511 research outputs found
Control of trapped-ion quantum states with optical pulses
We present new results on the quantum control of systems with infinitely
large Hilbert spaces. A control-theoretic analysis of the control of trapped
ion quantum states via optical pulses is performed. We demonstrate how resonant
bichromatic fields can be applied in two contrasting ways -- one that makes the
system completely uncontrollable, and the other that makes the system
controllable. In some interesting cases, the Hilbert space of the
qubit-harmonic oscillator can be made finite, and the Schr\"{o}dinger equation
controllable via bichromatic resonant pulses. Extending this analysis to the
quantum states of two ions, a new scheme for producing entangled qubits is
discovered.Comment: Submitted to Physical Review Letter
Double bracket dissipation in kinetic theory for particles with anisotropic interactions
We derive equations of motion for the dynamics of anisotropic particles
directly from the dissipative Vlasov kinetic equations, with the dissipation
given by the double bracket approach (Double Bracket Vlasov, or DBV). The
moments of the DBV equation lead to a nonlocal form of Darcy's law for the mass
density. Next, kinetic equations for particles with anisotropic interaction are
considered and also cast into the DBV form. The moment dynamics for these
double bracket kinetic equations is expressed as Lie-Darcy continuum equations
for densities of mass and orientation. We also show how to obtain a
Smoluchowski model from a cold plasma-like moment closure of DBV. Thus, the
double bracket kinetic framework serves as a unifying method for deriving
different types of dynamics, from density--orientation to Smoluchowski
equations. Extensions for more general physical systems are also discussed.Comment: 19 pages; no figures. Submitted to Proc. Roy. Soc.
Knee joint neuromuscular activation performance during muscle damage and superimposed fatigue
This study examined the concurrent effects of exercise-induced muscle damage and superimposed acute fatigue on the neuromuscular activation performance of the knee flexors of nine males (age: 26.7 ± 6.1yrs; height 1.81 ± 0.05m; body mass 81.2 ± 11.7kg [mean ± SD]). Measures were obtained during three experimental conditions: (i) FAT-EEVID, involving acute fatiguing exercise performed on each assessment occasion plus a single episode of eccentric exercise performed on the first occasion and after the fatigue trial; (ii) FAT, involving the fatiguing exercise only and; (iii) CON consisting of no exercise. Assessments were performed prior to (pre) and at lh, 24h, 48h, 72h, and 168h relative to the eccentric exercise. Repeated-measures ANOVAs showed that muscle damage within the FAT-EEVID condition elicited reductions of up to 38%, 24%) and 65%> in volitional peak force, electromechanical delay and rate of force development compared to baseline and controls, respectively (F[io, 80] = 2.3 to 4.6; p to 30.7%>) following acute fatigue (Fp; i6] = 4.3 to 9.1; p ; Fp, iq = 3.9; p <0.05). The safeguarding of evoked muscle activation capability despite compromised volitional performance might reveal aspects of capabilities for emergency and protective responses during episodes of fatigue and antecedent muscle damaging exercise
Hamiltonian statistical mechanics
A framework for statistical-mechanical analysis of quantum Hamiltonians is
introduced. The approach is based upon a gradient flow equation in the space of
Hamiltonians such that the eigenvectors of the initial Hamiltonian evolve
toward those of the reference Hamiltonian. The nonlinear double-bracket
equation governing the flow is such that the eigenvalues of the initial
Hamiltonian remain unperturbed. The space of Hamiltonians is foliated by
compact invariant subspaces, which permits the construction of statistical
distributions over the Hamiltonians. In two dimensions, an explicit dynamical
model is introduced, wherein the density function on the space of Hamiltonians
approaches an equilibrium state characterised by the canonical ensemble. This
is used to compute quenched and annealed averages of quantum observables.Comment: 8 pages, 2 figures, references adde
Educating for urban sustainability: A transdisciplinary approach
An understanding of sustainability issues should be a key component of degree programmes. It is widely regarded as being a central attribute to professional practice and responsible global citizenship, arguably more so for the training of teachers since they potentially influence their students. This issue was brought to the fore when responsibility for delivering the 'design and the environment' course was transferred to the building discipline at the University of Newcastle in Australia as a result of restructuring. The attractiveness of the subject as an elective, the need to make it accessible to distance learning students and the desirability of applying transdisciplinary approaches to solving environmental problems presented the course designers with both challenges and opportunities, particularly in devising an assessment context within which students from multiple disciplines could be exposed to, and learn from each other's professional environmental evaluation norms. This paper describes an innovative holistic, multi-criteria problem-solving course design that allows a diverse mix of undergraduates to develop a transdisciplinary understanding of sustainability issues through the use of learning contracts. It reports the experiences of staff and students involved with the course, highlighting the beneficial outcomes
Time Optimal Control in Spin Systems
In this paper, we study the design of pulse sequences for NMR spectroscopy as
a problem of time optimal control of the unitary propagator. Radio frequency
pulses are used in coherent spectroscopy to implement a unitary transfer of
state. Pulse sequences that accomplish a desired transfer should be as short as
possible in order to minimize the effects of relaxation and to optimize the
sensitivity of the experiments. Here, we give an analytical characterization of
such time optimal pulse sequences applicable to coherence transfer experiments
in multiple-spin systems. We have adopted a general mathematical formulation,
and present many of our results in this setting, mindful of the fact that new
structures in optimal pulse design are constantly arising. Moreover, the
general proofs are no more difficult than the specific problems of current
interest. From a general control theory perspective, the problems we want to
study have the following character. Suppose we are given a controllable right
invariant system on a compact Lie group, what is the minimum time required to
steer the system from some initial point to a specified final point? In NMR
spectroscopy and quantum computing, this translates to, what is the minimum
time required to produce a unitary propagator? We also give an analytical
characterization of maximum achievable transfer in a given time for the two
spin system.Comment: 20 Pages, 3 figure
Decoherence Control in Open Quantum System via Classical Feedback
In this work we propose a novel strategy using techniques from systems theory
to completely eliminate decoherence and also provide conditions under which it
can be done so. A novel construction employing an auxiliary system, the bait,
which is instrumental to decoupling the system from the environment is
presented. Our approach to decoherence control in contrast to other approaches
in the literature involves the bilinear input affine model of quantum control
system which lends itself to various techniques from classical control theory,
but with non-trivial modifications to the quantum regime. The elegance of this
approach yields interesting results on open loop decouplability and Decoherence
Free Subspaces(DFS). Additionally, the feedback control of decoherence may be
related to disturbance decoupling for classical input affine systems, which
entails careful application of the methods by avoiding all the quantum
mechanical pitfalls. In the process of calculating a suitable feedback the
system has to be restructured due to its tensorial nature of interaction with
the environment, which is unique to quantum systems. The results are
qualitatively different and superior to the ones obtained via master equations.
Finally, a methodology to synthesize feedback parameters itself is given, that
technology permitting, could be implemented for practical 2-qubit systems to
perform decoherence free Quantum Computing.Comment: 17 pages, 4 Fig
A Quantum Langevin Formulation of Risk-Sensitive Optimal Control
In this paper we formulate a risk-sensitive optimal control problem for
continuously monitored open quantum systems modelled by quantum Langevin
equations. The optimal controller is expressed in terms of a modified
conditional state, which we call a risk-sensitive state, that represents
measurement knowledge tempered by the control purpose. One of the two
components of the optimal controller is dynamic, a filter that computes the
risk-sensitive state.
The second component is an optimal control feedback function that is found by
solving the dynamic programming equation. The optimal controller can be
implemented using classical electronics.
The ideas are illustrated using an example of feedback control of a two-level
atom
Quantum control of the hyperfine-coupled electron and nuclear spins in alkali atoms
We study quantum control of the full hyperfine manifold in the
ground-electronic state of alkali atoms based on applied radio frequency and
microwave fields. Such interactions should allow essentially decoherence-free
dynamics and the application of techniques for robust control developed for NMR
spectroscopy. We establish the conditions under which the system is
controllable in the sense that one can generate an arbitrary unitary on the
system. We apply this to the case of Cs with its dimensional
Hilbert space of magnetic sublevels in the state, and design control
waveforms that generate an arbitrary target state from an initial fiducial
state. We develop a generalized Wigner function representation for this space
consisting of the direct sum of two irreducible representation of SU(2),
allowing us to visualize these states. The performance of different control
scenarios is evaluated based on the ability to generate high-fidelity operation
in an allotted time with the available resources. We find good operating points
commensurate with modest laboratory requirements.Comment: 14 pages, 7 figures; corrected typo
New directions for lifelong learning using network technologies
Please refer only to original source: Koper, R., Tattersall, C. (2004). New directions for lifelong learning using network technologies. British Journal of Educational Technology, 35 (6), 689-700.The requirements placed on learning technologies to support lifelong learning differ considerably from those placed on technologies to support particular fragments of a learning lifetime. The time scales involved in lifelong learning, together with its multi-institutional and episodic nature are not reflected in today’s mainstream learning technologies and their associated architectures. The article presents an integrated model and architecture to serve as the basis for the realization of networked learning technologies serving the specific needs and characteristics of lifelong learners. The integrative model is called a “Learning Network” (LN) and its requirements and architecture are explored, together with the ways in which its application can help in reducing barriers to lifelong learning
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