Biomedical subjects

P G Silvestrov

Publications and source records attributed to P G Silvestrov.

7 recordsLinked to original sources

PubMed2006

Spectrum of the Andreev billiard and giant fluctuations of the Ehrenfest time.

The density of states in the semiclassical Andreev billiard is theoretically studied and shown to be determined by the fluctuations of the classical Lyapunov exponent lambda. The rare trajectories with a small value of lambda give rise to an anomalous increase of the Ehrenfest time tauE approximately |lnvariant Planck's over 2pi|/lambda and, consequently, to the appearance of Andreev levels with small excitation energy. The gap in spectrum is obtained, and fluctuations of the value of the gap due to different positions of superconducting lead are considered.

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PubMed2003

Adiabatic quantization of Andreev quantum billiard levels.

We identify the time T between Andreev reflections as a classical adiabatic invariant in a ballistic chaotic cavity (Lyapunov exponent lambda), coupled to a superconductor by an N-mode constriction. Quantization of the adiabatically invariant torus in phase space gives a discrete set of periods T(n), which in turn generate a ladder of excited states epsilon (nm)=(m+1/2)pi(h) /T(n). The largest quantized period is the Ehrenfest time T(0)=lambda(-1)ln(N). Projection of the invariant torus onto the coordinate plane shows that the wave functions inside the cavity are squeezed to a transverse dimension W/sqrt[N], much below the width W of the constriction.

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PubMed2003

Enhanced sensitivity of the transmission phase of a quantum dot to Kondo correlations.

The strong sensitivity of the transmission phase through a quantum dot embedded into one arm of a two-wave Aharonov-Bohm interferometer to the Kondo effect is explained. The enhancement takes place because of the buildup of the exchange scattering on the dot due to Kondo correlations even much above T(K). The enhanced exchange competes with the potential scattering, which is always weak. Both cases of the Anderson impurity model and a multilevel quantum dot are considered. In the latter case in addition to the description of peculiar phase behavior a mechanism leading to ferromagnetic Kondo coupling in quantum dots is proposed.

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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

Ehrenfest times for classically chaotic systems.

We describe the quantum-mechanical spreading of a Gaussian wave packet by means of the semiclassical WKB approximation of Berry and Balazs [J. Phys. A 2, 625 (1979)]. We find that the time scale tau on which this approximation breaks down in a chaotic system is larger than the Ehrenfest times considered previously. In one dimension tau=7/6 lambda(-1)ln(A/Plancks over 2 pi), with lambda the Lyapunov exponent and A a typical classical action.

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PubMed2001

Golden rule decay versus Lyapunov decay of the quantum Loschmidt echo.

The overlap of two wave packets evolving in time with slightly different Hamiltonians decays exponentially approximate to e(-gammat), for perturbation strengths U greater than the level spacing Delta. We present numerical evidence for a dynamical system that the decay rate gamma is given by the smallest of the Lyapunov exponent lambda of the classical chaotic dynamics and the level broadening U(2)/Delta that follows from the golden rule of quantum mechanics. This implies the range of validity U > the square root of [lambdaDelta] for the perturbation-strength independent decay rate discovered by Jalabert and Pastawski [Phys. Rev. Lett. 86, 2490 (2001)].

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PubMed2001

Limits to error correction in quantum chaos.

We study the correction of errors that have accumulated in an entangled state of spins as a result of unknown local variations in the Zeeman energy ( B) and spin-spin interaction energy ( J). A nondegenerate code with error rate kappa can recover the original state with high fidelity within a time t(R) approximately Planck's over 2pikappa(1/2)/max(B,J)-independent of the number of encoded qubits. Whether the Hamiltonian is chaotic or not does not affect this time scale, but it does affect the complexity of the error-correcting code.

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