Quasi‐classical fluctuation–dissipation description of dielectric loss in oxides with implications for quantum information processing

One of the most important problems in developing devices for quantum computation is the coupling and dissipation of states by thermal noise. We present a study of a two‐state electric dipole in a crystal coupling to noise from a reservoir. As a realization of such an energy‐dissipating dipole, we report and analyze dielectric loss measurements in single crystal and polycrystalline Al₂O₃ over the temperature range 70–300 K. We are able to model the dielectric loss in terms of a quasi‐classical model that uses the fluctuation–dissipation theorem. Two key parameters in this model are the crystal oscillator energy and reservoir–lattice coupling constant. In polycrystalline samples, it is assumed that the main effect of structural disorder is a modification of the spectrum of the thermal phonons, so that acoustical vibrations acquire some optical mode character. The temperature dependence of the linewidth of the high dielectric strength infrared (IR) mode at 438 cm^?1 and the quasi‐degenerate Raman mode of the k = 0 (418 cm^?1) transition are also investigated and are shown to be related simply to the dielectric loss. The model reproduces the unusual temperature dependence of the dielectric loss observed experimentally. The implications for the coupling of quantum mechanical objects to noise and quantum information processing are discussed. © 2005 Wiley Periodicals, Inc. Int J Quantum Chem, 2006