74,869 research outputs found
How a protein searches for its specific site on DNA: the role of intersegment transfer
Proteins are known to locate their specific targets on DNA up to two orders
of magnitude faster than predicted by the Smoluchowski three-dimensional
diffusion rate. One of the mechanisms proposed to resolve this discrepancy is
termed "intersegment transfer". Many proteins have two DNA binding sites and
can transfer from one DNA segment to another without dissociation to water. We
calculate the target search rate for such proteins in a dense globular DNA,
taking into account intersegment transfer working in conjunction with DNA
motion and protein sliding along DNA. We show that intersegment transfer plays
a very important role in cases where the protein spends most of its time
adsorbed on DNA.Comment: 9 pages, 7 figure
Fluctuations of the vacuum energy density of quantum fields in curved spacetime via generalized zeta functions
For quantum fields on a curved spacetime with an Euclidean section, we derive
a general expression for the stress energy tensor two-point function in terms
of the effective action. The renormalized two-point function is given in terms
of the second variation of the Mellin transform of the trace of the heat kernel
for the quantum fields. For systems for which a spectral decomposition of the
wave opearator is possible, we give an exact expression for this two-point
function. Explicit examples of the variance to the mean ratio of the vacuum energy density of a
massless scalar field are computed for the spatial topologies of and , with results of , and
respectively. The large variance signifies the importance
of quantum fluctuations and has important implications for the validity of
semiclassical gravity theories at sub-Planckian scales. The method presented
here can facilitate the calculation of stress-energy fluctuations for quantum
fields useful for the analysis of fluctuation effects and critical phenomena in
problems ranging from atom optics and mesoscopic physics to early universe and
black hole physics.Comment: Uses revte
Generating entanglement with low Q-factor microcavities
We propose a method of generating entanglement using single photons and
electron spins in the regime of resonance scattering. The technique involves
matching the spontaneous emission rate of the spin dipole transition in bulk
dielectric to the modified rate of spontaneous emission of the dipole coupled
to the fundamental mode of an optical microcavity. We call this regime
resonance scattering where interference between the input photons and those
scattered by the resonantly coupled dipole transition result in a reflectivity
of zero. The contrast between this and the unit reflectivity when the cavity is
empty allow us to perform a non demolition measurement of the spin and to non
deterministically generate entanglement between photons and spins. The chief
advantage of working in the regime of resonance scattering is that the required
cavity quality factors are orders of magnitude lower than is required for
strong coupling, or Purcell enhancement. This makes engineering a suitable
cavity much easier particularly in materials such as diamond where etching high
quality factor cavities remains a significant challenge
Microspectroscopy and Imaging in the THz Range Using Coherent CW Radiation
A novel THz near-field spectrometer is presented which allows to perform
biological and medical studies with high spectral resolution combined with a
spatial resolution down to l/100. In the setup an aperture much smaller than
the used wavelength is placed in the beam very close to the sample. The sample
is probed by the evanescent wave behind the aperture. The distance is measured
extremely accurate by a confocal microscope. We use monochromatic sources which
provide powerful coherent cw radiation tuneable from 50 GHz up to 1.5 THz.
Transmission and reflection experiments can be performed which enable us to
study solids and molecules in aqueous solution. Examples for spectroscopic
investigations on biological tissues are presented.Comment: 4 pages, 5 figures, email: [email protected]
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