9,650 research outputs found
Social interaction of patients and personnel in a ward situation
Thesis (M.S.)--Boston Universit
Aharonov-Bohm Effect and Coordinate Transformations
Resorting to a Gedankenexperiment which is very similar to the famous
Aharonov-Bohm proposal it will be shown that, in the case of a Minkowskian
spacetime, we may use a nonrelativistic quantum particle and a noninertial
coordinate system and obtain geometric information of regions that are, to this
particle, forbidden. This shows that the outcome of a nonrelativistic quantum
process is determined not only by the features of geometry at those points at
which the process takes place, but also by geometric parameters of regions in
which the quantum system can not enter. From this fact we could claim that
geometry at the quantum level plays a non-local role. Indeed, the measurement
outputs of some nonrelativistic quantum experiments are determined not only by
the geometry of the region in which the experiment takes place, but also by the
geometric properties of spacetime volumes which are, in some way, forbidden in
the experiment.Comment: 11 pages, 1 figure, accepted in Mod. Phys. Letts.
Group-theoretical structure of quantum measurements and equivalence principle
The transverse group associated to some continuous quantum measuring
processes is analyzed in the presence of nonvanishing gravitational fields.
This is done considering, as an exmaple, the case of a particle whose
coordinates are being monitored. Employing the so called restricted path
integral formalism, it will be shown that the measuring process could always
contain information concerning the gravitational field. In other words, it
seems that with the presence of a measuring process the equivalence principle
may, in some cases, break down. The relation between the breakdown of the
equivalence principle, at quantum level, and the fact that the gravitational
field could act always as a decoherence environment, is also considered. The
phenomena of quantum beats of quantum optics will allow us to consider the
possibility that the experimental corroboration of the equivalence principle at
quantum level could be taken as an indirect evidence in favor of the
quantization of the gravitational field, i.e., the quantum properties of this
field avoid the violation of the equivalence principle.Comment: 13 pages, accepted in Modern Physics Letters
A General Transfer-Function Approach to Noise Filtering in Open-Loop Quantum Control
We present a general transfer-function approach to noise filtering in
open-loop Hamiltonian engineering protocols for open quantum systems. We show
how to identify a computationally tractable set of fundamental filter
functions, out of which arbitrary transfer filter functions may be assembled up
to arbitrary high order in principle. Besides avoiding the infinite recursive
hierarchy of filter functions that arises in general control scenarios, this
fundamental filter-functions set suffices to characterize the error suppression
capabilities of the control protocol in both the time and frequency domain. We
prove that the resulting notion of filtering order reveals conceptually
distinct, albeit complementary, features of the controlled dynamics as compared
to the order of error cancellation, traditionally defined in the Magnus sense.
Examples and implications are discussed.Comment: Paper plus supplementary material. 10 pages, 1 figure. Unnumbered
equation between 2 and 3 corrected. Results are unchange
Arbitrarily Accurate Dynamical Control in Open Quantum Systems
We show that open-loop dynamical control techniques may be used to synthesize
unitary transformations in open quantum systems in such a way that decoherence
is perturbatively compensated for to a desired (in principle arbitrarily high)
level of accuracy, which depends only on the strength of the relevant errors
and the achievable rate of control modulation. Our constructive and fully
analytical solution employs concatenated dynamically corrected gates, and is
applicable independently of detailed knowledge of the system-environment
interactions and environment dynamics. Explicit implications for boosting
quantum gate fidelities in realistic scenarios are addressed.Comment: 4 pages and 20 characters, 1 figure [improvements and fixes, PRL
version
Description of Quantum Entanglement with Nilpotent Polynomials
We propose a general method for introducing extensive characteristics of
quantum entanglement. The method relies on polynomials of nilpotent raising
operators that create entangled states acting on a reference vacuum state. By
introducing the notion of tanglemeter, the logarithm of the state vector
represented in a special canonical form and expressed via polynomials of
nilpotent variables, we show how this description provides a simple criterion
for entanglement as well as a universal method for constructing the invariants
characterizing entanglement. We compare the existing measures and classes of
entanglement with those emerging from our approach. We derive the equation of
motion for the tanglemeter and, in representative examples of up to four-qubit
systems, show how the known classes appear in a natural way within our
framework. We extend our approach to qutrits and higher-dimensional systems,
and make contact with the recently introduced idea of generalized entanglement.
Possible future developments and applications of the method are discussed.Comment: 40 pages, 7 figures, 1 table, submitted for publication. v2: section
II.E has been changed and the Appendix on "Four qubit sl-entanglement
measure" has been removed. There are changes in the notation of section IV.
Typos and language mistakes has been corrected. A figure has been added and a
figure has been replaced. The references have been update
Asymmetric Totally-corrective Boosting for Real-time Object Detection
Real-time object detection is one of the core problems in computer vision.
The cascade boosting framework proposed by Viola and Jones has become the
standard for this problem. In this framework, the learning goal for each node
is asymmetric, which is required to achieve a high detection rate and a
moderate false positive rate. We develop new boosting algorithms to address
this asymmetric learning problem. We show that our methods explicitly optimize
asymmetric loss objectives in a totally corrective fashion. The methods are
totally corrective in the sense that the coefficients of all selected weak
classifiers are updated at each iteration. In contract, conventional boosting
like AdaBoost is stage-wise in that only the current weak classifier's
coefficient is updated. At the heart of the totally corrective boosting is the
column generation technique. Experiments on face detection show that our
methods outperform the state-of-the-art asymmetric boosting methods.Comment: 14 pages, published in Asian Conf. Computer Vision 201
Quantum tensor product structures are observable-induced
It is argued that the partition of a quantum system into subsystems is
dictated by the set of operationally accessible interactions and measurements.
The emergence of a multi-partite tensor product structure of the state-space
and the associated notion of quantum entanglement are then relative and
observable-induced. We develop a general algebraic framework aimed to formalize
this concept. We discuss several cases relevant to quantum information
processing and decoherence control.Comment: 5 Pages LaTe
Dynamically Error-Corrected Gates for Universal Quantum Computation
Scalable quantum computation in realistic devices requires that precise
control can be implemented efficiently in the presence of decoherence and
operational errors. We propose a general constructive procedure for designing
robust unitary gates on an open quantum system without encoding or measurement
overhead. Our results allow for a low-level error correction strategy solely
based on Hamiltonian engineering using realistic bounded-strength controls and
may substantially reduce implementation requirements for fault-tolerant quantum
computing architectures.Comment: 5 pages, 3 figure
Fault-Tolerant Quantum Dynamical Decoupling
Dynamical decoupling pulse sequences have been used to extend coherence times
in quantum systems ever since the discovery of the spin-echo effect. Here we
introduce a method of recursively concatenated dynamical decoupling pulses,
designed to overcome both decoherence and operational errors. This is important
for coherent control of quantum systems such as quantum computers. For
bounded-strength, non-Markovian environments, such as for the spin-bath that
arises in electron- and nuclear-spin based solid-state quantum computer
proposals, we show that it is strictly advantageous to use concatenated, as
opposed to standard periodic dynamical decoupling pulse sequences. Namely, the
concatenated scheme is both fault-tolerant and super-polynomially more
efficient, at equal cost. We derive a condition on the pulse noise level below
which concatenated is guaranteed to reduce decoherence.Comment: 5 pages, 4 color eps figures. v3: Minor changes. To appear in Phys.
Rev. Let
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