  High-rate, high-fidelity entanglement distribution is essential to the
creation of a quantum internet, but recent achievements in fiber and
satellite-based entanglement distribution fall far short of what is needed.
Chen et al. [Phys. Rev. Appl. 19, 054209 (2023)] proposed a means for
dramatically increasing entanglement-distribution rates via zero added-loss
multiplexing (ZALM). ZALM's quantum transmitter employs a pair of
Sagnac-configured spontaneous parametric downconverters (SPDCs), channelization
via dense wavelength-division multiplexing (DWDM) filtering, and partial
Bell-state measurements (BSMs) to realize a near-deterministic, heralded source
of frequency-multiplexed polarization-entangled biphotons. Each biphoton is
transmitted to Alice and Bob with a classical message identifying its frequency
channel and the heralded entangled state. Their quantum receivers use DWDM
filtering and mode conversion to interface their received biphotons to
intra-cavity color-center quantum memories. This paper delves deeply into
ZALM's SPDCs, partial-BSMs, and loading of Alice and Bob's quantum memories. It
derives the density operators for the SPDC sources and the quantum memories,
allowing heralding probability, heralding efficiency, and fidelity to be
evaluated for both the polarization-entangled biphotons and the loaded quantum
memories, thus enabling exploration of the parameter space for optimizing ZALM
performance. Even without optimization analysis, the paper already demonstrates
two critical features of the ZALM architecture: the necessity of achieving a
near-separable channelized biphoton wave function to ensure the biphoton sent
to Alice and Bob is of high purity; and the premium placed on Alice and Bob's
temporal-mode converters' enabling narrowband push-pull memory loading to
ensure the arriving biphoton's state is faithfully transferred to the
intra-cavity color centers.
