Decoherence without Dissipation

That decoherence can take place in the presence of energy conservation seems to be a poorly known fact. That lack of knowledge has for example bedevilled the discussion of the "black hole information" problem. I present a simple model which illustrates such energy free decoherence.Comment: 4 pages-- To appear in Phil Trans Roy Soc in special issue on Decoherence, ed. A. Haga

Maintaining coherence in Quantum Computers

The effect of the inevitable coupling to external degrees of freedom of a quantum computer are examined. It is found that for quantum calculations (in which the maintenance of coherence over a large number of states is important), not only must the coupling be small but the time taken in the quantum calculation must be less than the thermal time scale, $\hbar/k_B T$. For longer times the condition on the strength of the coupling to the external world becomes much more stringent.Comment: 13 page

Long-wavelength metric backreactions in slow-roll inflation

We examine the importance of second order corrections to linearized cosmological perturbation theory in an inflationary background, taken to be a spatially flat FRW spacetime. The full second order problem is solved in the sense that we evaluate the effect of the superhorizon second order corrections on the inhomogeneous and homogeneous modes of the linearized flucuations. These second order corrections enter in the form of a {\it cumulative} contribution from {\it all} of their Fourier modes. In order to quantify their physical significance we study their effective equation of state by looking at the perturbed energy density and isotropic pressure to second order. We define the energy density (isotropic pressure) in terms of the (averaged) eigenvalues associated with timelike (spacelike) eigenvectors of a total stress energy for the metric and matter fluctuations. Our work suggests that that for many parameters of slow-roll inflation, the second order contributions to these energy density and pressures may dominate over the first order effects for the case of super-Hubble evolution. These results hold in our choice of first and second order coordinate conditions however we also argue that other `reasonable` coordinate conditions do not alter the relative importance of the second order terms. We find that these second order contributions approximately take the form of a cosmological constant in this coordinate gauge, as found by others using effective methods.Comment: Submitted to Phys. Rev.