Modulation schemes for frequency coded quantum key distribution

Frequency coding has been presented as a new alternative for qubits transmission on QKD systems. In this work we present an analysis of the different possibilities for modulation (amplitude or phase) in face of the BB84 and B92 protocols within systems employing frequency coding. It will be shown that schemes employing phase modulation both at Alice and Bob (PM-PM) are equivalent, in the sense that the classical BB84 protocol cannot be applied, to schemes using solely amplitude modulation (AM-AM). We present a new modulation scheme employing a combination of phase and amplitude modulation (AM-PM) which can accommodate classical BB84.