Biomedical subjects

C H Lewenkopf

Publications and source records attributed to C H Lewenkopf.

7 recordsLinked to original sources

PubMed2006

Decay of the Loschmidt echo in a time-dependent environment.

We study the decay rate of the Loschmidt echo or fidelity in a chaotic system under a time-dependent perturbation V(q,t) with typical strength Planck's/tau(v) . The perturbation represents the action of an uncontrolled environment interacting with the system, and is characterized by a correlation length xi(0) and a correlation time tau(0). For small perturbation strengths or rapid fluctuating perturbations, the Loschmidt echo decays exponentially with a rate predicted by the Fermi "golden rule," 1/approximately tau =tau(c)/tau(v)(2), where tau(c) approximately min[tau(0), xi(0)/upsilon] and upsilon is the typical particle velocity. Whenever the rate 1/approximately tau is larger than the Lyapunov exponent of the system, a perturbation independent Lyapunov decay regime arises. We also find that by speeding up the fluctuations (while keeping the perturbation strength fixed) the fidelity decay becomes slower, and hence one can protect the system against decoherence.

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PubMed2005

Universal transport properties of open microwave cavities with and without time-reversal symmetry.

We measure the transmission through asymmetric and reflection-symmetric chaotic microwave cavities in dependence on the number of attached waveguides. Ferrite cylinders are placed inside the cavities to break time-reversal symmetry. The phase-breaking properties of the ferrite and its range of applicability are discussed in detail. We use the random matrix theory accounting for absorption effects to calculate the universal distribution of transmission coefficients T and their energy derivatives dT/depsilon. Using the absorption strength as a fitting parameter, we find good agreement between universal transmission fluctuations predicted by the theory and the experimental data.

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PubMed2003

Distribution of reflection coefficients in absorbing chaotic microwave cavities.

The distribution of reflection coefficients P(R) for chaotic microwave cavities with time-reversal symmetry is investigated in different absorption and antenna coupling regimes. For all regimes the agreement between experimental distributions and random-matrix theory predictions is very good, provided both the antenna coupling T(a) and the wall absorption strength T(w) are taken into account in an appropriate way. These parameters are determined by independent experimental quantities.

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PubMed2003

Coherent versus sequential electron tunneling in quantum dots.

Manifestations of quantum coherence in the electronic conductance through nearly closed quantum dots in the Coulomb-blockade regime are addressed. We show that quantum coherent tunneling processes explain some puzzling statistical features of the conductance peak heights observed in recent experiments at low temperatures. We employ the constant interaction model and the random matrix theory to model the quantum dot electronic interactions and its single-particle statistical fluctuations, taking full account of the finite decay width of the quantum dot levels.

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PubMed2002

Conductance fluctuations and weak localization in chaotic quantum dots.

We study the conductance statistical features of ballistic electrons flowing through a chaotic quantum dot. We show how the temperature affects the universal conductance fluctuations by analyzing the influence of dephasing and thermal smearing. This leads us to two main findings. First, we show that the energy correlations in the transmission, which were overlooked so far, are important for calculating the variance and higher moments of the conductance. Second, we show that there is an ambiguity in the method of determination of the dephasing rate from the size of the weak localization. We find that the dephasing times obtained at low temperatures from quantum dots are underestimated.

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PubMed2002

Measuring the Lyapunov exponent using quantum mechanics.

We study the time evolution of two wave packets prepared at the same initial state, but evolving under slightly different Hamiltonians. For chaotic systems, we determine the circumstances that lead to an exponential decay with time of the wave packet overlap function. We show that for sufficiently weak perturbations, the exponential decay follows a Fermi golden rule, while by making the difference between the two Hamiltonians larger, the characteristic exponential decay time becomes the Lyapunov exponent of the classical system. We illustrate our theoretical findings by investigating numerically the overlap decay function of a two-dimensional dynamical system.

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PubMed2001

Transmission fluctuations in chaotic microwave billiards with and without time-reversal symmetry.

Transmission fluctuations have been studied in a microwave billiard in dependence to the number of attached wave guides on its entrance and exit. To investigate the influence of breaking time-reversal symmetry, ferrite cylinders were introduced into the billiard. The obtained transmission intensity distributions are compared with predictions from the random matrix theory. Because of the strong absorption caused by the ferrites, the existing statistical scattering theories had to be modified, by incorporating a number of additional absorbing scattering channels.

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