Quantum key distribution by phase flipping of coherent states of light

In this paper we present quantum key distribution protocol that, instead of single qubits, uses coherent states of light $|alpharangle$ to encode bit values of a randomly generated key. Given the reference value $alphainmathbb C$, and a string of phase rotations each randomly taken from a set of $2M$ equidistant phases, Alice prepares a quantum state given by a product of coherent states of light, such that a complex phase of each pulse is rotated by the corresponding phase rotation. The encoding of $i$-th bit of the key $r=r_1 dots r_ell$ is done by further performing phase rotation $r_i pi$ (with $r_i = 0,1$) on the $i$-th coherent state pulse. In order to protect the protocol against the man-in-the-middle and the collective attack, we introduce two types of verification procedures, and analyse the protocolu0027s security using the Holevo bound.