Biomedical subjects

J Tworzydło

Publications and source records attributed to J Tworzydło.

4 recordsLinked to original sources

PubMed2006

Sub-Poissonian shot noise in graphene.

We calculate the mode-dependent transmission probability of massless Dirac fermions through an ideal strip of graphene (length L, width W, no impurities or defects) to obtain the conductance and shot noise as a function of Fermi energy. We find that the minimum conductivity of order e2/h at the Dirac point (when the electron and hole excitations are degenerate) is associated with a maximum of the Fano factor (the ratio of noise power and mean current). For short and wide graphene strips the Fano factor at the Dirac point equals 1/3, 3 times smaller than for a Poisson process. This is the same value as for a disordered metal, which is remarkable since the classical dynamics of the Dirac fermions is ballistic.

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PubMed2004

Exponential sensitivity to dephasing of electrical conduction through a quantum dot.

According to random-matrix theory, interference effects in the conductance of a ballistic chaotic quantum dot should vanish proportional to (tau(phi)/tau(D))(p) when the dephasing time tau(phi) becomes small compared to the mean dwell time tau(D). Aleiner and Larkin have predicted that the power law crosses over to an exponential suppression proportional to exp((-tau(E)/tau(phi)) when tau(phi) drops below the Ehrenfest time tau(E). We report the first observation of this crossover in a computer simulation of universal conductance fluctuations. Their theory also predicts an exponential suppression proportional to exp((-tau(E)/tau(D)) in the absence of dephasing--which is not observed. We show that the effective random-matrix theory proposed previously for quantum dots without dephasing explains both observations.

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PubMed2003

Hypersensitivity to perturbations of quantum-chaotic wave-packet dynamics.

We reexamine the problem of the "Loschmidt echo," that measures the sensitivity to perturbation of quantum-chaotic dynamics. The overlap squared M(t) of two wave packets evolving under slightly different Hamiltonian is shown to have the double-exponential initial decay proportional to exp(-constant x e(2lambda(0)t)) in the main part of the phase space. The coefficient lambda(0) is the self-averaging Lyapunov exponent. The average decay (-)M proportional to e(-lambda(1)t) is single exponential with a different coefficient lambda(1). The volume of phase space that contributes to (-)M vanishes in the classical limit variant Planck-->0 for times less than the Ehrenfest time tau(E)=1/2lambda0(-1)|ln Planck|. It is only after the Ehrenfest time that the average decay is representative for a typical initial condition.

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PubMed2002

Quantum optical communication rates through an amplifying random medium.

We study the competing effects of stimulated and spontaneous emission on the information capacity of an amplifying disordered waveguide. At the laser threshold the capacity reaches a "universal" limit, independent of the degree of disorder. Whether or not this limit is larger or smaller than the capacity without amplification depends on the disorder, as well as on the input power. Explicit expressions are obtained for heterodyne detection of coherent states, and generalized for an arbitrary detection scheme.

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