This problem is based on the \"B92\" method of quantum key distribution. See Phy

Question

This problem is based on the "B92" method of quantum key distribution. See Physics World. May 1999. Photons are directed at a polaroid sheet held by Alice. The Transmission Axis (TA) of Alice's sheet is either vertical (V) or at 45degree. When Alice wants to send a "1" bit she uses the 45degree orientation and when she wants to send a "0" bit she uses the V orientation. Some distance away Bob has two polaroid sheets. His polaroid TA can be either horizontal (H) or at -45degree. When Bob uses his H sheet he assigns a "1" bit to any photon he observes. When Bob uses his -45degree sheet he assigns a "0" bit to any photon he observes. For each of the two types of photons that Alice can send make a table showing what Bob sees with each of his two polaroid orientations. For each case give the probability that Bob sees a photon. Bob and Alice keep the bits for which Bob saw a photon and they use these as the key. In this method they never discuss the polaroid orientations but just note the cases where Bob observed a photon. Show that for the kept subset of bits Bob and Alice agree on the bit values. Alice and Bob choose their polaroid sheets at random. What fraction of bits sent by Alice will be seen by Bob and therefore used? What happens if an evesdropper Eve, places a polaroid sheet (for example in the H direction) between Alice and Bob. What fraction of photons will Alice and Bob agree on now?

Explanation / Answer

If Alice and Bob use the sheets at random, Bob would see 50% of bits sent by Alice on an average.

if a polaroid with H direction is placed in between Alice and Bob, he would see a photon always. The phtons coming out of the intermdiate would be alinged along H, which can be consdiered as composed of +45 and -45. Thus Bob will always see a photon, whether he uses H sheet or -45 sheet.

Alice Alice Bob Bob Bob.Porb. V 0 H 0 0 0 -45 1 1 +45 1 H 1 1 1 -45 0 0

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