Phase randomization improves the security of quantum key distribution

Abstract

Ideal quantum key distribution (QKD) protocols call for a source that emits single photon signals, but the sources used in typical practical realizations emit weak coherent states instead. A weak coherent state may contain more than one photon, which poses a potential security risk. QKD with weak coherent state signals has nevertheless been proven to be secure, but only under the assumption that the phase of each signal is random (and completely unknown to the adversary). Since this assumption need not be fully justified in practice, it is important to know whether phase randomization is really a requirement for security rather than a convenient technical assumption that makes the security proof easier. Here, we exhibit an explicit attack in which the eavesdropper exploits knowledge of the phase of the signals, and show that this attack allows the eavesdropper to learn every key bit in a parameter regime where a protocol using phase-randomized signals is provably secure. Thus we demonstrate that phase randomization really does enhance the security of QKD using weak coherent states. This result highlights the importance of a careful characterization of the source for proofs of the security of quantum key distribution.

5 pages, REVTeX

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• Differential phase shift quantum key distribution experiment over 105 km fibre
• Quantum key distribution based on private states: unconditional security over untrusted channels with zero quantum capacity
• Manipulating the non-Gaussianity of phase-randomized coherent states
• Unconditional security at a low cost
• Quantum cryptography: theory and practice
• Security of quantum key distribution using weak coherent states with nonrandom phases
• A consolidated and accessible security proof for finite-size decoy-state quantum key distribution
• Optical transmitter for time-bin encoding Quantum Key Distribution