Optical coherence molecular photonics, quantum

Photons in quantum optical cavities also constitute excellent qubit candidates [52]. Resonant coupling of atoms with a single mode of the radiation field was experimentally achieved 25 years ago [53], and eventually the coherent coupling of quantum optical cavities with atoms or (simple) molecules was suggested as a means to achieve stable quantum memories in a hybrid quantum processor [54]. There might be a role to play for molecular spin qubits in this kind of hybrid quantum devices that combine solid-state with flying qubits. 

In this review we have described some of the advances in the quantum electrodynamical formulation of theory for molecular photonics. We have shown how the framework described in an earlier review has now been extended to new areas of application, and reformulated for application to real dispersive media—as reflected in the new treatment of refractive, dissipative, and resonance properties. With all its conceptual splendor, conventional quantum optics has not generally been pursued at this level of detail on its dielectric host, and it is our hope that this work will help match its precepts with quantitative accuracy. Applications of the new theory have revealed new quantum optical features in two quite different aspects of the familiar process of second harmonic generation, one operating through local coherence within small particles and the other, a coherence between the quantum amplitudes for fundamental and harmonic excitation. Where the salient experiments have been performed, they exactly match the theoretical predictions. The theoretical foundation we have discussed therefore shows promise for the delivery of accurate insights into other optical processes yet to be characterized, and it should be well placed to facilitate the determination of meaningful data from the associated experiments. 

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