2,444 research outputs found
Decoherence-free quantum information in the presence of dynamical evolution
We analyze decoherence-free (DF) quantum information in the presence of an
arbitrary non-nearest-neighbor bath-induced system Hamiltonian using a
Markovian master equation. We show that the most appropriate encoding for N
qubits is probably contained within the ~(2/9) N excitation subspace. We give a
timescale over which one would expect to apply other methods to correct for the
system Hamiltonian. In order to remain applicable to experiment, we then focus
on small systems, and present examples of DF quantum information for three and
four qubits. We give an encoding for four qubits that, while quantum
information remains in the two-excitation subspace, protects against an
arbitrary bath-induced system Hamiltonian. Although our results are general to
any system of qubits that satisfies our assumptions, throughout the paper we
use dipole-coupled qubits as an example physical system.Comment: 8 pages, 4 figure
Super- and subradiant emission of two-level systems in the near-Dicke limit
We analyze the stability of super- and subradiant states in a system of
identical two-level atoms in the near-Dicke limit, i.e., when the atoms are
very close to each other compared to the wavelength of resonant light. The
dynamics of the system are studied using a renormalized master equation, both
with multipolar and minimal-coupling interaction schemes. We show that both
models lead to the same result and, in contrast to unrenormalized models,
predict that the relative orientation of the (co-aligned) dipoles is
unimportant in the Dicke limit. Our master equation is of relevance to any
system of dipole-coupled two-level atoms, and gives bounds on the strength of
the dipole-dipole interaction for closely spaced atoms. Exact calculations for
small atom systems in the near-Dicke limit show the increased emission times
resulting from the evolution generated by the strong dipole-dipole interaction.
However, for large numbers of atoms in the near-Dicke limit, it is shown that
as the number of atoms increases, the effect of the dipole-dipole interaction
on collective emission is reduced.Comment: 14 pages, 6 figures, published versio
Decoherence-free quantum-information processing using dipole-coupled qubits
We propose a quantum-information processor that consists of decoherence-free
logical qubits encoded into arrays of dipole-coupled qubits. High-fidelity
single-qubit operations are performed deterministically within a
decoherence-free subsystem without leakage via global addressing of bichromatic
laser fields. Two-qubit operations are realized locally with four physical
qubits, and between separated logical qubits using linear optics. We show how
to prepare cluster states using this method. We include all
non-nearest-neighbor effects in our calculations, and we assume the qubits are
not located in the Dicke limit. Although our proposal is general to any system
of dipole-coupled qubits, throughout the paper we use nitrogen-vacancy (NV)
centers in diamond as an experimental context for our theoretical results.Comment: 7 pages, 5 figure
Spontaneous emission of atomic systems in the presence of incident fields
We characterise the spontaneous emission time and direction from small
numbers of dipole-coupled two-level atoms (2LAs) in the presence of incident
fields. We show how to use adiabatic passage to admit population transfer
between states in the one-quantum subspace for two and three 2LAs. Our method
is a multi-atom generalisation of stimulated-Raman-adiabatic-passage (STIRAP)
for a single multi-level atom. We use numerical results to justify an ansatz
that enables us to give analytical expressions for the directional emission
which depends on the incident fields. Our results admit a characterisation of
the efficacy of population transfer in small numbers of dipole-coupled 2LAs,
and are applicable to proof-of-principle experiments involving dipole-coupled
2LAs.Comment: 10 pages, 6 figure
Transverse instability for non-normal parameters
We consider the behaviour of attractors near invariant subspaces on varying a
parameter that does not preserve the dynamics in the invariant subspace but is
otherwise generic, in a smooth dynamical system. We refer to such a parameter
as ``non-normal''. If there is chaos in the invariant subspace that is not
structurally stable, this has the effect of ``blurring out'' blowout
bifurcations over a range of parameter values that we show can have positive
measure in parameter space.
Associated with such blowout bifurcations are bifurcations to attractors
displaying a new type of intermittency that is phenomenologically similar to
on-off intermittency, but where the intersection of the attractor by the
invariant subspace is larger than a minimal attractor. The presence of distinct
repelling and attracting invariant sets leads us to refer to this as ``in-out''
intermittency. Such behaviour cannot appear in systems where the transverse
dynamics is a skew product over the system on the invariant subspace.
We characterise in-out intermittency in terms of its structure in phase space
and in terms of invariants of the dynamics obtained from a Markov model of the
attractor. This model predicts a scaling of the length of laminar phases that
is similar to that for on-off intermittency but which has some differences.Comment: 15 figures, submitted to Nonlinearity, the full paper available at
http://www.maths.qmw.ac.uk/~eo
AVERT2(a very early rehabilitation trial, a very effective reproductive trigger): retrospective observational analysis of the number of babies born to trial staff
Objective: To report the number of participants needed to recruit per baby born to trial staff during AVERT, a large international trial on acute stroke, and to describe trial management consequences.
Design: Retrospective observational analysis.
Setting: 56 acute stroke hospitals in eight countries.
Participants: 1074 trial physiotherapists, nurses, and other clinicians.
Outcome measures: Number of babies born during trial recruitment per trial participant recruited.
Results: With 198 site recruitment years and 2104 patients recruited during AVERT, 120 babies were born to trial staff. Births led to an estimated 10% loss in time to achieve recruitment. Parental leave was linked to six trial site closures. The number of participants needed to recruit per baby born was 17.5 (95% confidence interval 14.7 to 21.0); additional trial costs associated with each birth were estimated at 5736 Australian dollars on average.
Conclusion: The staff absences registered in AVERT owing to parental leave led to delayed trial recruitment and increased costs, and should be considered by trial investigators when planning research and estimating budgets. However, the celebration of new life became a highlight of the annual AVERT collaboratorsâ meetings and helped maintain a cohesive collaborative group
Decoherence-free quantum information in Markovian systems
Decoherence in Markovian systems can result indirectly from the action of a
system Hamiltonian which is usually fixed and unavoidable. Here, we show that
in general in Markovian systems, because of the system Hamiltonian, quantum
information decoheres. We give conditions for the system Hamiltonian that must
be satisfied if coherence is to be preserved. Finally, we show how to construct
robust subspaces for quantum information processing.Comment: 4 pages, published versio
Combined Effects of Acute Temperature Change and Elevated pCO2 on the Metabolic Rates and Hypoxia Tolerances of Clearnose Skate (Rostaraja eglanteria), Summer Flounder (Paralichthys dentatus), and Thorny Skate (Amblyraja radiata)
Understanding how rising temperatures, ocean acidification, and hypoxia affect the performance of coastal fishes is essential to predicting species-specific responses to climate change. Although a populationâs habitat influences physiological performance, little work has explicitly examined the multi-stressor responses of species from habitats differing in natural variability. Here, clearnose skate (Rostaraja eglanteria) and summer flounder (Paralichthys dentatus) from mid-Atlantic estuaries, and thorny skate (Amblyraja radiata) from the Gulf of Maine, were acutely exposed to current and projected temperatures (20, 24, or 28 °C; 22 or 30 °C; and 9, 13, or 15 °C, respectively) and acidification conditions (pH 7.8 or 7.4). We tested metabolic rates and hypoxia tolerance using intermittent-flow respirometry. All three species exhibited increases in standard metabolic rate under an 8 °C temperature increase (Q10 of 1.71, 1.07, and 2.56, respectively), although this was most pronounced in the thorny skate. At the lowest test temperature and under the low pH treatment, all three species exhibited significant increases in standard metabolic rate (44â105%; p \u3c 0.05) and decreases in hypoxia tolerance (60â84% increases in critical oxygen pressure; p \u3c 0.05). This study demonstrates the interactive effects of increasing temperature and changing ocean carbonate chemistry are species-specific, the implications of which should be considered within the context of habitat.
Associated dataset:
Gail D. Schweiterman, Daniel P. Crear et al. 2019. Metabolic Rates and Hypoxia Tolerences of clearnose skate (Rostaraja eglanteria), summer flounder (Paralichthys dentatus), and thorny skate (Amblyraja radiata)
https://doi.org/10.25773/qmew-c18
Metabolic Rates and Hypoxia Tolerences of clearnose skate (Rostaraja eglanteria), summer flounder (Paralichthys dentatus), and thorny skate (Amblyraja radiata)
These data were collected following methods described in the associated publication: LINK
âCombined Effects of Acute Temperature Change and Elevated pCO2 on the Metabolic Rates and Hypoxia Tolerances of Clearnose Skate (Rostaraja eglanteria), Summer Flounder (Paralichthys dentatus), and Thorny Skate (Amblyraja radiata)â. Schweiterman, G.D. et al. 2019 Biology, 8(3), 56
Coordinated Transcription of Key Pathways in the Mouse by the Circadian Clock
AbstractIn mammals, circadian control of physiology and behavior is driven by a master pacemaker located in the suprachiasmatic nuclei (SCN) of the hypothalamus. We have used gene expression profiling to identify cycling transcripts in the SCN and in the liver. Our analysis revealed âŒ650 cycling transcripts and showed that the majority of these were specific to either the SCN or the liver. Genetic and genomic analysis suggests that a relatively small number of output genes are directly regulated by core oscillator components. Major processes regulated by the SCN and liver were found to be under circadian regulation. Importantly, rate-limiting steps in these various pathways were key sites of circadian control, highlighting the fundamental role that circadian clocks play in cellular and organismal physiology
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