1,645 research outputs found
Non-Markovian dynamics of double quantum dot charge qubits due to acoustic phonons
We investigate the dynamics of a double quantum dot charge qubit which is
coupled to piezoelectric acoustic phonons, appropriate for GaAs
heterostructures. At low temperatures, the phonon bath induces a non-Markovian
dynamical behavior of the oscillations between the two charge states of the
double quantum dot. Upon applying the numerically exact quasiadiabatic
propagator path-integral scheme, the reduced density matrix of the charge qubit
is calculated, thereby avoiding the Born-Markov approximation. This allows a
systematic study of the dependence of the Q-factor on the lattice temperature,
on the size of the quantum dots, as well as on the interdot coupling. We
calculate the Q-factor for a recently realized experimental setup and find that
it is two orders of magnitudes larger than the measured value, indicating that
the decoherence due to phonons is a subordinate mechanism.Comment: 5 pages, 7 figures, replaced with the version to appear in Phys. Rev.
Full density matrix dynamics for large quantum systems: Interactions, Decoherence and Inelastic effects
We develop analytical tools and numerical methods for time evolving the total
density matrix of the finite-size Anderson model. The model is composed of two
finite metal grains, each prepared in canonical states of differing chemical
potential and connected through a single electronic level (quantum dot or
impurity). Coulomb interactions are either excluded all together, or allowed on
the dot only. We extend this basic model to emulate decoherring and inelastic
scattering processes for the dot electrons with the probe technique. Three
methods, originally developed to treat impurity dynamics, are augmented to
yield global system dynamics: the quantum Langevin equation method, the well
known fermionic trace formula, and an iterative path integral approach. The
latter accommodates interactions on the dot in a numerically exact fashion. We
apply the developed techniques to two open topics in nonequilibrium many-body
physics: (i) We explore the role of many-body electron-electron repulsion
effects on the dynamics of the system. Results, obtained using exact path
integral simulations, are compared to mean-field quantum Langevin equation
predictions. (ii) We analyze aspects of quantum equilibration and
thermalization in large quantum systems using the probe technique, mimicking
elastic-dephasing effects and inelastic interactions on the dot. Here, unitary
simulations based on the fermionic trace formula are accompanied by quantum
Langevin equation calculations
Vibration-enhanced quantum transport
In this paper, we study the role of collective vibrational motion in the
phenomenon of electronic energy transfer (EET) along a chain of coupled
electronic dipoles with varying excitation frequencies. Previous experimental
work on EET in conjugated polymer samples has suggested that the common
structural framework of the macromolecule introduces correlations in the energy
gap fluctuations which cause coherent EET. Inspired by these results, we
present a simple model in which a driven nanomechanical resonator mode
modulates the excitation energy of coupled quantum dots and find that this can
indeed lead to an enhancement in the transport of excitations across the
quantum network. Disorder of the on-site energies is a key requirement for this
to occur. We also show that in this solid state system phase information is
partially retained in the transfer process, as experimentally demonstrated in
conjugated polymer samples. Consequently, this mechanism of vibration enhanced
quantum transport might find applications in quantum information transfer of
qubit states or entanglement.Comment: 7 pages, 6 figures, new material, included references, final
published versio
Exact dynamics of interacting qubits in a thermal environment: Results beyond the weak coupling limit
We demonstrate an exact mapping of a class of models of two interacting
qubits in thermal reservoirs to two separate spin-bath problems. Based on this
mapping, exact numerical simulations of the qubits dynamics can be performed,
beyond the weak system-bath coupling limit. Given the time evolution of the
system, we study, in a numerically exact way, the dynamics of entanglement
between pair of qubits immersed in boson thermal baths, showing a rich
phenomenology, including an intermediate oscillatory behavior, the entanglement
sudden birth, sudden death, and revival. We find that stationary entanglement
develops between the qubits due to their coupling to a thermal environment,
unlike the isolated qubits case in which the entanglement oscillates. We also
show that the occurrence of entanglement sudden death in this model depends on
the portion of the zero and double excitation states in the subsystem initial
state. In the long-time limit, analytic expressions are presented at weak
system-bath coupling, for a range of relevant qubit parameters
Matching schemes and public goods : a review
Matching schemes, where a party matches the contribution of others, reduce the effective price of a good and aim to foster its demand. We review the empirical literature on the effectiveness of these schemes in the context of public goods, especially in the field of charitable giving. As different measures of effectiveness are used, we classify results according to (i) the level of public good provision, (ii) the amount of individuals' contributions, (iii) the likelihood to give and (iv) the contribution conditional on contributing a positive amount. Generalizing results is challenging, since context specific factors matter. Predominantly, a match is found to create a significant increase in public good provision without crowding out individuals' contributions, while the effect on the likelihood of giving and contribution condition on contributing a positive amount is nonnegative. The discussion reveals several avenues for future research, as putting stronger emphasizes on long term effects, public good competition or heterogeneity in responses
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