Abstract: We disclose optical entanglement distribution in quantum networks based on a quasi-deterministic entangled photon pair source. Combining heralded photonic Bell pair generation with spectral mode conversion to interface with quantum memories eliminates switching losses due to multiplexing in the source. This zero-added-loss multiplexing (ZALM) Bell pair source is especially useful 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 ? of downlinks versus uplinks with realistic adaptive optics, and (ii) photon loss occurring before interaction with the quantum memory—i.e., Alice and Bob receiving rather than transmitting—improve entanglement generation rate scaling by (?{square root over (?)}).