Person: Luis Aina, Alfredo
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First Name
Alfredo
Last Name
Luis Aina
Affiliation
Universidad Complutense de Madrid
Faculty / Institute
Ciencias Físicas
Department
Óptica
Area
Optica
Identifiers
44 results
Search Results
Now showing 1 - 10 of 44
Item Nonclassicality in weak measurements(Physical review A, 2004) Johansen, Lars M.; Luis Aina, AlfredoWe examine weak measurements of arbitrary observables where the object is prepared in a mixed state and on which measurements with imperfect detectors are made. The weak value of an observable can be expressed as a conditional expectation value over an infinite class of different generalized Kirkwood quasiprobability distributions. "Strange" weak values for which the real part exceeds the eigenvalue spectrum of the observable can only be found if the Terletsky-Margenau-Hill distribution is negative or, equivalently, if the real part of the weak value of the density operator is negative. We find that a classical model of a weak measurement exists whenever the Terletsky-Margenau-Hill representation of the observable equals the classical representation of the observable and the Terletsky-Margenau-Hill distribution is non-negative. Strange weak values alone are not sufficient to obtain a contradiction with classical models. We propose feasible weak measurements of photon number of the radiation field. Negative weak values of energy contradict all classical stochastic models, whereas negative weak values of photon number contradict all classical stochastic models where the energy is bounded from below by the zero-point energy. We examine coherent states in particular and find negative weak values with probabilities of 16% for kinetic energy (or squared field quadrature), 8% for harmonic oscillator energy, and 50% for photon number. These experiments are robust against detector inefficiency and thermal noise.Item Complementarity for generalized observables(Physical review letters, 2002) Luis Aina, AlfredoWe examine basic properties of complementarity by using the most general description of quantum observables as positive-operator measures. We show that, in general, two observables can be complementary or not depending on the measure of fluctuations adopted and that complementarity is not a symmetric relation. This occurs because the states that determine the measured statistics do not necessarily coincide with the minimum uncertainty states for the same observable. We also show that there are observables without a complementary observable and that complementarity is not preserved by the Neumark extensions.Item Degree of polarization of type-II unpolarized light(Physical review A, 2007) Luis Aina, AlfredoWe address a quantitative determination of the degree of polarization of type-II unpolarized light via the computation of the distance between the polarization distribution and the uniform distribution associated with fully unpolarized light (i.e., type-I unpolarized light or natural light). We determine the maximum degree of polarization for type-II unpolarized light and the states reaching it. We show that the degree of polarization can be arbitrarily large, approaching complete polarization for increasing mean photon numbers.Item Zeno and anti-Zeno effects in two-level systems(Physical review A, 2003) Luis Aina, AlfredoWe examine the appearance of Zeno and anti-Zeno effects when the unitary evolution of a two-level system is observed. We demonstrate that, contrary to previous claims, the anti-Zeno effect can occur in two-level systems so that the observation can enhance the transition probability. Moreover, for nonresonant couplings we show that the Zeno and anti-Zeno effects always appear sequentially in the evolution of the same system: there is Zeno effect in the short-time regime and anti-Zeno effect in the long-time regime.Item Nonclassicality of states and measurements by breaking classical bounds on statistics(Physical review A, 2009) Rivas, Ángel; Luis Aina, AlfredoWe derive exceedingly simple practical procedures revealing the quantum nature of states and measurements by the violation of classical upper bounds on the statistics of arbitrary measurements. Data analysis is minimum, and definite conclusions are obtained without evaluation of moments or any other more sophisticated procedures. These nonclassical tests are independent of other typical quantum signatures such as sub-Poissonian statistics, quadrature squeezing, or oscillatory statistics. This approach can be equally well applied to very diverse situations such as single- and two-mode fields, observables with continuous and discrete spectra, finite- and infinite-dimensional systems, and ideal and noisy measurements.Item Quantum and classical operational complementarity for single systems(Physical review A, 2005) Luis Aina, AlfredoWe investigate duality relations between conjugate observables after measurements performed on a single realization of the system. The application of standard inference methods implies the existence of duality relations for single systems when using classical as well as quantum physics.Item Quantum-state preparation and control via the Zeno effect(Physical review A, 2001) Luis Aina, AlfredoWe present different schemes for preparing and maintaining the quantum state of multilevel systems. This is achieved via a suitable combination of coherent trapping and the Zeno effect which serves to protect the prepared state against undesired jumps to other states.Item Quantum properties of exponential states(Physical review A, 2007) Luis Aina, AlfredoThe use of Rényi entropy as an uncertainty measure alternative to variance leads to the study of states with quantum fluctuations below the levels established by Gaussian states, which are the position-momentum minimum uncertainty states according to variance. We examine the quantum properties of states with exponential wave functions, which combine reduced fluctuations with practical feasibility.Item Maximum likelihood state between measurements(Physical review A, 2009) Luis Aina, AlfredoWe estimate the quantum state of a system between two measurements via a maximum likelihood strategy. This incorporates all the information provided by preselection and postselection in a legitimate quantum state. Moreover, it allows to include any other constraint derived from practical considerations.Item Phase-shift amplification for precision measurements without nonclassical states(Physical review A, 2002) Luis Aina, AlfredoWe propose a practical arrangement that allows one to reach the Heisenberg limit in precision phase-shift measurements. This is achieved via phase-shift amplification. The arrangement we propose is based on experimental processes already carried out and does not require the use of any special quantum state.