Kinematic spin decoherence of a wave packet in a gravitational field

The spin state of a wave packet of a spin-1/2 particle in a gravitational field is a mixed state, obtained by integrating out the momentum degree of freedom. The entropy of this spin mixed state depends on the deviation of spacetime from the flatness. In terms of a succession of local inertial frames, it is shown that spin decoherence occurs even if the particle is uniformly moving or static in a gravitational field.