8,874 research outputs found
Optimal phase estimation for qubit mixed states
We address the problem of optimal estimation of the relative phase for
two-dimensional quantum systems in mixed states. In particular, we derive the
optimal measurement procedures for an arbitrary number of qubits prepared in
the same mixed state.Comment: revised version accepted for publicatio
Extremal covariant measurements
We characterize the extremal points of the convex set of quantum measurements
that are covariant under a finite-dimensional projective representation of a
compact group, with action of the group on the measurement probability space
which is generally non-transitive. In this case the POVM density is made of
multiple orbits of positive operators, and, in the case of extremal
measurements, we provide a bound for the number of orbits and for the rank of
POVM elements. Two relevant applications are considered, concerning state
discrimination with mutually unbiased bases and the maximization of the mutual
information.Comment: 11 pages, no figure
Metastability and anomalous fixation in evolutionary games on scale-free networks
We study the influence of complex graphs on the metastability and fixation
properties of a set of evolutionary processes. In the framework of evolutionary
game theory, where the fitness and selection are frequency-dependent and vary
with the population composition, we analyze the dynamics of snowdrift games
(characterized by a metastable coexistence state) on scale-free networks. Using
an effective diffusion theory in the weak selection limit, we demonstrate how
the scale-free structure affects the system's metastable state and leads to
anomalous fixation. In particular, we analytically and numerically show that
the probability and mean time of fixation are characterized by stretched
exponential behaviors with exponents depending on the network's degree
distribution.Comment: 5 pages, 4 figures, to appear in Physical Review Letter
Flow boiling of carbon dioxide: Heat transfer for smooth and enhanced geometries and effect of oil. state of the art review
This paper presents a state-of-the-art review on flow boiling of carbon dioxide, including experimen- tal studies and correlations for smooth and enhanced tubes, with pure CO 2 and CO 2 /lubricant mixtures. Specifically, 5223 CO 2 heat transfer coefficient data in smooth tubes are collected, and the effect of the operating conditions is discussed. Additional 883 data points in microfin tubes and 1184 experimental heat transfer coefficients in smooth tubes with CO 2 /oil mixture are also collected, and the influence of the microfin structure and of the oil presence on the heat transfer mechanism is analyzed. The statistical analysis has highlighted that the CO 2 -based correlation of Fang et al. is very accurate ( MAE = 5.1%) for the smooth tube database, whereas the heat transfer coefficients in microfin tubes are satisfactorily predicted ( MAE = 30.5%) with the model of Mehendale. Among the available cor- relations for CO 2 /oil mixture in smooth tubes, the method of Gao et al. provides the highest accuracy ( MAE = 63.2%)
Flow boiling of R452A: Heat transfer data, dry-out characteristics and a correlation
This paper presents an experimental investigation on two-phase heat transfer and dry-out occurrence for refrigerant R452A in a single horizontal circular stainless-steel tube having an internal diameter of 6.0 mm. The effects of mass flux (from 150 to 600 kg/m2s), saturation (bubble) temperature (from 23 to 55 °C) and heat flux (from 10 to 65 kW/m2) are investigated and discussed. Heat transfer coefficient and dry-out vapor quality data are then compared to R404A results in the same operating conditions, observing that the nucleate boiling contribution of the new blend is penalized by its very high temperature glide during evaporation. The assessment of some dry-out and flow boiling heat transfer coefficient prediction methods is finally carried-out and a correction factor on the nucleate boiling term is proposed to take into account the negative effect of the temperature glide difference on the mass diffusion in the liquid. By implementing this modification on two chosen asymptotic models, the statistical error analysis is considerably improved
How continuous quantum measurements in finite dimension are actually discrete
We show that in finite dimension a quantum measurement with continuous set of
outcomes is always equivalent to a continuous random choice of measurements
with only finite outcomes.Comment: 4 pages, 1 figur
Adiabatic frictional pressure gradient during flow boiling of pure refrigerant R1233zd and non-azeotropic mixtures R448A, R452A and R455A
The research on two-phase flow characteristics of refrigerants is of primary importance in several fields, such as air conditioning and refrigeration systems. Therefore, the determination of the pressure drop during flow boiling is important for the correct design of evaporators and heat spreaders systems. This paper presents a collection of experiments on flow boiling pressure drop using pure refrigerant R1233zd and new low-GWP refrigerant mixtures R448A, R452A and R455A. All tests were performed in adiabatic conditions, in a smooth horizontal stainless-steel tube having an internal diameter of 6.0 mm and a thickness of 1.0 mm. The effect of operating parameters, such as (bubble) saturation temperature (from 25 to 65 °C) and mass flux (from 150 to 600 kg/m2s) is investigated and discussed, and the performance of the chosen fluids is also compared. Finally, an assessment of existing prediction methods is carried-out to find the most suitable correlations for two-phase pressure drop evaluation
A simple trapped-ion architecture for high-fidelity Toffoli gates
We discuss a simple architecture for a quantum Toffoli gate implemented using
three trapped ions. The gate, which in principle can be implemented with a
single laser-induced operation, is effective under rather general conditions
and is strikingly robust (within any experimentally realistic range of values)
against dephasing, heating and random fluctuations of the Hamiltonian
parameters. We provide a full characterization of the unitary and
noise-affected gate using three-qubit quantum process tomography
Geometry of Empty Space is the Key to Near-Arrest Dynamics
We study several examples of kinetically constrained lattice models using
dynamically accessible volume as an order parameter. Thereby we identify two
distinct regimes exhibiting dynamical slowing, with a sharp threshold between
them. These regimes are identified both by a new response function in
dynamically available volume, as well as directly in the dynamics. Results for
the selfdiffusion constant in terms of the connected hole density are
presented, and some evidence is given for scaling in the limit of dynamical
arrest.Comment: 11 page
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