Decoherence: Intrinsic, Extrinsic, and Environmental

Environmental decoherence times have been difficult to predict in solid-state systems. In spin systems, environmental decoherence is predicted to arise from nuclear spins, spin-phonon interactions, and long-range dipolar interactions [1]. Recent experiments have confirmed these predictions quantitatively in crystals of Fe$_8$ molecules [2]. Coherent spin dynamics was observed over macroscopic volumes, with a decoherence $Q$-factor $Q_{\phi} = 1.5 \times 10^6$ (the upper predicted limit in this system being $Q_{\phi} = 6 \times 10^7$). Decoherence from dipolar interactions is particularly complex, and depends on the shape and the quantum state of the system. No extrinsic ``noise'' decoherence was observed. The generalization to quantum dot and superconducting qubit systems is also discussed. We then discuss searches for ``intrinsic'' decoherence [3,4], coming from non-linear corrections to quantum mechanics. Particular attention is paid to condensed matter tests of such intrinsic decoherence, in hybrid spin/optomechanical systems, and to ways of distinguishing intrinsic decoherence from environmental and extrinsic decoherence sources. \\[4pt] [1] Morello, A. Stamp, P. C. E. \& Tupitsyn, Phys. Rev. Lett. {\bf 97}, 207206 (2006).\\[0pt] [2] S. Takahashi et al., Nature {\bf 476}, 76 (2011).\\[0pt] [3] Stamp, P. C. E., Stud. Hist. Phil. Mod. Phys. {\bf 37}, 467 (2006). \\[0pt] [4] Stamp, P.C.E., Phil. Trans. Roy. Soc. A (to be published)