  Orthogonal temporal modes (TMs) form a field-orthogonal, continuous-variable
degree of freedom that is in principle infinite dimensional, and create a
promising resource for quantum information science and technology. The ideal
quantum pulse gate (QPG) is a device that multiplexes and demultiplexes
temporally orthogonal optical pulses that have the same carrier frequency,
spatial mode, and polarization. The QPG is the chief enabling technology for
usage of orthogonal temporal modes as a basis for high-dimensional quantum
information storage and processing. The greatest hurdle for QPG implementation
using nonlinear-optical, parametric processes with time-varying pump or control
fields is the limitation on achievable temporal mode selectivity, defined as
perfect TM discrimination combined with unity efficiency. We propose the use of
pulsed nonlinear frequency conversion in an optical cavity having greatly
different finesses for different frequencies to implement a nearly perfectly
TM-selective QPG in a low-loss integrated-optics platform.
