19,396 research outputs found
Quantum computation with the Jaynes-Cummings model
In this paper, we propose a method for building a two-qubit gate with the
Jaynes-Cummings model (JCM). In our scheme, we construct a qubit from a pair of
optical paths where a photon is running. Generating Knill, Laflamme and
Milburn's nonlinear sign-shift gate by the JCM, we construct the conditional
sign-flip gate, which works with small error probability in principle. We also
discuss two experimental setups for realizing our scheme. In the first
experimental setup, we make use of coherent lights to examine whether or not
our scheme works. In the second experimental setup, an optical loop circuit
made out of the polarizing beam splitter and the Pockels cell takes an
important role in the cavity.Comment: 4 pages, 2 eps figures, latex2e; v2: Figure 1 and its caption are
modified; v3: one new section is added; v4: experimental setups are
completely rewritten; v5: minor corrections; v6: two references added; v7: 18
peges, 11 eps figures, PTPTeX, LaTeX2e, Section 4 is rewritte
Interaction-free measurement with an imperfect absorber
In this paper, we consider interaction-free measurement (IFM) with imperfect
interaction. In the IFM proposed by Kwiat et al., we assume that interaction
between an absorbing object and a probe photon is imperfect, so that the photon
is absorbed with probability 1-\eta (0\leq\eta\leq 1) and it passes by the
object without being absorbed with probability \eta when it approaches close to
the object. We derive the success probability P that we can find the object
without the photon absorbed under the imperfect interaction as a power series
in 1/N, and show the following result: Even if the interaction between the
object and the photon is imperfect, we can let the success probability P of the
IFM get close to unity arbitrarily by making the reflectivity of the beam
splitter larger and increasing the number of the beam splitters. Moreover, we
obtain an approximating equation of P for large N from the derived power series
in 1/N.Comment: 6 pages, 3 eps figures, latex2e; v2: minor corrections; v3: the title
is change
Brownian Dynamics Studies on DNA Gel Electrophoresis. II. `Defect' Dynamics in the Elongation-Contraction Motion
By means of the Brownian dynamics (BD) method of simulations we have
developed, we study dynamics of individual DNA undergoing constant field gel
electrophoresis (CFGE), focusing on the relevance of the `defect' concept due
to de Gennes in CFGE. The corresponding embodiment, which we call {\it slack
beads} (s-beads) is explicitly introduced in our BD model. In equilibrium under
a vanishing field the distance between s-beads and their hopping range are
found to be randomly distributed following a Poisson distribution. In the
strong field range, where a chain undergoes the elongation-contraction motion,
s-beads are observed to be alternatively annihilated in elongation and created
in contraction of the chain. On the other hand, the distribution of hopping
range of s-beads does not differ much from that in equilibrium. The results
indicate that the motion of the chain elongated consists of a huge number of
random movements of s-beads. We have also confirmed that these features of
s-beads agree qualitatively with those of s-monomers in the extended bond
fluctuation model (EBFM) which we recently proposed. The coincidence of the two
simulations strongly supports the stochastic semi-local movement of s-monomers
which we {\it a priori} introduced into the EBFM.Comment: 14 pages, 11 figure
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