I' m trying to understand attacks on Quantum key distribution protocols. In the COW QKD protocol, pulses from Alice are passed through an asymmetric coupler and a fraction 'T' of the photons are sent to generate key, and the remaining fraction '1 − T' is sent into an interferometer to monitor coherence of consecutive pulses. In one of the references "Zero-Error Attacks and Detection Statistics in the Coherent One-Way Protocol for Quantum Cryptography"by Gisin et al. I came across the sentence:
If any two non-empty pulses are coherent and all pulses are equidistant, any attempt of measuring the number of photons on a finite number of pulses breaks some coherence and contributes to errors.
which I'm unable to understand.