  In classical coherence theory, coherence time is typically related to the
bandwidth of the optical field. Narrowing the bandwidth will result in the
lengthening of the coherence time. This will erase temporal distinguishability
of photons due to time delay in pulsed photon interference. However, this is
changed in an SU(1,1)-type quantum interferometer where quantum entanglement is
involved. In this paper, we investigate how the temporal coherence of the
fields in a pulse-pumped SU(1,1) interferometer changes with the bandwidth of
optical filtering. We find that, because of the quantum entanglement, the
coherence of the fields does not improve when optical filtering is applied, in
contrary to the classical coherence theory, and quantum entanglement plays a
crucial role in quantum interference in addition to distinguishability.
