1,720,963 research outputs found
Quantum optomechanical piston engines powered by heat
No full textWe study two different models of optomechanical systems where a temperature
gradient between two radiation baths is exploited for inducing self-sustained
coherent oscillations of a mechanical resonator. Viewed from a thermodynamic
perspective, such systems represent quantum instances of self-contained thermal
machines converting heat into a periodic mechanical motion and thus they can be
interpreted as nano-scale analogues of macroscopic piston engines. Our models
are potentially suitable for testing fundamental aspects of quantum
thermodynamics in the laboratory and for applications in energy efficient
nanotechnology
Discriminating strength: a bona fide measure of non-classical correlations
Full text linkA new measure of non-classical correlations is introduced and characterized.
It tests the ability of using a state
ho of a composite system AB as a
probe for a quantum illumination task [e.g. see S. Lloyd, Science 321, 1463
(2008)], in which one is asked to remotely discriminate among the two following
scenarios: i) either nothing happens to the probe, or ii) the subsystem A is
transformed via a local unitary R_A whose properties are partially unspecified
when producing
ho. This new measure can be seen as the discrete version of
the recently introduced Intereferometric Power measure [G. Girolami et al.
e-print arXiv:1309.1472 (2013)] and, at least for the case in which A is a
qubit, it is shown to coincide (up to an irrelevant scaling factor) with the
Local Quantum Uncertainty measure of D. Girolami, T. Tufarelli, and G. Adesso,
Phys. Rev. Lett. 110, 240402 (2013). Analytical expressions are derived which
allow us to formally prove that, within the set of separable configurations,
the maximum value of our non-classicality measure is achieved over the set of
quantum-classical states (i.e. states
ho which admit a statistical
unravelling where each element of the associated ensemble is distinguishable
via local measures on B).A new measure of non-classical correlations is introduced and characterized. It tests the ability of using a state ρ of a composite system AB as a probe for a quantum illumination task (e.g. see Lloyd 2008 Science 321 1463), in which one is asked to remotely discriminate between the two following scenarios: (i) either nothing happens to the probe, or (ii) the subsystem A is transformed via a local unitary whose properties are partially unspecified when producing ρ. This new measure can be seen as the discrete version of the recently introduced interferometric power measure (Girolami et al 2013 e-print arXiv:1309.1472) and, at least for the case in which A is a qubit, it is shown to coincide (up to an irrelevant scaling factor) with the local quantum uncertainty measure of Girolami, Tufarelli and Adesso (2013 Phys. Rev. Lett. 110 240402). Analytical expressions are derived which allow us to formally prove that, within the set of separable configurations, the maximum value of our non-classicality measure is achieved over the set of quantum-classical states (i.e. states ρ which admit a statistical unravelling where each element of the associated ensemble is distinguishable via local measures on B)
Gaussian discriminating strength
No full textWe present a quantifier of non-classical correlations for bipartite,
multi-mode Gaussian states. It is derived from the Discriminating Strength
measure, introduced for finite dimensional systems in A. Farace et al., New. J.
Phys. 16, 073010 (2014). As the latter the new measure exploits the Quantum
Chernoff Bound to gauge the susceptibility of the composite system with respect
to local perturbations induced by unitary gates extracted from a suitable set
of allowed transformations (the latter being identified by posing some general
requirements). Closed expressions are provided for the case of two-mode
Gaussian states obtained by squeezing or by linearly mixing via a beam-splitter
a factorized two-mode thermal state. For these density matrices, we study how
non-classical correlations are related with the entanglement present in the
system and with its total photon number
Building versatile bipartite probes for quantum metrology
Full text linkWe consider bipartite systems as versatile probes for the estimation of transformations acting locally on one of the subsystems. We investigate what resources are required for the probes to offer a guaranteed level of metrological performance, when the latter is averaged over specific sets of local transformations. We quantify such a performance via the average skew information (AvSk), a convex quantity which we compute in closed form for bipartite states of arbitrary dimensions, and which is shown to be strongly dependent on the degree of local purity of the probes. Our analysis contrasts and complements the recent series of studies focused on the minimum, rather than the average, performance of bipartite probes in local estimation tasks, which was instead determined by quantum correlations other than entanglement. We provide explicit prescriptions to characterize the most reliable states maximizing the AvSk, and elucidate the role of state purity, separability and correlations in the classification of optimal probes. Our results can help in the identification of useful resources for sensing, estimation and discrimination applications when complete knowledge of the interaction mechanism realizing the local transformation is unavailable, and access to pure entangled probes is technologically limited
Mutual information as an order parameter for quantum synchronization
No full textSpontaneous synchronization is a fundamental phenomenon, important in many
theoretical studies and applications. Recently this effect has been analyzed
and observed in a number of physical systems close to the quantum mechanical
regime. In this work we propose the mutual information as a useful order
parameter which can capture the emergence of synchronization in very different
contexts, ranging from semi-classical to intrinsically quantum mechanical
systems. Specifically we first study the synchronization of two coupled Van der
Pol oscillators in both classical and quantum regimes and later we consider the
synchronization of two qubits inside two coupled optical cavities. In all these
contexts, we find that mutual information can be used as an appropriate figure
of merit for determining the synchronization phases, independently of the
specific details of the system
Steady-state entanglement activation in optomechanical cavities
No full textQuantum discord, and related indicators, are raising a relentless interest as a novel paradigm of nonclassical correlations beyond entanglement. Here, we discover a discord-activated mechanism yielding steady-state entanglement production in a realistic continuous-variable setup. This comprises two coupled optomechanical cavities, where the optical modes (OMs) communicate through a fiber. We first use a simplified model to highlight the creation of steady-state discord between the OMs. We show next that such discord improves the level of stationary optomechanical entanglement attainable in the system, making it more robust against temperature and thermal noise
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
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