21,549 research outputs found
Quasi-Topological Ricci Polynomial Gravities
Quasi-topological terms in gravity can be viewed as those that give no
contribution to the equations of motion for a special subclass of metric
ans\"atze. They therefore play no r\^ole in constructing these solutions, but
can affect the general perturbations. We consider Einstein gravity extended
with Ricci tensor polynomial invariants, which admits Einstein metrics with
appropriate effective cosmological constants as its vacuum solutions. We
construct three types of quasi-topological gravities. The first type is for the
most general static metrics with spherical, toroidal or hyperbolic isometries.
The second type is for the special static metrics where is
constant. The third type is the linearized quasi-topological gravities on the
Einstein metrics. We construct and classify results that are either dependent
on or independent of dimensions, up to the tenth order. We then consider a
subset of these three types and obtain Lovelock-like quasi-topological
gravities, that are independent of the dimensions. The linearized gravities on
Einstein metrics on all dimensions are simply Einstein and hence ghost free.
The theories become quasi-topological on static metrics in one specific
dimension, but non-trivial in others. We also focus on the quasi-topological
Ricci cubic invariant in four dimensions as a specific example to study its
effect on holography, including shear viscosity, thermoelectric DC
conductivities and butterfly velocity. In particular, we find that the
holographic diffusivity bounds can be violated by the quasi-topological terms,
which can induce an extra massive mode that yields a butterfly velocity unbound
above.Comment: Latex, 56 pages, discussion on shear viscosity revise
Cooling mechanical resonators to quantum ground state from room temperature
Ground-state cooling of mesoscopic mechanical resonators is a fundamental
requirement for test of quantum theory and for implementation of quantum
information. We analyze the cavity optomechanical cooling limits in the
intermediate coupling regime, where the light-enhanced optomechanical coupling
strength is comparable with the cavity decay rate. It is found that in this
regime the cooling breaks through the limits in both the strong and weak
coupling regimes. The lowest cooling limit is derived analytically at the
optimal conditions of cavity decay rate and coupling strength. In essence,
cooling to the quantum ground state requires , with being the mechanical quality factor and
being the thermal phonon number. Remarkably, ground-state
cooling is achievable starting from room temperature, when mechanical
-frequency product , and both of the
cavity decay rate and the coupling strength exceed the thermal decoherence
rate. Our study provides a general framework for optimizing the backaction
cooling of mesoscopic mechanical resonators
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