= Solution
A herald tells the experimenter when an entangled resource has actually been prepared while leaving the two resource <photons> available for subsequent operations. This permits synchronization, conditional storage or switching, and feed-forward. Direct down-conversion emission is probabilistic; inferring that a pair existed by destructively detecting both <photons> does not leave that same pair available as an input resource.
The second network uses passive optical elements, auxiliary <entanglement> and detection rather than a strong deterministic <photon>–<photon> <CNOT gate> interaction. Its smaller success rate can be acceptable if trials are inexpensive and successful events are reliably announced. Heralding is still probabilistic, not deterministic on demand; losses, dark counts and multiple-pair emission can give false or degraded heralds. The ideal $1/64$ result assumes the specified input pairs and valid detection model, so it should not be confused with a complete laboratory production efficiency.
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