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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.

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    $\begingroup$ Better on the Physics stack? $\endgroup$
    – Solar Mike
    May 10 '19 at 9:37
  • $\begingroup$ I concur, Mike. This is definitely physics $\endgroup$
    – Rhodie
    Dec 19 '19 at 1:09

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