9,938 research outputs found
Decay of correlations in the dissipative two-state system
We study the equilibrium correlation function of the polaron-dressed
tunnelling operator in the dissipative two-state system and compare the
asymptoptic dynamics with that of the position correlations. For an Ohmic
spectral density with the damping strength , the correlation functions
are obtained in analytic form for all times at any and any bias. For ,
the asymptotic dynamics is found by using a diagrammatic approach within a
Coulomb gas representation. At T=0, the tunnelling or coherence correlations
drop as , whereas the position correlations show universal decay
. The former decay law is a signature of unscreened attractive
charge-charge interactions, while the latter is due to unscreened dipole-dipole
interactions.Comment: 5 pages, 5 figures, to be published in Europhys. Let
Duality Relation for Quantum Ratchets
A duality relation between the long-time dynamics of a quantum Brownian
particle in a tilted ratchet potential and a driven dissipative tight-binding
model is reported. It relates a situation of weak dissipation in one model to
strong dissipation in the other one, and vice versa. We apply this duality
relation to investigate transport and rectification in ratchet potentials: From
the linear mobility we infer ground-state delocalization for weak dissipation.
We report reversals induced by adiabatic driving and temperature in the ratchet
current and its dependence on the potential shape.Comment: Modified content, corrected typo
Ultraslow quantum dynamics in a sub-Ohmic heat bath
We show that the low-frequency modes of a sub-Ohmic bosonic heat bath
generate an effective dynamical asymmetry for an intrinsically symmetric
quantum spin -1/2. An initially fully polarized spin first decays towards a
quasiequilibrium determined by the dynamical asymmetry, thereby showing
coherent damped oscillations on the (fast) time scale of the spin splitting. On
top of this, the dynamical asymmetry itself decays on an ultraslow time scale
and vanishes asymptotically since the global equilibrium phase is symmetric. We
quantitatively study the nature of the initial fast decay to the
quasiequilibrium and discuss the features of ultraslow dynamics of the
quasiequilibrium itself. The dynamical asymmetry is more pronounced for smaller
values of the sub-Ohmic exponent and for lower temperatures, which emphasizes
the quantum many-body nature of the effect. The symmetry breaking is related to
the dynamic crossover between coherent and overdamped relaxation of the spin
polarization and is not connected to the localization quantum phase transition.
In addition to this delocalized phase, we identify a novel phase which is
characterized by damped coherent oscillations in the localized phase. This
allows for a sketch of the zero-temperature phase diagram of the sub-Ohmic
spin-boson model with four distinct phases.Comment: published version (minor changes), 8 pages, 5 figure
Characterization of coherent impurity effects in solid state qubits
We propose a characterisation of the effects of bistable coherent impurities
in solid state qubits. We introduce an effective impurity description in terms
of a tunable spin-boson environment and solve the dynamics for the qubit
coherences. The dominant rate characterizing the asymptotic time limit is
identified and signatures of non-Gaussian behavior of the quantum impurity at
intermediate times are pointed out. An alternative perspective considering the
qubit as a measurement device for the spin-boson impurity is proposed.Comment: 4 pages, 5 figures. Replaced with published version, minor change
A Single Impurity in Tomonaga-Luttinger Liquids
The problem of a single impurity in one dimensional Tomonaga-Luttinger
liquids with a repulsive electron-electron interaction is discussed. We find
that the renormalization group flow diagram for the parameters characterizing
the impurity is rather complex. Apart from the fixed points corresponding to
two weakly connected semi-infinite wires, the flow diagram contains additional
fixed points which control the low temperature physics when the bare potential
of the impurity is not strong.Comment: To be published in the Philosophical Magazine in the Proceedings of
the "MINERVA WORKSHOP on MESOSCOPICS, FRACTALS and NEURAL NETWORKS", Eilat,
Israel, March 199
Entanglement spectroscopy of a driven solid-state qubit and its detector
We study the asymptotic dynamics of a driven quantum two level system coupled
via a quantum detector to the environment. We find multi-photon resonances
which are due to the entanglement of the qubit and the detector. Different
regimes are studied by employing a perturbative Floquet-Born-Markov approach
for the qubit+detector system, as well as non-perturbative real-time path
integral schemes for the driven spin-boson system. We find analytical results
for the resonances, including the red and the blue sidebands. They agree well
with those of exact ab-initio calculations.Comment: 4 pages, 4 figure
Detecting entanglement of two electron spin qubits with witness operators
We propose a scheme for detecting entanglement between two electron spin
qubits in a double quantum dot using an entanglement witness operator. We first
calculate the optimal configuration of the two electron spins, defined as the
position in the energy level spectrum where, averaged over the nuclear spin
distribution, 1) the probability to have two separated electrons, and 2) the
degree of entanglement of the quantum state quantified by the concurrence are
both large. Using a density matrix approach, we then calculate the evolution of
the expectation value of the witness operator for the two-spin singlet state,
taking into account the effect of decoherence due to quantum charge
fluctuations modeled as a boson bath. We find that, for large interdot
coupling, it is possible to obtain a highly entangled and robust ground state.Comment: 4 pages, 3 figure
Magnetic Miniband Structure and Quantum Oscillations in Lateral Semiconductor Superlattices
We present fully quantum-mechanical magnetotransport calculations for
short-period lateral superlattices with one-dimensional electrostatic
modulation. A non-perturbative treatment of both magnetic field and modulation
potential proves to be necessary to reproduce novel quantum oscillations in the
magnetoresistance found in recent experiments in the resistance component
parallel to the modulation potential. In addition, we predict oscillations of
opposite phase in the component perpendicular to the modulation not yet
observed experimentally. We show that the new oscillations originate from the
magnetic miniband structure in the regime of overlapping minibands.Comment: 6 pages with 4 figure
Non-Markovian incoherent quantum dynamics of a two-state system
We present a detailed study of the non-Markovian two-state system dynamics
for the regime of incoherent quantum tunneling. Using perturbation theory in
the system tunneling amplitude , and in the limit of strong system-bath
coupling, we determine the short time evolution of the reduced density matrix
and thereby find a general equation of motion for the non-Markovian evolution
at longer times. We relate the nonlocality in time due to the non-Markovian
effects with the environment characteristic response time. In addition, we
study the incoherent evolution of a system with a double-well potential, where
each well consists several quantized energy levels. We determine the crossover
temperature to a regime where many energy levels in the wells participate in
the tunneling process, and observe that the required temperature can be much
smaller than the one associated with the system plasma frequency. We also
discuss experimental implications of our theoretical analysis.Comment: 10 pages, published versio
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