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The ideal Bennett-Brassard 1984 (BB84) quantum key distribution protocol isbased on the preparation and measurement of qubits in two alternative basesdiffering by an angle of pi/2. Any real implementation of the protocol, though,will inevitably introduce misalignments in the preparation of the states and inthe alignment of the measurement bases with respect to this ideal situation.Various security proofs take into account (at least partially) such errors,i.e., show how Alice and Bob can still distil a secure key in the presence ofthese imperfections. Here, we consider the complementary problem: how can Eveexploit misalignments to obtain more information about the key than would bepossible in an ideal implementation? Specifically, we investigate the effectsof misalignment errors on the security of the BB84 protocol in the case ofindividual attacks, where necessary and sufficient conditions for security areknown. Though the effects of these errors are small for expected deviationsfrom the perfect situation, our results nevertheless show that Alice and Bobcan incorrectly conclude that they have established a secure key if theinevitable experimental errors in the state preparation and in the alignment ofthe measurements are not taken into account. This gives further weight to theidea that the formulation and security analysis of any quantum cryptographyprotocol should be based on realistic assumptions about the properties of theapparatus used. Additionally, we note that BB84 seems more robust againstalignment imperfections if both the x and z bases are used to generate the key.

Effects of preparation and measurement misalignments on the security of the BB84 quantum key distribution protocol