3,480 research outputs found
Monsters, black holes and the statistical mechanics of gravity
We review the construction of monsters in classical general relativity.
Monsters have finite ADM mass and surface area, but potentially unbounded
entropy. From the curved space perspective they are objects with large proper
volume that can be glued on to an asymptotically flat space. At no point is the
curvature or energy density required to be large in Planck units, and quantum
gravitational effects are, in the conventional effective field theory
framework, small everywhere. Since they can have more entropy than a black hole
of equal mass, monsters are problematic for certain interpretations of black
hole entropy and the AdS/CFT duality.
In the second part of the paper we review recent developments in the
foundations of statistical mechanics which make use of properties of
high-dimensional (Hilbert) spaces. These results primarily depend on kinematics
-- essentially, the geometry of Hilbert space -- and are relatively insensitive
to dynamics. We discuss how this approach might be adopted as a basis for the
statistical mechanics of gravity. Interestingly, monsters and other highly
entropic configurations play an important role.Comment: 9 pages, 4 figures, revtex; invited Brief Review to be published in
Modern Physics Letters
Two Forms of Inconsistency in Quantum Foundations
Recently, there has been some discussion of how Dutch Book arguments might be
used to demonstrate the rational incoherence of certain hidden variable models
of quantum theory (Feintzeig and Fletcher 2017). In this paper, we argue that
the 'form of inconsistency' underlying this alleged irrationality is deeply and
comprehensively related to the more familiar 'inconsistency' phenomenon of
contextuality. Our main result is that the hierarchy of contextuality due to
Abramsky and Brandenburger (2011) corresponds to a hierarchy of
additivity/convexity-violations which yields formal Dutch Books of different
strengths. We then use this result to provide a partial assessment of whether
these formal Dutch Books can be interpreted normatively.Comment: 26 pages, 5 figure
Efficient Decomposition of Image and Mesh Graphs by Lifted Multicuts
Formulations of the Image Decomposition Problem as a Multicut Problem (MP)
w.r.t. a superpixel graph have received considerable attention. In contrast,
instances of the MP w.r.t. a pixel grid graph have received little attention,
firstly, because the MP is NP-hard and instances w.r.t. a pixel grid graph are
hard to solve in practice, and, secondly, due to the lack of long-range terms
in the objective function of the MP. We propose a generalization of the MP with
long-range terms (LMP). We design and implement two efficient algorithms
(primal feasible heuristics) for the MP and LMP which allow us to study
instances of both problems w.r.t. the pixel grid graphs of the images in the
BSDS-500 benchmark. The decompositions we obtain do not differ significantly
from the state of the art, suggesting that the LMP is a competitive formulation
of the Image Decomposition Problem. To demonstrate the generality of the LMP,
we apply it also to the Mesh Decomposition Problem posed by the Princeton
benchmark, obtaining state-of-the-art decompositions
Cluster state preparation using gates operating at arbitrary success probabilities
Several physical architectures allow for measurement-based quantum computing
using sequential preparation of cluster states by means of probabilistic
quantum gates. In such an approach, the order in which partial resources are
combined to form the final cluster state turns out to be crucially important.
We determine the influence of this classical decision process on the expected
size of the final cluster. Extending earlier work, we consider different
quantum gates operating at various probabilites of success. For finite
resources, we employ a computer algebra system to obtain the provably optimal
classical control strategy and derive symbolic results for the expected final
size of the cluster. We identify two regimes: When the success probability of
the elementary gates is high, the influence of the classical control strategy
is found to be negligible. In that case, other figures of merit become more
relevant. In contrast, for small probabilities of success, the choice of an
appropriate strategy is crucial.Comment: 7 pages, 9 figures, contribution to special issue of New J. Phys. on
"Measurement-Based Quantum Information Processing". Replaced with published
versio
An experimental test of all theories with predictive power beyond quantum theory
According to quantum theory, the outcomes of future measurements cannot (in
general) be predicted with certainty. In some cases, even with a complete
physical description of the system to be measured and the measurement
apparatus, the outcomes of certain measurements are completely random. This
raises the question, originating in the paper by Einstein, Podolsky and Rosen,
of whether quantum mechanics is the optimal way to predict measurement
outcomes. Established arguments and experimental tests exclude a few specific
alternative models. Here, we provide a complete answer to the above question,
refuting any alternative theory with significantly more predictive power than
quantum theory. More precisely, we perform various measurements on distant
entangled photons, and, under the assumption that these measurements are chosen
freely, we give an upper bound on how well any alternative theory could predict
their outcomes. In particular, in the case where quantum mechanics predicts two
equally likely outcomes, our results are incompatible with any theory in which
the probability of a prediction is increased by more than ~0.19. Hence, we can
immediately refute any already considered or yet-to-be-proposed alternative
model with more predictive power than this.Comment: 13 pages, 4 figure
Optimal quantum operations at zero energy cost
Quantum technologies are developing powerful tools to generate and manipulate
coherent superpositions of different energy levels. Envisaging a new generation
of energy-efficient quantum devices, here we explore how coherence can be
manipulated without exchanging energy with the surrounding environment. We
start from the task of converting a coherent superposition of energy
eigenstates into another. We identify the optimal energy-preserving operations,
both in the deterministic and in the probabilistic scenario. We then design a
recursive protocol, wherein a branching sequence of energy-preserving filters
increases the probability of success while reaching maximum fidelity at each
iteration. Building on the recursive protocol, we construct efficient
approximations of the optimal fidelity-probability trade-off, by taking
coherent superpositions of the different branches generated by probabilistic
filtering. The benefits of this construction are illustrated in applications to
quantum metrology, quantum cloning, coherent state amplification, and
ancilla-driven computation. Finally, we extend our results to transitions where
the input state is generally mixed and we apply our findings to the task of
purifying quantum coherence.Comment: 35 pages, 10 figures; published versio
Repeat-Until-Success quantum computing using stationary and flying qubits
We introduce an architecture for robust and scalable quantum computation
using both stationary qubits (e.g. single photon sources made out of trapped
atoms, molecules, ions, quantum dots, or defect centers in solids) and flying
qubits (e.g. photons). Our scheme solves some of the most pressing problems in
existing non-hybrid proposals, which include the difficulty of scaling
conventional stationary qubit approaches, and the lack of practical means for
storing single photons in linear optics setups. We combine elements of two
previous proposals for distributed quantum computing, namely the efficient
photon-loss tolerant build up of cluster states by Barrett and Kok [Phys. Rev.
A 71, 060310(R) (2005)] with the idea of Repeat-Until-Success (RUS) quantum
computing by Lim et al. [Phys. Rev. Lett. 95, 030505 (2005)]. This idea can be
used to perform eventually deterministic two-qubit logic gates on spatially
separated stationary qubits via photon pair measurements. Under non-ideal
conditions, where photon loss is a possibility, the resulting gates can still
be used to build graph states for one-way quantum computing. In this paper, we
describe the RUS method, present possible experimental realizations, and
analyse the generation of graph states.Comment: 14 pages, 7 figures, minor changes, references and a discussion on
the effect of photon dark counts adde
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