10 research outputs found
Selective coherence transfers in homonuclear dipolar coupled spin systems
Mapping the physical dipolar Hamiltonian of a solid-state network of nuclear
spins onto a system of nearest-neighbor couplings would be extremely useful for
a variety of quantum information processing applications, as well as NMR
structural studies. We demonstrate such a mapping for a system consisting of an
ensemble of spin pairs, where the coupling between spins in the same pair is
significantly stronger than the coupling between spins on different pairs. An
amplitude modulated RF field is applied on resonance with the Larmor frequency
of the spins, with the frequency of the modulation matched to the frequency of
the dipolar coupling of interest. The spin pairs appear isolated from each
other in the regime where the RF power (omega_1) is such that omega_weak <<
omega_1 << omega_strong. Coherence lifetimes within the two-spin system are
increased from 19 us to 11.1 ms, a factor of 572.Comment: 4 pages. Paper re-submitted with minor changes to clarify that the
scheme demonstrated is not an exact mapping onto a nearest neighbor system.
However, this is the first demonstration of a controlled evolution in a
subspace of an extended spin system, on a timescale that is much larger than
the dipolar dephasing tim
Role of trans-Planckian modes in cosmology
Motivated by the old trans-Planckian (TP) problem of inflationary cosmology,
it has been conjectured that any consistent effective field theory should keep
TP modes `hidden' behind the Hubble horizon, so as to prevent them from turning
classical and thereby affecting macroscopic observations. In this paper we
present two arguments against the Hubble horizon being a scale of singular
significance as has been put forward in the TP Censorship Conjecture (TCC).
First, refinements of TCC are presented that allow for the TP modes to grow
beyond the horizon while still keeping the de-Sitter conjecture valid. Second,
we show that TP modes can turn classical even well within the Hubble horizon,
which, as such, negates this rationale behind keeping them from crossing it.
The role of TP modes is known to be less of a problem in warm inflation,
because fluctuations start out usually as classical. This allows warm inflation
to be more resilient to the TP problem compared to cold inflation. To
understand how robust this is, we identity limits where quantum modes can
affect the primordial power spectrum in one specific case.Comment: 33 pages, comments welcome; v2: References updated, matches published
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