946 research outputs found
Fidelity of the surface code in the presence of a bosonic bath
We study the resilience of the surface code to decoherence caused by the
presence of a bosonic bath. This approach allows us to go beyond the standard
stochastic error model commonly used to quantify decoherence and error
threshold probabilities in this system. The full quantum mechanical system-bath
dynamics is computed exactly over one quantum error correction cycle. Since all
physical qubits interact with the bath, space-time correlations between errors
are taken into account. We compute the fidelity of the surface code as a
function of the quantum error correction time. The calculation allows us to map
the problem onto an Ising-like statistical spin model with two-body
interactions and a fictitious temperature which is related to the inverse bath
coupling constant. The model departs from the usual Ising model in the sense
that interactions can be long ranged and can involve complex exchange
couplings; in addition, the number of allowed configurations is restricted by
the syndrome extraction. Using analytical estimates and numerical calculations,
we argue that, in the limit of an infinite number of physical qubits, the spin
model sustain a phase transition which can be associated to the existence of an
error threshold in the surface code. An estimate of the transition point is
given for the case of nearest-neighbor interactions.Comment: 15 pages, 5 figure
Historical Roots of Histrionic Personality Disorder
Histrionic Personality Disorder is one of the most ambiguous diagnostic categories in psychiatry. Hysteria is a classical term that includes a wide variety of psychopathological states. Ancient Egyptians and Greeks blamed a displaced womb, for many women's afflictions. Several researchers from the 18th and 19th centuries studied this theme, namely, Charcot who defined hysteria as a "neurosis" with an organic basis and Sigmund Freud who redefined "neurosis" as a re-experience of past psychological trauma. Histrionic personality disorder (HPD) made its first official appearance in the Diagnostic and Statistical Manual of Mental Disorders II (DSM-II) and since the DSM-III, HPD is the only disorder that kept the term derived from the old concept of hysteria. The subject of hysteria has reflected positions about health, religion and relationships between the sexes in the last 4000 years, and the discussion is likely to continue
Decoherence by Correlated Noise and Quantum Error Correction
We study the decoherence of a quantum computer in an environment which is
inherently correlated in time and space. We first derive the nonunitary time
evolution of the computer and environment in the presence of a stabilizer error
correction code, providing a general way to quantify decoherence for a quantum
computer. The general theory is then applied to the spin-boson model. Our
results demonstrate that effects of long-range correlations can be
systematically reduced by small changes in the error correction codes.Comment: 4 pages, 1 figure, Phys. Rev. Lett. in pres
Properties of magnetic nanodots with perpendicular anisotropy
Nanodots with magnetic vortices have many potential applications, such as
magnetic memories (VRAMs) and spin transfer nano-oscillators (STNOs). Adding a
perpendicular anisotropy term to the magnetic energy of the nanodot it becomes
possible to tune the vortex core properties. This can be obtained, e.g., in Co
nanodots by varying the thickness of the Co layer in a Co/Pt stack. Here we
discuss the spin configuration of circular and elliptical nanodots for
different perpendicular anisotropies; we show for nanodisks that micromagnetic
simulations and analytical results agree. Increasing the perpendicular
anisotropy, the vortex core radii increase, the phase diagrams are modified and
new configurations appear; the knowledge of these phase diagrams is relevant
for the choice of optimum nanodot dimensions for applications. MFM measurements
on Co/Pt multilayers confirm the trend of the vortex core diameters with
varying Co layer thicknesses.Comment: 7 pages, 8 figure
New methods for stress assessment and monitoring at the workplace
The topic of stress is nowadays a very important one, not only in research but on social life in general. People are increasingly aware of this problem and its consequences at several levels: health, social life, work, quality of life, etc. This resulted in a significant increase in the search for devices and applications to measure and manage stress in real-time. Recent technological and scientific evolution fosters this interest with the development of new methods and approaches. In this paper we survey these new methods for stress assessment, focusing especially on those that are suited for the workplace: one of today’s major sources of stress. We contrast them with more traditional methods and compare them between themselves, evaluating nine characteristics. Given the diversity of methods that exist nowadays, this work facilitates the stakeholders’ decision towards which one to use, based on how much their organization values aspects such as privacy, accuracy, cost-effectiveness or intrusivenes
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