Digital communication using a cyclic level scheme in an atomic radio-over-fiber device

We experimentally demonstrate binary phase-shift keying and multi-stage four phase shift keying of a microwave carrier and its corresponding demodulation in the optical regime using room temperature Rb atoms. We use a cyclic three-level scheme in 85Rb atoms to achieve this. The importance of our scheme is that the cyclic, closed interaction of the atomic levels with electromagnetic fields makes our system inherently sensitive to the phase of the microwave field. This enables our system to directly encode a phase modulated digital signal in the microwave field and decode it as intensity modulation in the optical field. We measure the correlation of our demodulated optical signal with an ideal template and establish a viable signal bandwidth of about 1 MHz. Our atomic scheme also enables phase dependent amplification of the demodulated optical signal through a hybrid second order nonlinearity. This phase dependent atomic antenna has inherent features of demodulation, radio-to-optical conversion, and amplification. The ground states used in our scheme are quantum memory storage spin states, which makes our scheme inherently suitable for applications involving communication and storage and retrieval of quantum signals.