  The Minkowski vacuum state is expressed as an entangled state between the
left and right Rindler wedges when it is constructed on the Rindler vacuum. In
this paper, we further examine the entanglement structure and extend the
expression to the future (expanding) and past (shrinking) Kasner spacetimes.
This clarifies the origin of the quantum radiation produced by an Unruh--DeWitt
detector in uniformly accelerated motion in the four-dimensional Minkowski
spacetime. We also investigate the two-dimensional massless case where the
quantum radiation vanishes but the same entanglement structure exists.
