578 research outputs found
On small Mixed Pattern Ramsey numbers
We call the minimum order of any complete graph so that for any coloring of
the edges by colors it is impossible to avoid a monochromatic or rainbow
triangle, a Mixed Ramsey number. For any graph with edges colored from the
above set of colors, if we consider the condition of excluding in the
above definition, we produce a \emph{Mixed Pattern Ramsey number}, denoted
. We determine this function in terms of for all colored -cycles
and all colored -cliques. We also find bounds for when is a
monochromatic odd cycles, or a star for sufficiently large . We state
several open questions.Comment: 16 page
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An Astrophysical Laboratory: Understanding and Exploiting the Young Massive Cluster Westerlund 1
Westerlund 1 provides a unique opportunity to probe the physics of massive stars, from birth to death and beyond, as well as the formation and evolution of a super star cluster that appears destined to evolve into a globular cluster. We highlight the result of current studies of this cluster, its diverse stellar constituents and immediate environment, concluding with a summary of future research avenues enabled by ESO facilities
Self-Consistent Measurement and State Tomography of an Exchange-Only Spin Qubit
We report initialization, complete electrical control, and single-shot
readout of an exchange-only spin qubit. Full control via the exchange
interaction is fast, yielding a demonstrated 75 qubit rotations in under 2 ns.
Measurement and state tomography are performed using a maximum-likelihood
estimator method, allowing decoherence, leakage out of the qubit state space,
and measurement fidelity to be quantified. The methods developed here are
generally applicable to systems with state leakage, noisy measurements, and
non-orthogonal control axes.Comment: contains Supplementary Informatio
Fast spin exchange between two distant quantum dots
The Heisenberg exchange interaction between neighboring quantum dots allows
precise voltage control over spin dynamics, due to the ability to precisely
control the overlap of orbital wavefunctions by gate electrodes. This allows
the study of fundamental electronic phenomena and finds applications in quantum
information processing. Although spin-based quantum circuits based on
short-range exchange interactions are possible, the development of scalable,
longer-range coupling schemes constitutes a critical challenge within the
spin-qubit community. Approaches based on capacitative coupling and
cavity-mediated interactions effectively couple spin qubits to the charge
degree of freedom, making them susceptible to electrically-induced decoherence.
The alternative is to extend the range of the Heisenberg exchange interaction
by means of a quantum mediator. Here, we show that a multielectron quantum dot
with 50-100 electrons serves as an excellent mediator, preserving speed and
coherence of the resulting spin-spin coupling while providing several
functionalities that are of practical importance. These include speed (mediated
two-qubit rates up to several gigahertz), distance (of order of a micrometer),
voltage control, possibility of sweet spot operation (reducing susceptibility
to charge noise), and reversal of the interaction sign (useful for dynamical
decoupling from noise).Comment: 6 pages including 4 figures, plus 8 supplementary pages including 5
supplementary figure
Efficient quantum state tomography
Quantum state tomography, the ability to deduce the state of a quantum system
from measured data, is the gold standard for verification and benchmarking of
quantum devices. It has been realized in systems with few components, but for
larger systems it becomes infeasible because the number of quantum measurements
and the amount of computation required to process them grows exponentially in
the system size. Here we show that we can do exponentially better than direct
state tomography for a wide range of quantum states, in particular those that
are well approximated by a matrix product state ansatz. We present two schemes
for tomography in 1-D quantum systems and touch on generalizations. One scheme
requires unitary operations on a constant number of subsystems, while the other
requires only local measurements together with more elaborate post-processing.
Both schemes rely only on a linear number of experimental operations and
classical postprocessing that is polynomial in the system size. A further
strength of the methods is that the accuracy of the reconstructed states can be
rigorously certified without any a priori assumptions.Comment: 9 pages, 4 figures. Combines many of the results in arXiv:1002.3780,
arXiv:1002.3839, and arXiv:1002.4632 into one unified expositio
Spin of a Multielectron Quantum Dot and Its Interaction with a Neighboring Electron
We investigate the spin of a multielectron GaAs quantum dot in a sequence of
nine charge occupancies, by exchange coupling the multielectron dot to a
neighboring two-electron double quantum dot. For all nine occupancies, we make
use of a leakage spectroscopy technique to reconstruct the spectrum of spin
states in the vicinity of the interdot charge transition between a single- and
a multielectron quantum dot. In the same regime we also perform time-resolved
measurements of coherent exchange oscillations between the single- and
multielectron quantum dot. With these measurements, we identify distinct
characteristics of the multielectron spin state, depending on whether the dot's
occupancy is even or odd. For three out of four even occupancies we do not
observe any exchange interaction with the single quantum dot, indicating a
spin-0 ground state. For the one remaining even occupancy, we observe an
exchange interaction that we associate with a spin-1 multielectron quantum dot
ground state. For all five of the odd occupancies, we observe an exchange
interaction associated with a spin-1/2 ground state. For three of these odd
occupancies, we clearly demonstrate that the exchange interaction changes sign
in the vicinity of the charge transition. For one of these, the exchange
interaction is negative (i.e. triplet-preferring) beyond the interdot charge
transition, consistent with the observed spin-1 for the next (even) occupancy.
Our experimental results are interpreted through the use of a Hubbard model
involving two orbitals of the multielectron quantum dot. Allowing for the spin
correlation energy (i.e. including a term favoring Hund's rules) and different
tunnel coupling to different orbitals, we qualitatively reproduce the measured
exchange profiles for all occupancies.Comment: 20 pages, 13 figures, 2 table
A Delphi study to strengthen research-methods training in undergraduate psychology programs
Psychology programs often emphasize inferential statistical tests over a solid understanding of data and research design. This imbalance may leave graduates underequipped to effectively interpret research and employ data to answer questions. We conducted a two-round modified Delphi to identify the research-methods skills that the UK psychology community deems essential for undergraduates to learn. Participants included 103 research-methods instructors, academics, students, and nonacademic psychologists. Of 78 items included in the consensus process, 34 reached consensus. We coupled these results with a qualitative analysis of 707 open-ended text responses to develop nine recommendations for organizations that accredit undergraduate psychology programs—such as the British Psychological Society. We recommend that accreditation standards emphasize (1) data skills, (2) research design, (3) descriptive statistics, (4) critical analysis, (5) qualitative methods, and (6) both parameter estimation and significance testing; as well as (7) give precedence to foundational skills, (8) promote transferable skills, and (9) create space in curricula to enable these recommendations. Our data and findings can inform modernized accreditation standards to include clearly defined, assessable, and widely encouraged skills that foster a competent graduate body for the contemporary world
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Beginning to explore the experience of managing a direct payment for someone with dementia: The perspectives of suitable people and adult social care practitioners
Following legal improvements made around mental capacity together with the Health and Social Care Act, it is now possible for a direct payment to be paid to a 'Suitable Person' to manage on someone's behalf to purchase directly care and support services. People with dementia are a key group affected by this change in England of adult social care. We interviewed nine social care practitioners and seven Suitable People for people with dementia across five English local authorities to begin to examine their experiences of this new method of social care provision. Findings from thematic analyses suggest positive outcomes and multiple beneficiaries, but some challenges: potentially inappropriate processes, support planning, divergence in attitudes towards care and support outcomes. Implications for practice include obfuscation of recipients' and Suitable People's best interests and supporting practitioners to explore fully clients' aspirations for care and support
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