243 research outputs found
On entropy production for controlled Markovian evolution
We consider thermodynamic systems with finitely many degrees of freedom and
subject to an external control action. We derive some basic results on the
dependence of the relative entropy production rate on the controlling force.
Applications to macromolecular cooling and to controlling the convergence to
equilibrium rate are sketched. Analogous results are derived for closed and
open n-level quantum systems.Comment: 19 page
Protecting subspaces by acting on the outside
Many quantum control tasks aim at manipulating the state of a quantum
mechanical system within a finite subspace of states. However, couplings to the
outside are often inevitable. Here we discuss strategies which keep the system
in the controlled subspace by applying strong interactions onto the outside.
This is done by drawing analogies to simple toy models and to the quantum Zeno
effect. Special attention is paid to the constructive use of dissipation in the
protection of subspaces.Comment: 16 pages, 10 figure
Quantum Control of Two-Qubit Entanglement Dissipation
We investigate quantum control of the dissipation of entanglement under
environmental decoherence. We show by means of a simple two-qubit model that
standard control methods - coherent or open-loop control - will not in general
prevent entanglement loss. However, we propose a control method utilising a
Wiseman-Milburn feedback/measurement control scheme which will effectively
negate environmental entanglement dissipation.Comment: 11 pages,4 figures, minor correctio
Cooling atoms into entangled states
We discuss the possibility of preparing highly entangled states by simply
cooling atoms into the ground state of an applied interaction Hamiltonian. As
in laser sideband cooling, we take advantage of a relatively large detuning of
the desired state, while all other qubit states experience resonant laser
driving. Once spontaneous emission from excited atomic states prepares the
system in its ground state, it remains there with a very high fidelity for a
wide range of experimental parameters and all possible initial states. After
presenting the general theory, we discuss concrete applications with one and
two qubits.Comment: 16 pages, 6 figures, typos correcte
Generation and propagation of entanglement in driven coupled-qubit systems
In a bipartite system subject to decoherence from two separate reservoirs,
the entanglement is typically destroyed faster than for single reservoirs.
Surprisingly however, the existence of separate reservoirs can also have a
beneficial entangling effect: if the qubits are coupled and driven externally
by a classical field, the system ends up in a stationary state characterized by
a finite degree of entanglement. This phenomenon occurs only in a certain
region of the parameter space and the structure of the stationary state has a
universal form which does not depend on the initial state or on the specific
physical realization of the qubits. We show that the entanglement thus
generated can be propagated within a quantum network using simple local unitary
operations. We suggest the use of such systems as "batteries of entanglement"
in quantum circuits.Comment: 14 pages, 7 figure
Brachistochrone of Entanglement for Spin Chains
We analytically investigate the role of entanglement in time-optimal state
evolution as an appli- cation of the quantum brachistochrone, a general method
for obtaining the optimal time-dependent Hamiltonian for reaching a target
quantum state. As a model, we treat two qubits indirectly cou- pled through an
intermediate qubit that is directly controllable, which represents a typical
situation in quantum information processing. We find the time-optimal unitary
evolution law and quantify residual entanglement by the two-tangle between the
indirectly coupled qubits, for all possible sets of initial pure quantum states
of a tripartite system. The integrals of the motion of the brachistochrone are
determined by fixing the minimal time at which the residual entanglement is
maximized. Entan- glement plays a role for W and GHZ initial quantum states,
and for the bi-separable initial state in which the indirectly coupled qubits
have a nonzero value of the 2-tangle.Comment: 9 pages, 4 figure
Reverse quantum state engineering using electronic feedback loops
We propose an all-electronic technique to manipulate and control interacting
quantum systems by unitary single-jump feedback conditioned on the outcome of a
capacitively coupled electrometer and in particular a single-electron
transistor. We provide a general scheme to stabilize pure states in the quantum
system and employ an effective Hamiltonian method for the quantum master
equation to elaborate on the nature of stabilizable states and the conditions
under which state purification can be achieved. The state engineering within
the quantum feedback scheme is shown to be linked with the solution of an
inverse eigenvalue problem. Two applications of the feedback scheme are
presented in detail: (i) stabilization of delocalized pure states in a single
charge qubit and (ii) entanglement stabilization in two coupled charge qubits.
In the latter example we demonstrate the stabilization of a maximally entangled
Bell state for certain detector positions and local feedback operations.Comment: 23 pages, 6 figures, to be published by New Journal of Physics (2013
Brain-Computer Interface for Clinical Purposes : Cognitive Assessment and Rehabilitation
Alongside the best-known applications of brain-computer interface (BCI) technology for restoring communication abilities and controlling external devices, we present the state of the art of BCI use for cognitive assessment and training purposes. We first describe some preliminary attempts to develop verbal-motor free BCI-based tests for evaluating specific or multiple cognitive domains in patients with Amyotrophic Lateral Sclerosis, disorders of consciousness, and other neurological diseases. Then we present the more heterogeneous and advanced field of BCI-based cognitive training, which has its roots in the context of neurofeedback therapy and addresses patients with neurological developmental disorders (autism spectrum disorder and attention-deficit/hyperactivity disorder), stroke patients, and elderly subjects. We discuss some advantages of BCI for both assessment and training purposes, the former concerning the possibility of longitudinally and reliably evaluating cognitive functions in patients with severe motor disabilities, the latter regarding the possibility of enhancing patients' motivation and engagement for improving neural plasticity. Finally, we discuss some present and future challenges in the BCI use for the described purposes
A novel nonsense ATP7A pathogenic variant in a family exhibiting a variable occipital horn syndrome phenotype
We report on a family with occipital horn syndrome (OHS) diagnosed in the proband's late fifties. A novel ATP7A pathogenic variant (c.4222A > T, p.(Lys1408*)), representing the first nonsense variant and the second late truncation causing OHS rather than classic Menkes disease, was found to segregate in the family. The predicted maintenance of transmembrane domains is consistent with a residual protein activity, which may explain the mild clinical presentation
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