Person: Sánchez Soto, Luis Lorenzo
Universidad Complutense de Madrid
Faculty / Institute
Now showing 1 - 10 of 80
PublicationCapacitance of Josephson junctions made on bicrystalline substrates of different geometries(American Physical Society, 2005-01) Lucía Mulas, María Luisa; Sánchez Quesada, Francisco; Sánchez Soto, Luis Lorenzo; Navacerrada, M.A.The electromagnetic parameters of YBa2Cu3O7-x (YBCO) grain boundary Josephson junctions (JJs) fabricated on four different tilt bicrystal geometries: 12degrees  asymmetric, 24degrees  symmetric, 24degrees  asymmetric, and 45degrees  asymmetric, have been studied. While the Swihart velocity ((c) over bar) is slightly affected by the nature of the barrier and mainly fixed by the junction width, a notable influence of the barrier structure, of the geometry of the bicrystal substrate, on the relative dielectric constant to the barrier thickness ratio (epsilon/t) values has been found. Interesting barrier information can be deduced from the study of the dependence of the junction capacitance on the junction resistance. We have observed that the capacitance values deduced by means of Fiske steps in YBCO 24degrees  symmetric and 45degrees  asymmetric JJs scales with junction resistance in the opposite direction. This result could reveal the presence of a tunnel barrier in the YBCO 45degrees  asymmetric JJs. On the other hand, different capacitance values have been obtained by means of Fiske steps and hysteresis observed in the current-voltage characteristics in 45degrees  asymmetric JJs. An interpretation of this result can be made taking into account the contribution of the depleted YBCO layers close to the crystallographic grain boundary. PublicationDynamics of a two-level atom observed via an interaction-free measurement(American Physical Society, 1999-07) Luis Aina, Alfredo; Sánchez Soto, Luis LorenzoWe examine the evolution of a two-level atom when its population is monitored. The detection proposed is an interaction-fret measurement, so that the observation occurs without photon exchange between the atom and the apparatus. It is shown that the observed dynamics exhibits features that an isolated two-level atom cannot accommodate. Such phenomena are explained in terms of an atom-apparatus effective interaction. PublicationObjective compressive quantum process tomography(Amer Physical Soc, 2020-02-01) Teo, Yong Siah; Struchalin, G. I.; Kovlakov, E. V.; Ahn, Daekun; Jeong, Hyunseok; Straupe, S. S.; Kulik, S. P.; Leuchs, Gerd; Sánchez Soto, Luis LorenzoWe present a compressive quantum process tomography scheme that fully characterizes any rank-deficient completely positive process with no spurious a priori information. It uses randomly chosen input states and adaptive output von Neumann measurements. Both entangled and tensor-product configurations are flexibly employable in our scheme, the latter of which are especially compatible with many-body quantum computing. Two main features of this scheme are the certification protocol that verifies whether the accumulated data uniquely characterize the quantum process and a compressive reconstruction method for the output states. We emulate multipartite scenarios with high-order transverse modes and optical fibers to demonstrate that, in terms of measurement resources, our assumption-free compressive strategy can reconstruct quantum processes almost equally efficiently using all types of input states and basis measurements. PublicationVariations on the adiabatic invariance: the Lorentz pendulum(American Association of Physics Teachers, 2012-12-20) Sánchez Soto, Luis Lorenzo; Zoido Chamorro, Jesús ManuelWe analyze a very simple variant of the Lorentz pendulum, in which the length is varied exponentially, instead of uniformly, as it is assumed in the standard case. We establish quantitative criteria for the condition of adiabatic changes in both pendula and put in evidence their substantially different physical behavior with regard to adiabatic invariance. PublicationCoarse graining the phase space of N qubits(Amer Physical Soc, 2017-02-28) Di Matteo, Olivia; Sánchez Soto, Luis Lorenzo; Leuchs, Gerd; Grassl, M.We develop a systematic coarse-graining procedure for systems of N qubits. We exploit the underlying geometrical structures of the associated discrete phase space to produce a coarse-grained version with reduced effective size. Our coarse-grained spaces inherit key properties of the original ones. In particular, our procedure naturally yields a subset of the original measurement operators, which can be used to construct a coarse discrete Wigner function. These operators also constitute a systematic choice of incomplete measurements for the tomographer wishing to probe an intractably large system. PublicationComplete characterization of arbitrary quantum measurement processes(American Physical Society, 1999-11-01) Luis Aina, Alfredo; Sánchez Soto, Luis LorenzoWe examine two simple and feasible practical schemes allowing the complete determination of any quantum measuring arrangement. This is illustrated with the example of parity measurement. PublicationLeast-bias state estimation with incomplete unbiased measurements(American Physical Society, 2015-11) Řeháček, Jaroslav; Hradil, Zdenek; Teo, Yong Siah; Sánchez Soto, Luis Lorenzo; Ng, Hui Khoon; Chai, Jing Hao; Englert, Berthold-GeorgMeasuring incomplete sets of mutually unbiased bases constitutes a sensible approach to the tomography of high-dimensional quantum systems. The unbiased nature of these bases optimizes the uncertainty hypervolume. However, imposing unbiasedness on the probabilities for the unmeasured bases does not generally yield the estimator with the largest von Neumann entropy, a popular figure of merit in this context. Furthermore, this imposition typically leads to mock density matrices that are not even positive definite. This provides a strong argument against perfunctory applications of linear estimation strategies. We propose to use instead the physical state estimators that maximize the Shannon entropy of the unmeasured outcomes, which quantifies our lack of knowledge fittingly and gives physically meaningful statistical predictions. PublicationPhysical mechanisms underpinning the vacuum permittivity(MDPI, 2023-03) Leuchs, Gerd; Hawton, Margaret; Sánchez Soto, Luis LorenzoThe debate about the emptiness of space goes back to the prehistory of science and is epitomized by the Aristotelian 'horror vacui', which can be seen as the precursor of the ether, whose modern version is the dynamical quantum vacuum. In this paper, we suggest to change a common view to 'gaudium vacui' and discuss how the vacuum fluctuations fix the value of the permittivity, e(0), and permeability, mu(0), by modelling their dynamical response by three-dimensional harmonic oscillators. PublicationAnticaustics in a Fabry-Perot interferometer(Optical Society of America, 2022-12-01) Sánchez Soto, Luis Lorenzo; Monzón Serrano, Juan José; Leuchs, GerdWe address the response of a Fabry-Perot interferometer to a monochromatic point source. We calculate the anti -caustics (that is, the virtual wavefronts of null path difference) resulting from the successive internal reflections occurring in the system. They turn out to be a family of ellipsoids (or hyperboloids) of revolution, which allows us to reinterpret the operation of the Fabry-Perot interferometer from a geometrical point of view that facilitates comparison with other apparently disparate arrangements, such as Young's double slit. PublicationRadial quantum number of Laguerre-Gauss modes(American Physical Society, 2014-06-16) Karimi, E.; Boyd, R.W.; Hoz Iglesias, Pablo de la; de Guise, H.; Řeháček, J.; Hradil, Z.; Aiello, A.; Leuchs, Gerd; Sánchez Soto, Luis LorenzoWe introduce an operator linked with the radial index in the Laguerre-Gauss modes of a two-dimensional harmonic oscillator in cylindrical coordinates. We discuss ladder operators for this variable, and confirm that they obey the commutation relations of the su(1,1) algebra. Using this fact, we examine how basic quantum optical concepts can be recast in terms of radial modes.