  We investigate decoherence of quantum superpositions induced by gravitational
time dilation and spontaneous emission between two atomic levels. It has been
shown that gravitational time dilation can be an universal decoherence source.
Here, we consider decoherence induced by gravitational time dilation only in
the situation of spontaneous emission. Then, we obtain that the coherence of
particle's position state depends on reference frame due to the time dilation
changing the distinguishability of emission photon from two positions of
particle. Changing the direction of light field can also result in the
difference about the coherence of quantum superpositions. For observing the
decoherence effect mainly due to gravitational time dilation, time-delayed
feedback can be utilized to increase the decoherence of particle's
superpositions.
