. When the degrees of freedom of
orbital angular momentum (OAM) are considered, all OAM modes share the same
expected particle number. Here, we examine the OAM spectrum in a rotating
accelerated reference frame to see how the spectrum differs from the linear
accelerated case. When the observer starts to rotate, not all OAM modes are
allowed and some negative energy modes show up. To understand how a rotating
accelerated observer actually perceives these particles, the Unruh-DeWitt
detector and its detailed balance are studied. This relation is studied both in
the comoving inertial frame and in the rest frame. Based on these results, the
OAM entanglement degradation is explored in two-dimensional and
high-dimensional cases, respectively. The results indicate that the
entanglement dimension and the highest order of OAM modes are mainly related to
the acceleration and the rotation, respectively. It is then demonstrated that
these results can be generalized to all stationary trajectories.
