Polarization in Quantum Optics

Ravi Shankar Singh, Hari Prakash

The classical definition of degree of polarization (DOP) is expressed in the quantum domain by replacing intensities through quantum mechanical average values of relevant number operators and is viewed as the first generalization of intensity. This definition assigns inaccurately the unpolarized status to some typical optical fields, e.g., amplitude-coherent phase-randomized and hidden-polarized light, which are not truly unpolarized light. The apparent paradoxical trait is circumvented by proposing a new definition of DOP in quantum optics through the second generalization of intensity. The correspondence of a new DOP to the usual DOP in quantum optics is established. It is seen that the two definitions disagree significantly for intense optical fields but coincide for weak light (thermal light) or for optical fields in which occupancy of photons in the orthogonal mode is very feeble. Our proposed definition of DOP, similar to other proposals in literature, reveals an interesting feature that states of polarization of optical quantum fields depend upon the average photons (intensity) present therein.

Alfredo Luis

J. E. Nordholt

Quantum communication transmitters include beacon lasers that transmit a beacon optical signal in a predetermined state of polarization such as one of the states of polarization of a quantum communication basis. Changes in the beacon polarization are detected at a receiver, and a retarder is adjusted so that the states of polarization in a received quantum communication optical signal are matched to basis polarizations. The beacon and QC signals can be at different wavelengths so that the beacon does not interfere with detection and decoding of the QC optical signal.

Klaus M. Gheri, Daniel F. Walls

Huan Zhao, Ma Hai-Qiang, Ling-An Wu

A method is proposed for measuring and controlling optical polarization by using circular fiber polarization controllers. A detailed matrix analysis of the modulation of the optical polarization state is given for the two-disc type controller. It is experimentally verified that this method is effective for controlling polarization in the quantum channel of a quantum cryptography system over a transmission distance of 25 km.