101 research outputs found
Decoherent quantum walks driven by a generic coin operation
We consider the effect of different unitary noise mechanisms on the evolution
of a quantum walk (QW) on a linear chain with a generic coin operation: (i)
bit-flip channel noise, restricted to the coin subspace of the QW, and (ii)
topological noise caused by randomly broken links in the linear chain.
Similarities and differences in the respective decoherent dynamics of the
walker as a function of the probability per unit time of a decoherent event
taking place are discussed
State permutations from manipulation of near level-crossings
We discuss some systematic methods for implementing state manipulations in
systems formally similar to chains of a few spins with nearest-neighbor
interactions, arranged such that there are strong and weak scales of coupling
links. States are permuted by means of bias potentials applied to a few
selected sites. This generic structure is then related to an atoms-in-a-cavity
model that has been proposed in the literature as a way of achieving a
decoherence free subspace. A new method using adiabatically varying laser
detuning to implement a CNOT gate in this model is proposed.Comment: 6 pages, 5 figures. Substantial revision and extension of the
introduction and the atoms-in-a-cavity section
Temperature dependence of the anomaly in the excitation spectrum of the 2D quantum Heisenberg antiferromagnet
It is well established that in the low-temperature limit, the two-dimensional
quantum Heisenberg antiferromagnet on a square lattice (2DQHAFSL) exhibits an
anomaly in its spectrum at short-wavelengths on the zone-boundary. In the
vicinity of the point the pole in the one-magnon response exhibits a
downward dispersion, is heavily damped and attenuated, giving way to an
isotropic continuum of excitations extending to high energies. The origin of
the anomaly and the presence of the continuum are of current theoretical
interest, with suggestions focused around the idea that the latter evidences
the existence of spinons in a two-dimensional system. Here we present the
results of neutron inelastic scattering experiments and Quantum Monte Carlo
calculations on the metallo-organic compound Cu(DCOO)DO (CFTD),
an excellent physical realisation of the 2DQHAFSL, designed to investigate how
the anomaly at evolves up to finite temperatures . Our
data reveal that on warming the anomaly survives the loss of long-range,
three-dimensional order, and is thus a robust feature of the two-dimensional
system. With further increase of temperature the zone-boundary response
gradually softens and broadens, washing out the anomaly. This is
confirmed by a comparison of our data with the results of finite-temperature
Quantum Monte Carlo simulations where the two are found to be in good accord.
At lower energies, in the vicinity of the antiferromagnetic zone centre, there
was no significant softening of the magnetic excitations over the range of
temperatures investigated.Comment: Dedicated to the life and work of Professor Roger Cowley. 22 pages, 8
figure
Holonomic quantum computation with neutral atoms
We propose an all-geometric implementation of quantum computation using
neutral atoms in cavity QED. We show how to perform generic single- and
two-qubit gates, the latter by encoding a two-atom state onto a single,
many-level atom. We compare different strategies to overcome limitations due to
cavity imperfections.Comment: 14 pages, 9 figure
Recurrence of biased quantum walks on a line
The Polya number of a classical random walk on a regular lattice is known to
depend solely on the dimension of the lattice. For one and two dimensions it
equals one, meaning unit probability to return to the origin. This result is
extremely sensitive to the directional symmetry, any deviation from the equal
probability to travel in each direction results in a change of the character of
the walk from recurrent to transient. Applying our definition of the Polya
number to quantum walks on a line we show that the recurrence character of
quantum walks is more stable against bias. We determine the range of parameters
for which biased quantum walks remain recurrent. We find that there exist
genuine biased quantum walks which are recurrent.Comment: Journal reference added, minor corrections in the tex
Ferromagnetism in Co7(TeO3)4Br6: A byproduct of complex antiferromagnetic order and single-ion anisotropy
Pronounced anisotropy of magnetic properties and complex magnetic order of a
new oxi-halide compound Co7(TeO3)4Br6 has been investigated by powder and
single crystal neutron diffraction, magnetization and ac susceptibility
techniques. Anisotropy of susceptibility extends far into the paramagnetic
temperature range. A principal source of anisotropy are anisotropic properties
of the involved octahedrally coordinated single Co(2+) ions, as confirmed by
angular-overlap-model calculations presented in this work. Incommensurate
antiferromagnetic order sets in at TN=34 K. Propagation vector is strongly
temperature dependent reaching k1=(0.9458(6), 0, 0.6026(5)) at 30 K. A
transition to a ferrimagnetic structure with k2=0 takes place at TC=27 K.
Magnetically ordered phase is characterized by very unusual anisotropy as well:
while M-H scans along b-axis reveals spectacularly rectangular but otherwise
standard ferromagnetic hysteresis loops, M-H studies along other two principal
axes are perfectly reversible, revealing very sharp spin flop (or spin flip)
transitions, like in a standard antiferromagnet (or metamagnet). Altogether,
the observed magnetic phenomenology is interpreted as an evidence of competing
magnetic interactions permeating the system, first of all of the single ion
anisotropy energy and the exchange interactions. Different coordinations of the
Co(2+)-ions involved in the low-symmetry C2/c structure of Co7(TeO3)4Br6 render
the exchange-interaction network very complex by itself. Temperature dependent
changes in the magnetic structure, together with an abrupt emergence of a
ferromagnetic component, are ascribed to continual spin reorientations
described by a multi-component, but yet unknown, spin Hamiltonian.Comment: 12 pages, 13 figures; submitted to PR
Fractional recurrence in discrete-time quantum walk
Quantum recurrence theorem holds for quantum systems with discrete energy
eigenvalues and fails to hold in general for systems with continuous energy. We
show that during quantum walk process dominated by interference of amplitude
corresponding to different paths fail to satisfy the complete quantum
recurrence theorem. Due to the revival of the fractional wave packet, a
fractional recurrence characterized using quantum P\'olya number can be seen.Comment: 10 pages, 11 figure : Accepted to appear in Central European Journal
of Physic
Socioeconomic predictors and consequences of depression among primary care attenders with non-communicable diseases in the Western Cape, South Africa:Cohort study within a randomised trial
Background: Socioeconomic predictors and consequences of depression and its treatment were investigated in 4393 adults with specified non-communicable diseases attending 38 public sector primary care clinics in the Eden and Overberg districts of the Western Cape, South Africa. Methods: Participants were interviewed at baseline in 2011 and 14 months later, as part of a randomised controlled trial of a guideline-based intervention to improve diagnosis and management of chronic diseases. The 10-item Center for Epidemiologic Studies Depression Scale (CESD-10) was used to assess depression symptoms, with higher scores representing more depressed mood. Results: Higher CESD-10 scores at baseline were independently associated with being less educated (p=0.004) and having lower income (p=0.003). CESD-10 scores at follow-up were higher in participants with less education (p=0.010) or receiving welfare grants (p=0.007) independent of their baseline scores. Participants with CESD-10 scores of 10 or more at baseline (56% of all participants) had 25% higher odds of being unemployed at follow-up (p=0.016), independently of baseline CESD-10 score and treatment status. Among participants with baseline CESD-10 scores of 10 or more, antidepressant medication at baseline was independently more likely in participants who had more education (p=0.002), higher income (p<0.001), or were unemployed (p=0.001). Antidepressant medication at follow up was independently more likely in participants with higher income (p=0.023), and in clinics with better access to pharmacists (p=0.053) and off-site drug delivery (p=0.013). Conclusions: Socioeconomic disadvantage appears to be both a cause and consequence of depression, and may also be a barrier to treatment. There are opportunities for improving the prevention, diagnosis and treatment of depression in primary care in inequitable middle income countries like South Africa. Trial registration: The trial is registered with Current Controlled Trials (ISRCTN20283604) and the Office for Human Research Protections Database (IRB00001938, FWA00001637)
Evaluating the use of the Child and Adolescent Intellectual Disability Screening Questionnaire (CAIDS-Q) to estimate IQ in children with low intellectual ability
In situations where completing a full intellectual assessment is not possible or desirable the clinician or researcher may require an alternative means of accurately estimating intellectual functioning. There has been limited research in the use of proxy IQ measures in children with an intellectual disability or low IQ. The present study aimed to provide a means of converting total scores from a screening tool (the Child and Adolescent Intellectual Disability Screening Questionnaire: CAIDS-Q) to an estimated IQ. A series of linear regression analyses were conducted on data from 428 children and young people referred to clinical services, where FSIQ was predicted from CAIDS-Q total scores. Analyses were conducted for three age groups between ages 6 and 18 years. The study presents a conversion table for converting CAIDS-Q total scores to estimates of FSIQ, with corresponding 95% prediction intervals to allow the clinician or researcher to estimate FSIQ scores from CAIDS-Q total scores. It is emphasised that, while this conversion may offer a quick means of estimating intellectual functioning in children with a below average IQ, it should be used with caution, especially in children aged between 6 and 8 years old
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