  We propose a scheme for optical entanglement distribution in quantum networks
based on a quasi-deterministic entangled photon pair source. By combining
heralded photonic Bell pair generation with spectral mode conversion to
interface with quantum memories, the scheme eliminates switching losses due to
multiplexing. We analyze this `zero-added-loss multiplexing' (ZALM) Bell pair
source for the particularly challenging problem of long-baseline entanglement
distribution via satellites and ground-based memories, where it unlocks
additional advantages: (i) the substantially higher channel efficiency $\eta$
of \textit{downlinks} vs.\ \textit{uplinks} with realistic adaptive optics, and
(ii) photon loss occurring \textit{before} interaction with the quantum memory
-- i.e., Alice and Bob receiving rather than transmitting -- improve
entanglement generation rate scaling by $\mathcal{O}(\sqrt{\eta})$. Based on
numerical analyses, we estimate our protocol to achieve $>$10$~$ebits/s at
memory multiplexing of $10^2$ spin qubits for ground distance $>$10$^2~$km,
with the spin-spin Bell state fidelity exceeding 99$\%$. Our architecture
presents a blueprint for realizing global-scale quantum networks in the
near-term.
