 If more than formula_1 bits differ they abort the key and try again, possibly with a different quantum channel, as the security of the key cannot be guaranteed.formula_1 is chosen so that if the number of bits known to Eve is less than this, privacy amplification can be used to reduce Eve's knowledge of the key to an arbitrarily small amount at the cost of reducing the length of the key.Artur Ekert's scheme  uses entangled pairs of photons.