1,447 research outputs found
Fighting Decoherence by Feedback-controlled Dissipation
Repeated closed-loop control operations acting as piecewise-constant
Liouville superoperators conditioned on the outcomes of regularly performed
measurements may effectively be described by a fixed-point iteration for the
density matrix. Even when all Liouville superoperators point to the completely
mixed state, feedback of the measurement result may lead to a pure state, which
can be interpreted as selective dampening of undesired states. Using a
microscopic model, we exemplify this for a single qubit, which can be purified
in an arbitrary single-qubit state by tuning the measurement direction and two
qubits that may be purified towards a Bell state by applying a special
continuous two-local measurement. The method does not require precise knowledge
of decoherence channels and works for large reservoir temperatures provided
measurement, processing, and control can be implemented in a continuous
fashion.Comment: to appear in PR
Coherently controlled entanglement generation in a binary Bose-Einstein condensate
Considering a two-component Bose-Einstein condensate in a double-well
potential, a method to generate a Bell state consisting of two spatially
separated condensates is suggested. For repulsive interactions, the required
tunnelling control is achieved numerically by varying the amplitude of a
sinusoidal potential difference between the wells. Both numerical and
analytical calculations reveal the emergence of a highly entangled mesoscopic
state.Comment: 6 pages, 6 figures, epl2.cl
Non-markovian dynamics of double quantum dot charge qubit with static bias
The dynamics of charge qubit in double quantum dot coupled to phonons is
investigated theoretically. The static bias is considered. By means of the
perturbation approach based on unitary transformations, the dynamical tunneling
current is obtained explicitly. The biased system displays broken symmetry and
a significantly larger coherence-incoherence transition critical point . We also analyzed the decoherence induced by piezoelectric coupling
phonons in detail. The results show that reducing the coupling between system
and bath make coherence frequency increase and coherence time prolong. To
maintain quantum coherence, applying static bias also is a good means.Comment: 13 pages, 5 figure
Optimal Dynamical Decoupling Sequence for Ohmic Spectrum
We investigate the optimal dynamical decoupling sequence for a qubit coupled
to an ohmic environment. By analytically computing the derivatives of the
decoherence function, the optimal pulse locations are found to satisfy a set of
nonlinear equations which can be easily solved. These equations incorporates
the environment information such as high-energy (UV) cutoff frequency \omega_c,
giving a complete description of the decoupling process. The solutions explain
previous experimental and theoretical results of locally optimized dynamical
decoupling (LODD) sequence in high-frequency dominated environment, which were
obtained by purely numerical computation and experimental feedback. As shown in
numerical comparison, these solutions outperform the Uhrig dynamical decoupling
(UDD) sequence by one or more orders of magnitude in the ohmic case.Comment: 5 pages, 4 figures, to appear in Phys. Rev.
Stationary entanglement in strongly coupled qubits
The dynamics of two superconducting flux qubits coupled to each other and to
a common bath is discussed. We focus on the case in which the qubit-qubit
coupling strength dominates over the respective qubit transition frequencies.
We derive the master equation including collective effect by modeling the bath
as 1D open space in this ultra-strong coupling regime, and find that the
coupling greatly modifies both the coherent and the incoherent dynamics of the
system, giving rise to qualitatively different properties. By analyzing the
steady-state and the dynamics governed by the master equation, we show that
ground state entanglement and maximum coherence between the two qubits can be
induced by the environment alone. By employing in addition a single external
driving field, both the entangled anti-symmetric and symmetric collective
states can be populated and preserved with high fidelity. Similarly, entangled
states can be prepared using adiabatic passage techniques using two external
fields. Our results could find applications in entangling quantum gates and
quantum memories free from the decoherence.Comment: 19 pages, 21 figure
Non-Equilibrium Dynamics of Correlated Electron Transfer in Molecular Chains
The relaxation dynamics of correlated electron transport (ET) along molecular
chains is studied based on a substantially improved numerically exact path
integral Monte Carlo (PIMC) approach. As archetypical model we consider a
Hubbard chain containing two interacting electrons coupled to a bosonic bath.
For this generalization of the ubiquitous spin-boson model, the intricate
interdependence of correlations and dissipation leads to non-Boltzmann thermal
equilibrium distributions for many-body states. By mapping the multi-particle
dynamics onto an isomorphic single particle motion this phenomenon is shown to
be sensitive to the particle statistics and due to its robustness allows for
new control schemes in designed quantum aggregates.Comment: 5 pages, 4 figure
Nonequilibrium many-body dynamics along a dissipative Hubbard chain: Symmetries and Quantum Monte Carlo simulations
The nonequilibrium dynamics of correlated charge transfer along a
one-dimensional chain in presence of a phonon environment is investigated
within a dissipative Hubbard model. For this generalization of the ubiquitous
spin-boson model the crucial role of symmetries is analysed in detail and
corresponding invariant subspaces are identified. It is shown that the time
evolution typically occurs in each of the disjunct subspaces independently
leading e.g. asymptotically to a non-Boltzmann equilibrium state. Based on
these findings explicit results are obtained for two interacting electrons by
means of a substantially improved real-time quantum Monte Carlo approach. In
the incoherent regime an appropriate mapping of the many-body dynamics onto an
isomorphic single particle motion allows for an approximate description of the
numerical data in terms of rate equations. These results may lead to new
control schemes of charge transport in tailored quantum systems as e.g.
molecular chains or quantum dot arrays.Comment: 13 pages, 9 figures submitted to Phys. Rev.
Charge qubit entanglement in double quantum dots
We study entanglement of charge qubits in a vertical tunnel-coupled double
quantum dot containing two interacting electrons. Exact diagonalization is used
to compute the negativity characterizing entanglement. We find that
entanglement can be efficiently generated and controlled by sidegate voltages,
and describe how it can be detected. For large enough tunnel coupling, the
negativity shows a pronounced maximum at an intermediate interaction strength
within the Wigner molecule regime.Comment: revised version of the manuscript, as published in EPL, 7 pages, 4
figure
A practical scheme for error control using feedback
We describe a scheme for quantum error correction that employs feedback and
weak measurement rather than the standard tools of projective measurement and
fast controlled unitary gates. The advantage of this scheme over previous
protocols (for example Ahn et. al, PRA, 65, 042301 (2001)), is that it requires
little side processing while remaining robust to measurement inefficiency, and
is therefore considerably more practical. We evaluate the performance of our
scheme by simulating the correction of bit-flips. We also consider
implementation in a solid-state quantum computation architecture and estimate
the maximal error rate which could be corrected with current technology.Comment: 12 pages, 3 figures. Minor typographic change
A variational description of the quantum phase transition in the sub-Ohmic spin-boson model
The sub-ohmic spin-boson model is known to possess a novel quantum phase
transition at zero temperature between a localised and delocalised phase. We
present here an analytical theory based on a variational ansatz for the ground
state, which describes a continuous localization transition with mean-field
exponents for . Our results for the critical properties show good
quantitiative agreement with previous numerical results, and we present a
detailed description of all the spin observables as the system passes through
the transition. Analysing the ansatz itself, we give an intuitive microscopic
description of the transition in terms of the changing correlations between the
system and bath, and show that it is always accompanied by a divergence of the
low-frequency boson occupations. The possible relevance of this divergence for
some numerical approaches to this problem is discussed and illustrated by
looking at the ground state obtained using density matrix renormalisation group
methods
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