1,187 research outputs found
Autonomous quantum thermal machine for generating steady-state entanglement
We discuss a simple quantum thermal machine for the generation of
steady-state entanglement between two interacting qubits. The machine is
autonomous in the sense that it uses only incoherent interactions with thermal
baths, but no source of coherence or external control. By weakly coupling the
qubits to thermal baths at different temperatures, inducing a heat current
through the system, steady-state entanglement is generated far from thermal
equilibrium. Finally, we discuss two possible implementations, using
superconducting flux qubits or a semiconductor double quantum dot. Experimental
prospects for steady-state entanglement are promising in both systems.Comment: 14 pages, 4 figure
Quantifying photonic high-dimensional entanglement
High-dimensional entanglement offers promising perspectives in quantum
information science. In practice, however, the main challenge is to devise
efficient methods to characterize high-dimensional entanglement, based on the
available experimental data which is usually rather limited. Here we report the
characterization and certification of high-dimensional entanglement in photon
pairs, encoded in temporal modes. Building upon recently developed theoretical
methods, we certify an entanglement of formation of 2.09(7) ebits in a time-bin
implementation, and 4.1(1) ebits in an energy-time implementation. These
results are based on very limited sets of local measurements, which illustrates
the practical relevance of these methods.Comment: 5 pages, 3 figure
Entanglement enhances cooling in microscopic quantum fridges
Small self-contained quantum thermal machines function without external
source of work or control, but using only incoherent interactions with thermal
baths. Here we investigate the role of entanglement in a small self-contained
quantum refrigerator. We first show that entanglement is detrimental as far as
efficiency is concerned---fridges operating at efficiencies close to the Carnot
limit do not feature any entanglement. Moving away from the Carnot regime, we
show that entanglement can enhance cooling and energy transport. Hence a truly
quantum refrigerator can outperform a classical one. Furthermore, the amount of
entanglement alone quantifies the enhancement in cooling.Comment: 7 pages, 3 figure
Unifying paradigms of quantum refrigeration: fundamental limits of cooling and associated work costs
In classical thermodynamics the work cost of control can typically be
neglected. On the contrary, in quantum thermodynamics the cost of control
constitutes a fundamental contribution to the total work cost. Here, focusing
on quantum refrigeration, we investigate how the level of control determines
the fundamental limits to cooling and how much work is expended in the
corresponding process. \jona{We compare two extremal levels of control. First
coherent operations, where the entropy of the resource is left unchanged, and
second incoherent operations, where only energy at maximum entropy (i.e. heat)
is extracted from the resource. For minimal machines, we find that the lowest
achievable temperature and associated work cost depend strongly on the type of
control, in both single-cycle and asymptotic regimes. We also extend our
analysis to general machines.} Our work provides a unified picture of the
different approaches to quantum refrigeration developed in the literature,
including algorithmic cooling, autonomous quantum refrigerators, and the
resource theory of quantum thermodynamics.Comment: 17 + 28 pages, 10 figure
Signal calibration for an electrical impedance mammography system
Electrical Impedance Tomography (EIT) technology has been applied clinically since the 1980s. Numerous papers have addressed a variety of systematic error sources and indicated different calibration methods. The Sussex Mk4 Electrical Impedance Mammography (EIM) system has been developed for the investigation of early stage breast lesions. Investigations have shown that the system performance is subjected to a number of systematic errors: frequencies-dependant noise level due to both internal and external sources; stray capacitance within both PCB tracks and cable connections; and artefacts generated by patient movement during scanning etc. This paper reports upon several traditional and novel calibration methods utilized to reduce some of these errors in the acquired signals before image reconstruction. Techniques used include frequency spectrum analysis, filtering, phase calibration and other means of noise reduction. Results of both before and after calibration are presented and analyzed. The conclusion is reached that the signal quality of the Sussex Mk4 EIM system is such that the system is, post-calibrated, capable of producing images for the diagnosis of breast cancer
Extractable Work from Correlations
Work and quantum correlations are two fundamental resources in thermodynamics
and quantum information theory. In this work we study how to use correlations
among quantum systems to optimally store work. We analyse this question for
isolated quantum ensembles, where the work can be naturally divided into two
contributions: a local contribution from each system, and a global contribution
originating from correlations among systems. We focus on the latter and
consider quantum systems which are locally thermal, thus from which any
extractable work can only come from correlations. We compute the maximum
extractable work for general entangled states, separable states, and states
with fixed entropy. Our results show that while entanglement gives an advantage
for small quantum ensembles, this gain vanishes for a large number of systems.Comment: 5+6 pages; 1 figure. Some minor changes, close to published versio
Thermodynamic cost of creating correlations
We investigate the fundamental limitations imposed by thermodynamics for
creating correlations. Considering a collection of initially uncorrelated
thermal quantum systems, we ask how much classical and quantum correlations can
be obtained via a cyclic Hamiltonian process. We derive bounds on both the
mutual information and entanglement of formation, as a function of the
temperature of the systems and the available energy. While for a finite number
of systems there is a maximal temperature allowing for the creation of
entanglement, we show that genuine multipartite entanglement---the strongest
form of entanglement in multipartite systems---can be created at any
temperature when sufficiently many systems are considered. This approach may
find applications, e.g. in quantum information processing, for physical
platforms in which thermodynamic considerations cannot be ignored.Comment: 17 pages, 3 figures, substantially rewritten with some new result
On the contribution of the electromagnetic dipole operator to the decay amplitude
We construct a factorization theorem that allows to systematically include
QCD corrections to the contribution of the electromagnetic dipole operator in
the effective weak Hamiltonian to the decay
amplitude. We first rederive the known result for the leading-order QED box
diagram, which features a double-logarithmic enhancement associated to the
different rapidities of the light quark in the meson and the
energetic muons in the final state. We provide a detailed analysis of the
cancellation of the related endpoint divergences appearing in individual
momentum regions, and show how the rapidity logarithms can be isolated by
suitable subtractions applied to the corresponding bare factorization theorem.
This allows us to include in a straightforward manner the QCD corrections
arising from the renormalization-group running of the hard matching coefficient
of the electromagnetic dipole operator in soft-collinear effective theory, the
hard-collinear scattering kernel, and the -meson distribution amplitude.
Focusing on the contribution from the double endpoint logarithms, we derive a
compact formula that resums the leading-logarithmic QCD corrections.Comment: 33 pages, 3 figure
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