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When using the BB84 algorithm for QKD, you arbitrarily choose which of two filters (bases) to use when detecting a photon: rectilinear or diagonal. If you choose rectilinear, you can detect polarization with certainty if the photon is rectilinear, i.e., either vertically- or horizontally-polarized.

  1. But how? A polarization filter that allows transmission of vertically-polarized photons will block horizontally-polarized photons and vice-versa, no?
  2. Perhaps you use a vertically-polarized filter as the rectilinear filter and then the complete lack of a photon indicates horizontal polarization, but how would you distinguish that from, say, absorption of the photon into the fiber?
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  • $\begingroup$ This may be better to be asked Quantum.Se site or Physics.SE? $\endgroup$ – kelalaka Feb 21 at 20:36
  • $\begingroup$ Perhaps--it really is a physics question. $\endgroup$ – Evariste Feb 21 at 20:51

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