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