  We study the two-path interference of single-particle pulses measured by the
Unruh-DeWitt-type quantum detector, which itself is a quantum state as well as
the incoming pulse, and of which the interaction with the pulse is described by
unitary quantum evolution instead of a nonunitary collapsing process. Provided
that the quantum detector remains coherent in time long enough, the detection
probability still manifests the two-path interference pattern even if the
length difference between the two paths considerably exceeds the coherence
length of the single-particle pulse, contrary to the result measured by an
ordinary classical detector. Furthermore, it is formally shown that an ensemble
of identical Unruh-DeWitt-type quantum detectors collectively behaves as an
ordinary classical detector, if coherence in time of each individual quantum
detector becomes sufficiently short. Our study provides a concrete yet
manageable theoretical model to investigate the two-path interference measured
by a quantum detector and facilitates a quantitative analysis of the difference
between classical and quantum detectors. The analysis affirms the main idea of
decoherence theory: quantum behavior is lost as a result of quantum
decoherence.
