Quantum cryptography without switching

A coherent state quantum key distribution (QKD) protocol based on simultaneous quadrature measurements was proposed. A lower bound on the secret key rate for this protocol, was also calculated. A possible eavesdropping attack in the form of a simple feed forward scheme, which provided with an upper bound on the secret key rate, was considered. It was found that a significantly larger secret key rate than that obtained by the usual single quadrature measurements was achieved by performing simultaneous quadrature measurements in a coherent state quantum cryptography protocol.