635 research outputs found
Fidelity Between Unitary Operators and the Generation of Gates Robust Against Off-Resonance Perturbations
We perform a functional expansion of the fidelity between two unitary
matrices in order to find the necessary conditions for the robust
implementation of a target gate. Comparison of these conditions with those
obtained from the Magnus expansion and Dyson series shows that they are
equivalent in first order. By exploiting techniques from robust design
optimization, we account for issues of experimental feasibility by introducing
an additional criterion to the search for control pulses. This search is
accomplished by exploring the competition between the multiple objectives in
the implementation of the NOT gate by means of evolutionary multi-objective
optimization
Quantum control by von Neumann measurements
A general scheme is presented for controlling quantum systems using evolution
driven by non-selective von Neumann measurements, with or without an additional
tailored electromagnetic field. As an example, a 2-level quantum system
controlled by non-selective quantum measurements is considered. The control
goal is to find optimal system observables such that consecutive non-selective
measurement of these observables transforms the system from a given initial
state into a state which maximizes the expected value of a target operator (the
objective). A complete analytical solution is found including explicit
expressions for the optimal measured observables and for the maximal objective
value given any target operator, any initial system density matrix, and any
number of measurements. As an illustration, upper bounds on measurement-induced
population transfer between the ground and the excited states for any number of
measurements are found. The anti-Zeno effect is recovered in the limit of an
infinite number of measurements. In this limit the system becomes completely
controllable. The results establish the degree of control attainable by a
finite number of measurements
A Simplified Approach to Optimally Controlled Quantum Dynamics
A new formalism for the optimal control of quantum mechanical physical
observables is presented. This approach is based on an analogous classical
control technique reported previously[J. Botina, H. Rabitz and N. Rahman, J.
chem. Phys. Vol. 102, pag. 226 (1995)]. Quantum Lagrange multiplier functions
are used to preserve a chosen subset of the observable dynamics of interest. As
a result, a corresponding small set of Lagrange multipliers needs to be
calculated and they are only a function of time. This is a considerable
simplification over traditional quantum optimal control theory[S. shi and H.
Rabitz, comp. Phys. Comm. Vol. 63, pag. 71 (1991)]. The success of the new
approach is based on taking advantage of the multiplicity of solutions to
virtually any problem of quantum control to meet a physical objective. A family
of such simplified formulations is introduced and numerically tested. Results
are presented for these algorithms and compared with previous reported work on
a model problem for selective unimolecular reaction induced by an external
optical electric field.Comment: Revtex, 29 pages (incl. figures
Observation-assisted optimal control of quantum dynamics
This paper explores the utility of instantaneous and continuous observations
in the optimal control of quantum dynamics. Simulations of the processes are
performed on several multilevel quantum systems with the goal of population
transfer. Optimal control fields are shown to be capable of cooperating or
fighting with observations to achieve a good yield, and the nature of the
observations may be optimized to more effectively control the quantum dynamics.
Quantum observations also can break dynamical symmetries to increase the
controllability of a quantum system. The quantum Zeno and anti-Zeno effects
induced by observations are the key operating principles in these processes.
The results indicate that quantum observations can be effective tools in the
control of quantum dynamics
Evaluation of the Sustainability of an Intervention to Increase HIV Testing
BACKGROUND
Sustainability—the routinization and institutionalization of processes that improve the quality of healthcare—is difficult to achieve and not often studied.
OBJECTIVE
To evaluate the sustainability of increased rates of HIV testing after implementation of a multi-component intervention in two Veterans Health Administration healthcare systems.
DESIGN
Quasi-experimental implementation study in which the effect of transferring responsibility to conduct the provider education component of the intervention from research to operational staff was assessed.
PATIENTS
Persons receiving healthcare between 2005 and 2006 (intervention year) and 2006 and 2007 (sustainability year).
MEASUREMENTS
Monthly HIV testing rate, stratified by frequency of clinic visits
RESULTS
The monthly adjusted testing rate increased from 2% at baseline to 6% at the end intervention year and then declined reaching 4% at the end of the sustainability year. However, the stratified, visit-specific testing rate for persons newly exposed to the intervention (i.e., having their first through third visits during the study period) increased throughout the intervention and sustainability years. Increases in the proportion of visits by patients who remained untested despite multiple, prior exposures to the intervention accounted for the aggregate attenuation of testing during the sustainability year. Overall, the percentage of patients who received an HIV test in the sustainability year was 11.6%, in the intervention year 11.1%, and in the pre-intervention year 5.0%
CONCLUSIONS
Provider education combined with informatics and organizational support had a sustainable effect on HIV testing rates. The effect was most pronounced during patients' early contacts with the healthcare system.Health Services Research & Development Service (SDP 06–001
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