Quantum applications referred to here are those applications that are dependent on the preservation of quantum information. In the millimetre wave band this is an undeveloped area of technology as:
1) The natural (digital) information contain in packets of energy hf has an energy far smaller than the thermal quanta of energy kT, meaning this signal is swamped (or hidden) when processing using sensors operating at ambient temperature.
2) Quantum information in the form of entangled states has the potential for novel forms of digital processing, however entanglement can easily be lost through decoherence.
The above indicates a great potential for systems exploiting quantum physics. Problems of associated with the small energy of millimetre wave photons can be overcome by combining homodyne interferometers with parametric amplifiers. Decoherence is less than in the optical band, as waves interact less with materials per unit length. Development of these systems can then be used to explore powerful novel sensing capabilities using existing electronic technology, to explore secure communications, covert radar and imaging. Experimentation can provide new insights into the fundamental nature of entanglement and the clockwork universe in an as yet unchartered spectral band.
The objective of this area of research is to bring the science of quantum optics into the millimetre wave band. Potential novel sources of entangled photons to enable this require materials or devices with nonlinear responses to electromagnetic radiation. Phase matching, by the tuning of refractive indices, can increase the efficiencies of entangled photon creation. Diodes, surface acoustic wave devices, dielectrics or magnetic materials are potential sources.