823 research outputs found
Universal quantum control in irreducible state-space sectors: application to bosonic and spin-boson systems
We analyze the dynamical-algebraic approach to universal quantum control
introduced in P. Zanardi, S. Lloyd, quant-ph/0305013. The quantum state-space
encoding information decomposes into irreducible sectors and
subsystems associated to the group of available evolutions. If this group
coincides with the unitary part of the group-algebra \CC{\cal K} of some
group then universal control is achievable over the -irreducible components of . This general strategy is applied to
different kind of bosonic systems. We first consider massive bosons in a
double-well and show how to achieve universal control over all
finite-dimensional
Fock sectors. We then discuss a multi-mode massless case giving the
conditions for generating the whole infinite-dimensional multi-mode
Heisenberg-Weyl enveloping-algebra. Finally we show how to use an auxiliary
bosonic mode coupled to finite-dimensional systems to generate high-order
non-linearities needed for universal control.Comment: 10 pages, LaTeX, no figure
Universal control of quantum subspaces and subsystems
We describe a broad dynamical-algebraic framework for analyzing the quantum
control properties of a set of naturally available interactions. General
conditions under which universal control is achieved over a set of
subspaces/subsystems are found. All known physical examples of universal
control on subspaces/systems are related to the framework developed here.Comment: 4 Pages RevTeX, Some typos fixed, references adde
Revisiting consistency conditions for quantum states of systems on closed timelike curves: an epistemic perspective
There has been considerable recent interest in the consequences of closed
timelike curves (CTCs) for the dynamics of quantum mechanical systems. A vast
majority of research into this area makes use of the dynamical equations
developed by Deutsch, which were developed from a consistency condition that
assumes that mixed quantum states uniquely describe the physical state of a
system. We criticise this choice of consistency condition from an epistemic
perspective, i.e., a perspective in which the quantum state represents a state
of knowledge about a system. We demonstrate that directly applying Deutsch's
condition when mixed states are treated as representing an observer's knowledge
of a system can conceal time travel paradoxes from the observer, rather than
resolving them. To shed further light on the appropriate dynamics for quantum
systems traversing CTCs, we make use of a toy epistemic theory with a strictly
classical ontology due to Spekkens and show that, in contrast to the results of
Deutsch, many of the traditional paradoxical effects of time travel are
present.Comment: 10 pages, 6 figures, comments welcome; v2 added references and
clarified some points; v3 published versio
Representation in Westminster in the 1990s : The ghost of Edmund Burke
Why are 'trustee' notions of representation still invoked in the UK House of Commons in the 1990s? In answering this question this article analyses the premises of Burkean theory and the arguments that these premises are of little relevance in the late twentieth century. Despite these dismissals of trusteeship, Burkean ideas are still articulated in the Commons some 200 years after they were first voiced. The idea of trusteeship can prove extremely useful to justify the actions of representatives when those actions conflict with constituency 'opinion', party policy or the wishes of interest groups. Examples of the occasions when Burkean notions have been invoked in the 1990s are provided
Longitudinal Results With Intratympanic Dexamethasone in the Treatment of MĂ©niĂšreâs Disease
To assess patient satisfaction with vertigo control using
intratympanic (IT) dexamethasone (12 mg/mL) for medically refractory unilateral
Meniere's disease. STUDY DESIGN: Retrospective study. SETTING: Tertiary referral
neurotology clinic. PATIENTS: One hundred twenty-nine subjects diagnosed with
unilateral Meniere's disease still having vertigo despite medical therapy.
INTERVENTION: IT dexamethasone injections as needed to control vertigo attacks.
MAIN OUTCOME MEASURE: A Kaplan-Meier time-to-event method was used to determine
the rate of "survival," meaning sufficient satisfaction with vertigo control that
the subject did not wish to have subsequent ablative treatment. "Failure" was
defined as poor control and the choice to proceed to ablative treatment. RESULTS:
Acceptable vertigo control ("survival") was achieved in 117 (91%) of 129
subjects. Vertigo control required only one dexamethasone injection in 48 (37%),
2 injections in 26 (20%), 3 injections in 18 (14%), and 4 injections in 10 (8%).
More than 4 injections were needed in 15 subjects (21%). Of 12 failures (9%), 9
occurred within 6 months of the first IT dexamethasone injection. Follow-up data
for 2 years were available for 96 subjects. Of these, 87 (91%) had vertigo
control with IT dexamethasone, of whom 61 (70)% required no further injections
after 2 years, 23 (26%) continued to receive IT dexamethasone injections, and 3
(3%) chose IT gentamicin treatment. CONCLUSION: IT dexamethasone injection
therapy on an as-needed outpatient basis can provide vertigo control that is
satisfactory in patients with Meniere's disease. The Kaplan-Meier method
addresses the need for an outcome measure suited to repeated treatments and
variable lengths of follow-up. However, due to the retrospective nature of this
study, the presence of bias caused by loss of subjects from follow-up cannot be
ruled out
Quantum logic between atoms inside a high Q optical cavity
We propose a protocol for conditional quantum logic between two 4-state atoms
inside a high Q optical cavity. The process detailed in this paper utilizes a
direct 4-photon 2-atom resonant process and has the added advantage of commonly
addressing the two atoms when they are inside the high Q optical cavity.Comment: 8 pages, 3 figs. submitte
Perfect state distinguishability and computational speedups with postselected closed timelike curves
Bennett and Schumacher's postselected quantum teleportation is a model of
closed timelike curves (CTCs) that leads to results physically different from
Deutsch's model. We show that even a single qubit passing through a
postselected CTC (P-CTC) is sufficient to do any postselected quantum
measurement, and we discuss an important difference between "Deutschian" CTCs
(D-CTCs) and P-CTCs in which the future existence of a P-CTC might affect the
present outcome of an experiment. Then, based on a suggestion of Bennett and
Smith, we explicitly show how a party assisted by P-CTCs can distinguish a set
of linearly independent quantum states, and we prove that it is not possible
for such a party to distinguish a set of linearly dependent states. The power
of P-CTCs is thus weaker than that of D-CTCs because the Holevo bound still
applies to circuits using them regardless of their ability to conspire in
violating the uncertainty principle. We then discuss how different notions of a
quantum mixture that are indistinguishable in linear quantum mechanics lead to
dramatically differing conclusions in a nonlinear quantum mechanics involving
P-CTCs. Finally, we give explicit circuit constructions that can efficiently
factor integers, efficiently solve any decision problem in the intersection of
NP and coNP, and probabilistically solve any decision problem in NP. These
circuits accomplish these tasks with just one qubit traveling back in time, and
they exploit the ability of postselected closed timelike curves to create
grandfather paradoxes for invalid answers.Comment: 15 pages, 4 figures; Foundations of Physics (2011
The Detectability of Departures from the Inflationary Consistency Equation
We study the detectability, given CMB polarization maps, of departures from
the inflationary consistency equation, r \equiv T/S \simeq -5 n_T, where T and
S are the tensor and scalar contributions to the quadrupole variance,
respectively. The consistency equation holds if inflation is driven by a
slowly-rolling scalar field. Departures can be caused by: 1) higher-order terms
in the expansion in slow-roll parameters, 2) quantum loop corrections or 3)
multiple fields. Higher-order corrections in the first two slow-roll parameters
are undetectably small. Loop corrections are detectable if they are nearly
maximal and r \ga 0.1. Large departures (|\Delta n_T| \ga 0.1) can be seen if r
\ga 0.001. High angular resolution can be important for detecting non-zero
r+5n_T, even when not important for detecting non-zero r.Comment: 7 pages, 4 figures, submitted to PR
Electromigration-Induced Flow of Islands and Voids on the Cu(001) Surface
Electromigration-induced flow of islands and voids on the Cu(001) surface is
studied at the atomic scale. The basic drift mechanisms are identified using a
complete set of energy barriers for adatom hopping on the Cu(001) surface,
combined with kinetic Monte Carlo simulations. The energy barriers are
calculated by the embedded atom method, and parameterized using a simple model.
The dependence of the flow on the temperature, the size of the clusters, and
the strength of the applied field is obtained. For both islands and voids it is
found that edge diffusion is the dominant mass-transport mechanism. The rate
limiting steps are identified. For both islands and voids they involve
detachment of atoms from corners into the adjacent edge. The energy barriers
for these moves are found to be in good agreement with the activation energy
for island/void drift obtained from Arrhenius analysis of the simulation
results. The relevance of the results to other FCC(001) metal surfaces and
their experimental implications are discussed.Comment: 9 pages, 13 ps figure
Detectability of Tensor Perturbations Through CBR Anisotropy (final published version)
Detection of the tensor perturbations predicted in inflationary models is
important for testing inflation as well as for reconstructing the inflationary
potential. We show that because of cosmic variance the tensor contribution to
the square of the CBR quadrupole anisotropy must be greater than about 20\% of
the scalar contribution to ensure a statistically significant detection of
tensor perturbations. This sensitivity could be achieved by full-sky
measurements on angular scales of and .Comment: 10 pages, uu-encoded postscript file, FERMILAB-PUB-94/175-
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