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 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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