> If any of the pulses’ paths are interrupted and they don’t arrive at the endpoint at the expected nanosecond, the sender and receiver know their communication

> If any of the pulses’ paths are interrupted and they don’t arrive at the endpoint at the expected nanosecond, the sender and receiver know their communication has been compromised. This isn't how QKD works - it isn't based on timing or delays. The common BB84 protocol (and the decoy-state modification) are based on the fact that measuring a photon necessarily changes it. Essentially, it is easy to distinguish a photon that's been measured twice from a photon that's been measured once (statistically speaking). Because an eavesdropper would necessarily have to measure photons in order to extract any useful information about the communication, their presence can be easily detected.