36,772 research outputs found
Reconceptualising Personas Across Cultures: Archetypes, Stereotypes & Collective Personas in Pastoral Namibia
The paucity of projects where persona is the research foci and a lack of consensus on this artefact keep many reticent about its purpose and value. Besides crafting personas is expected to differ across cultures, which contrasts the advancements in Western theory with studies and progress in other sites. We postulate User-Created Personas reveal specific characteristics of situated contexts by allowing laypeople to design persona artefacts in their own terms. Hence analysing four persona sessions with an ethnic group in pastoral Namibia –ovaHerero– brought up a set of fundamental questions around the persona artefact regarding stereotypes, archetypes, and collective persona representations: (1) to what extent user depictions are stereotypical or archetypal? If stereotypes prime (2) to what degree are current personas a useful method to represent end-users in technology design? And, (3) how can we ultimately read accounts not conforming to mainstream individual persona descriptions but to collectives
Fast Two-Qubit Gates in Semiconductor Quantum Dots using a Photonic Microcavity
Implementations for quantum computing require fast single- and multi-qubit
quantum gate operations. In the case of optically controlled quantum dot qubits
theoretical designs for long-range two- or multi-qubit operations satisfying
all the requirements in quantum computing are not yet available. We have
developed a design for a fast, long-range two-qubit gate mediated by a photonic
microcavity mode using excited states of the quantum dot-cavity system that
addresses these needs. This design does not require identical qubits, it is
compatible with available optically induced single qubit operations, and it
advances opportunities for scalable architectures. We show that the gate
fidelity can exceed 90% in experimentally accessible systems
Charge qubits and limitations of electrostatic quantum gates
We investigate the characteristics of purely electrostatic interactions with
external gates in constructing full single qubit manipulations. The quantum bit
is naturally encoded in the spatial wave function of the electron system.
Single-electron{transistor arrays based on quantum dots or insulating
interfaces typically allow for electrostatic controls where the inter-island
tunneling is considered constant, e.g. determined by the thickness of an
insulating layer. A representative array of 3x3 quantum dots with two mobile
electrons is analyzed using a Hubbard Hamiltonian and a capacitance matrix
formalism. Our study shows that it is easy to realize the first quantum gate
for single qubit operations, but that a second quantum gate only comes at the
cost of compromising the low-energy two-level system needed to encode the
qubit. We use perturbative arguments and the Feshbach formalism to show that
the compromising of the two-level system is a rather general feature for
electrostatically interacting qubits and is not just related to the specific
details of the system chosen. We show further that full implementation requires
tunable tunneling or external magnetic fields.Comment: 7 pages, 5 figures, submitted to PR
Application of asymptotic expansions of maximum likelihood estimators errors to gravitational waves from binary mergers: the single interferometer case
In this paper we describe a new methodology to calculate analytically the
error for a maximum likelihood estimate (MLE) for physical parameters from
Gravitational wave signals. All the existing litterature focuses on the usage
of the Cramer Rao Lower bounds (CRLB) as a mean to approximate the errors for
large signal to noise ratios. We show here how the variance and the bias of a
MLE estimate can be expressed instead in inverse powers of the signal to noise
ratios where the first order in the variance expansion is the CRLB. As an
application we compute the second order of the variance and bias for MLE of
physical parameters from the inspiral phase of binary mergers and for noises of
gravitational wave interferometers . We also compare the improved error
estimate with existing numerical estimates. The value of the second order of
the variance expansions allows to get error predictions closer to what is
observed in numerical simulations. It also predicts correctly the necessary SNR
to approximate the error with the CRLB and provides new insight on the
relationship between waveform properties SNR and estimation errors. For example
the timing match filtering becomes optimal only if the SNR is larger than the
kurtosis of the gravitational wave spectrum
Quantum Phase Transitions and Bipartite Entanglement
We develop a general theory of the relation between quantum phase transitions
(QPTs) characterized by nonanalyticities in the energy and bipartite
entanglement. We derive a functional relation between the matrix elements of
two-particle reduced density matrices and the eigenvalues of general two-body
Hamiltonians of -level systems. The ground state energy eigenvalue and its
derivatives, whose non-analyticity characterizes a QPT, are directly tied to
bipartite entanglement measures. We show that first-order QPTs are signalled by
density matrix elements themselves and second-order QPTs by the first
derivative of density matrix elements. Our general conclusions are illustrated
via several quantum spin models.Comment: 5 pages, incl. 2 figures. v3: The version published in PRL, including
a few extra comments and clarifications for which there was no space in the
PR
Error correction in ensemble registers for quantum repeaters and quantum computers
We propose to use a collective excitation blockade mechanism to identify
errors that occur due to disturbances of single atoms in ensemble quantum
registers where qubits are stored in the collective population of different
internal atomic states. A simple error correction procedure and a simple
decoherence-free encoding of ensemble qubits in the hyperfine states of alkali
atoms are presented.Comment: 4 pages, 2 figure
Scaling laws for the decay of multiqubit entanglement
We investigate the decay of entanglement of generalized N-particle
Greenberger-Horne-Zeilinger (GHZ) states interacting with independent
reservoirs. Scaling laws for the decay of entanglement and for its finite-time
extinction (sudden death) are derived for different types of reservoirs. The
latter is found to increase with the number of particles. However, entanglement
becomes arbitrarily small, and therefore useless as a resource, much before it
completely disappears, around a time which is inversely proportional to the
number of particles. We also show that the decay of multi-particle GHZ states
can generate bound entangled states.Comment: Minor mistakes correcte
Law Behind Second Law of Thermodynamics --Unification with Cosmology--
In an abstract setting of a general classical mechanical system as a model
for the universe we set up a general formalism for a law behind the second law
of thermodynamics, i.e. really for "initial conditions". We propose a
unification with the other laws by requiring similar symmetry and locality
properties.Comment: 17 page
Entanglement Equivalence of -qubit Symmetric States
We study the interconversion of multipartite symmetric -qubit states under
stochastic local operations and classical communication (SLOCC). We demonstrate
that if two symmetric states can be connected with a nonsymmetric invertible
local operation (ILO), then they belong necessarily to the separable, W, or GHZ
entanglement class, establishing a practical method of discriminating subsets
of entanglement classes. Furthermore, we prove that there always exists a
symmetric ILO connecting any pair of symmetric -qubit states equivalent
under SLOCC, simplifying the requirements for experimental implementations of
local interconversion of those states.Comment: Minor correction
Extreme AO Observations of Two Triple Asteroid Systems with SPHERE
We present the discovery of a new satellite of asteroid (130) Elektra -
S/2014 (130) 1 - in differential imaging and in integral field spectroscopy
data over multiple epochs obtained with SPHERE/VLT. This new (second) moonlet
of Elektra is about 2 km across, on an eccentric orbit and about 500 km away
from the primary. For a comparative study, we also observed another triple
asteroid system (93) Minerva. For both systems, component-resolved reflectance
spectra of the satellites and primary were obtained simultaneously. No
significant spectral difference was observed between the satellites and the
primary for either triple system. We find that the moonlets in both systems are
more likely to have been created by sub-disruptive impacts as opposed to having
been captured.Comment: 8 pages, 4 figures, 1 table, accepted to be published in the
Astrophysical Journal Letter
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