689 research outputs found
Optimal simulation of two-qubit Hamiltonians using general local operations
We consider the simulation of the dynamics of one nonlocal Hamiltonian by
another, allowing arbitrary local resources but no entanglement nor classical
communication. We characterize notions of simulation, and proceed to focus on
deterministic simulation involving one copy of the system. More specifically,
two otherwise isolated systems and interact by a nonlocal Hamiltonian
. We consider the achievable space of Hamiltonians such
that the evolution can be simulated by the interaction
interspersed with local operations. For any dimensions of and , and any
nonlocal Hamiltonians and , there exists a scale factor such that
for all times the evolution can be simulated by acting for
time interspersed with local operations. For 2-qubit Hamiltonians and
, we calculate the optimal and give protocols achieving it. The optimal
protocols do not require local ancillas, and can be understood geometrically in
terms of a polyhedron defined by a partial order on the set of 2-qubit
Hamiltonians.Comment: (1) References to related work, (2) protocol to simulate one
two-qudit Hamiltonian with another, and (3) other related results added. Some
proofs are simplifie
Three-dimensional Acceleration Measurement Using Videogrammetry Tracking Data
In order to evaluate the feasibility of multi-point, non-contact, acceleration measurement, a high-speed, precision videogrammetry system has been assembled from commercially-available components and software. Consisting of three synchronized 640 X 480 pixel monochrome progressive scan CCD cameras each operated at 200 frames per second, this system has the capability to provide surface-wide position-versus-time data that are filtered and twice-differentiated to yield the desired acceleration tracking at multiple points on a moving body. The oscillating motion of targets mounted on the shaft of a modal shaker were tracked, and the accelerations calculated using the videogrammetry data were compared directly to conventional accelerometer measurements taken concurrently. Although differentiation is an inherently noisy operation, the results indicate that simple mathematical filters based on the well-known Savitzky and Golay algorithms, implemented using spreadsheet software, remove a significant component of the noise, resulting in videogrammetry-based acceleration measurements that are comparable to those obtained using the accelerometers
Relations for classical communication capacity and entanglement capability of two-qubit operations
Bipartite operations underpin both classical communication and entanglement
generation. Using a superposition of classical messages, we show that the
capacity of a two-qubit operation for error-free entanglement-assisted
bidirectional classical communication can not exceed twice the entanglement
capability. In addition we show that any bipartite two-qubit operation can
increase the communication that may be performed using an ensemble by twice the
entanglement capability.Comment: 4 page
A framework for understanding the workspace activity of design teams
Abstract ’ Small group design sessions were empirically studied to understand better collaborative workspace activity. A conventional view of workspace activity may be characterized as concerned only with storing information and conveying ideas through text and graphics. Empirical evidence shows that this view is deficient in not accounting for how the workspace is used: a) in a group setting, rather than by an individual, and b) as part of a process of constructing artifacts, rather than just a medium for the resulting artifacts themselves. An understanding of workspace activity needs to include the role of gestural activity, and the use of the workspace to develop ideas and mediate interaction. A framework that helps illustrate an expanded view of workspace activity is proposed and supported with empirical data
Deriving Bisimulation Congruences: 2-categories vs precategories
G-relative pushouts (GRPOs) have recently been proposed by the authors as a new foundation for Leifer and Milner’s approach to deriving labelled bisimulation congruences from reduction systems. This paper develops the theory of GRPOs further, arguing that they provide a simple and powerful basis towards a comprehensive solution. As an example, we construct GRPOs in a category of ‘bunches and wirings.’ We then examine the approach based on Milner’s precategories and Leifer’s functorial reactive systems, and show that it can be recast in a much simpler way into the 2-categorical theory of GRPOs
The lesson of causal discovery algorithms for quantum correlations: Causal explanations of Bell-inequality violations require fine-tuning
An active area of research in the fields of machine learning and statistics
is the development of causal discovery algorithms, the purpose of which is to
infer the causal relations that hold among a set of variables from the
correlations that these exhibit. We apply some of these algorithms to the
correlations that arise for entangled quantum systems. We show that they cannot
distinguish correlations that satisfy Bell inequalities from correlations that
violate Bell inequalities, and consequently that they cannot do justice to the
challenges of explaining certain quantum correlations causally. Nonetheless, by
adapting the conceptual tools of causal inference, we can show that any attempt
to provide a causal explanation of nonsignalling correlations that violate a
Bell inequality must contradict a core principle of these algorithms, namely,
that an observed statistical independence between variables should not be
explained by fine-tuning of the causal parameters. In particular, we
demonstrate the need for such fine-tuning for most of the causal mechanisms
that have been proposed to underlie Bell correlations, including superluminal
causal influences, superdeterminism (that is, a denial of freedom of choice of
settings), and retrocausal influences which do not introduce causal cycles.Comment: 29 pages, 28 figs. New in v2: a section presenting in detail our
characterization of Bell's theorem as a contradiction arising from (i) the
framework of causal models, (ii) the principle of no fine-tuning, and (iii)
certain operational features of quantum theory; a section explaining why a
denial of hidden variables affords even fewer opportunities for causal
explanations of quantum correlation
Towards a Formulation of Quantum Theory as a Causally Neutral Theory of Bayesian Inference
Quantum theory can be viewed as a generalization of classical probability
theory, but the analogy as it has been developed so far is not complete.
Whereas the manner in which inferences are made in classical probability theory
is independent of the causal relation that holds between the conditioned
variable and the conditioning variable, in the conventional quantum formalism,
there is a significant difference between how one treats experiments involving
two systems at a single time and those involving a single system at two times.
In this article, we develop the formalism of quantum conditional states, which
provides a unified description of these two sorts of experiment. In addition,
concepts that are distinct in the conventional formalism become unified:
channels, sets of states, and positive operator valued measures are all seen to
be instances of conditional states; the action of a channel on a state,
ensemble averaging, the Born rule, the composition of channels, and
nonselective state-update rules are all seen to be instances of belief
propagation. Using a quantum generalization of Bayes' theorem and the
associated notion of Bayesian conditioning, we also show that the remote
steering of quantum states can be described within our formalism as a mere
updating of beliefs about one system given new information about another, and
retrodictive inferences can be expressed using the same belief propagation rule
as is used for predictive inferences. Finally, we show that previous arguments
for interpreting the projection postulate as a quantum generalization of
Bayesian conditioning are based on a misleading analogy and that it is best
understood as a combination of belief propagation (corresponding to the
nonselective state-update map) and conditioning on the measurement outcome.Comment: v1 43 pages, revTeX4. v2 42 pages, edited for clarity, added
references and corrected minor errors, submitted to Phys. Rev. A. v3 41
pages, improved figures, added two new figures, added extra explanation in
response to referee comments, minor rewrites for readability. v4 44 pages,
added "towards" to title, rewritten abstract, rewritten introduction with new
table
Self-directed growth of AlGaAs core-shell nanowires for visible light applications
Al(0.37)Ga(0.63)As nanowires (NWs) were grown in a molecular beam epitaxy
system on GaAs(111)B substrates. Micro-photoluminescence measurements and
energy dispersive X-ray spectroscopy indicated a core-shell structure and Al
composition gradient along the NW axis, producing a potential minimum for
carrier confinement. The core-shell structure formed during the growth as a
consequence of the different Al and Ga adatom diffusion lengths.Comment: 20 pages, 7 figure
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