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Luis A.,Complutense University of Madrid | Sanz T.S.,Institute Fisica Fundamental IFF CSIC
Annals of Physics | Year: 2015

Weak-measurement-based experiments (Kocsis etal., 2011) have shown that, at least for pure states, the average evolution of independent photons in Young's two-slit experiment is in compliance with the trajectories prescribed by the Bohmian formulation of quantum mechanics. But, what happens if the same experiment is repeated assuming that the wave function associated with each particle is different, i.e., in the case of mixed (incoherent) states? This question is investigated here by means of two alternative numerical simulations of Young's experiment, purposely devised to be easily implemented and tested in the laboratory. Contrary to what could be expected a priori, it is found that even for conditions of maximal mixedness or incoherence (total lack of interference fringes), experimental data will render a puzzling and challenging outcome: the average particle trajectories will still display features analogous to those for pure states, i.e., independently of how mixedness arises, the associated dynamics is influenced by both slits at the same time. Physically this simply means that weak measurements are not able to discriminate how mixedness arises in the experiment, since they only provide information about the averaged system dynamics. © 2015 Elsevier Inc.

Luis A.,Complutense University of Madrid | Sanz A.S.,Institute Fisica Fundamental IFF CSIC | Sanz A.S.,University College London
Physical Review A - Atomic, Molecular, and Optical Physics | Year: 2013

The question of the representation of quantum stationary partially polarized waves as random superpositions of different polarization ellipses is addressed. To this end, the Bohmian formulation of quantum mechanics is considered and extended to quantum optical polarization. As is shown, this approach properly combines definite time-evolving trajectories with rigorous stationary quantum distributions via the topology displayed by the associated phase field. © 2013 American Physical Society.

Peropadre B.,Institute Fisica Fundamental IFF CSIC | Zueco D.,University of Zaragoza | Porras D.,Complutense University of Madrid | Porras D.,University of Sussex | Garcia-Ripoll J.J.,Institute Fisica Fundamental IFF CSIC
Physical Review Letters | Year: 2013

The time and space resolved dynamics of a qubit with an Ohmic coupling to propagating 1D photons is studied, from weak coupling to the ultrastrong coupling regime. A nonperturbative study based on matrix product states shows the following results, (i) The ground state of the combined systems contains excitations of both the qubit and the surrounding bosonic field. (ii) An initially excited qubit equilibrates through spontaneous emission to a state, which under certain conditions is locally close to that ground state, both in the qubit and the field. (iii) The resonances of the combined qubit-photon system match those of the spontaneous emission process and also the predictions of the adiabatic renormalization [A. J. Leggett et al., Rev. Mod. Phys. 59, 1 (1987)]. Finally, nonperturbative ab initio calculations show that this physics can be studied using a flux qubit galvanically coupled to a superconducting transmission line. © 2013 American Physical Society.

Luis A.,Complutense University of Madrid | Sanz A.S.,Institute Fisica Fundamental IFF CSIC
Physical Review A - Atomic, Molecular, and Optical Physics | Year: 2015

Quantum polarization is investigated by means of a trajectory picture based on the Bohmian formulation of quantum mechanics. Relevant examples of classical-like two-mode field states are thus examined, namely, Glauber and SU(2) coherent states. Although these states are often regarded as classical, the analysis here shows that the corresponding electric-field polarization trajectories display topologies very different from those expected from classical electrodynamics. Rather than incompatibility with the usual classical model, this result demonstrates the dynamical richness of quantum motions, determined by local variations of the system quantum phase in the corresponding (polarization) configuration space, absent in classical-like models. These variations can be related to the evolution in time of the phase, but also to its dependence on configurational coordinates, which is the crucial factor to generate motion in the case of stationary states like those considered here. In this regard, for completeness these results are compared with those obtained from nonclassical NOON states. ©2015 American Physical Society.

Sanz A.S.,Institute Fisica Fundamental IFF CSIC | Davidovic M.,University of Belgrade | Bozic M.,University of Belgrade
Annals of Physics | Year: 2015

Atomic three-grating Mach-Zehnder interferometry constitutes an important tool to probe fundamental aspects of the quantum theory. There is, however, a remarkable gap in the literature between the oversimplified models and robust numerical simulations considered to describe the corresponding experiments. Consequently, the former usually lead to paradoxical scenarios, such as the wave-particle dual behavior of atoms, while the latter make difficult the data analysis in simple terms. Here these issues are tackled by means of a simple grating working model consisting of evenly-spaced Gaussian slits. As is shown, this model suffices to explore and explain such experiments both analytically and numerically, giving a good account of the full atomic journey inside the interferometer, and hence contributing to make less mystic the physics involved. More specifically, it provides a clear and unambiguous picture of the wavefront splitting that takes place inside the interferometer, illustrating how the momentum along each emerging diffraction order is well defined even though the wave function itself still displays a rather complex shape. To this end, the local transverse momentum is also introduced in this context as a reliable analytical tool. The splitting, apart from being a key issue to understand atomic Mach-Zehnder interferometry, also demonstrates at a fundamental level how wave and particle aspects are always present in the experiment, without incurring in any contradiction or interpretive paradox. On the other hand, at a practical level, the generality and versatility of the model and methodology presented, makes them suitable to attack analogous problems in a simple manner after a convenient tuning. © 2014 Elsevier Inc.

Diaz-Camacho G.,Institute Fisica Fundamental IFF CSIC | Porras D.,University of Sussex | Garcia-Ripoll J.J.,Institute Fisica Fundamental IFF CSIC
Physical Review A - Atomic, Molecular, and Optical Physics | Year: 2015

In this work we study numerically and analytically the interaction of two qubits in a one-dimensional waveguide, as mediated by the photons that propagate through the guide. We develop strategies to assert the Markovianity of the problem, the effective qubit-qubit interactions, and their individual and collective spontaneous emission. We prove the existence of collective Lamb shifts that affect the qubit-qubit interactions and the dependency of coherent and incoherent interactions on the qubit separation. We also develop the scattering theory associated with these models and prove single-photon spectroscopy does probe the renormalized resonances of the single- and multiqubit models, in sharp contrast to earlier toy models in which individual and collective Lamb shifts cancel. © 2015 American Physical Society.

Sanz A.S.,Institute Fisica Fundamental IFF CSIC
Foundations of Physics | Year: 2015

Bohmian mechanics, a hydrodynamic formulation of the quantum theory, constitutes a useful tool to understand the role of the phase as the mechanism responsible for the dynamical evolution displayed by quantum systems. This role is analyzed and discussed here in the context of quantum interference, considering to this end two well-known scenarios, namely Young’s two-slit experiment and Wheeler’s delayed choice experiment. A numerical implementation of the first scenario is used to show how interference in a coherent superposition of two counter-propagating wave packets can be seen and explained in terms of an effective model consisting of a single wave packet scattered off an attractive hard wall. The outcomes from this model are then applied to the analysis of Wheeler’s delayed choice experiment, also recreated by means of a reliable realistic simulation. Both examples illustrate quite well how the Bohmian formulation helps to explain in a natural way (and therefore to demystify) aspects of the quantum theory typically regarded as paradoxical. In other words, they show that a proper understanding of quantum phase dynamics immediately removes any trace of unnecessary artificial wave-particle arguments. © 2015, Springer Science+Business Media New York.

Sanz A.S.,Institute Fisica Fundamental IFF CSIC | Sanz A.S.,University College London | Miret-Artes S.,Institute Fisica Fundamental IFF CSIC
Annals of Physics | Year: 2013

In its standard formulation, quantum mechanics presents a very serious inconvenience: given a quantum system, there is no possibility at all to unambiguously (causally) connect a particular feature of its final state with some specific section of its initial state. This constitutes a practical limitation, for example, in numerical analyses of quantum systems, which often make necessary the use of some extra assistance from classical methodologies. Here it is shown how the Bohmian formulation of quantum mechanics removes the ambiguity of quantum mechanics, providing a consistent and clear answer to such a question without abandoning the quantum framework. More specifically, this formulation allows to define probability tubes, along which the enclosed probability keeps constant in time all the way through as the system evolves in configuration space. These tubes have the interesting property that once their boundary is defined at a given time, they are uniquely defined at any time. As a consequence, it is possible to determine final restricted (or partial) probabilities directly from localized sets of (Bohmian) initial conditions on the system initial state. Here, these facts are illustrated by means of two simple yet physically insightful numerical examples: tunneling transmission and grating diffraction. © 2013 Elsevier Inc.

Sanz A.S.,Institute Fisica Fundamental IFF CSIC
Journal of Physics: Conference Series | Year: 2014

Mermin's "shut up and calculate!" somehow summarizes the most widely accepted view on quantum mechanics. This conception has led to a rather constraining way to think and understand the quantum world. Nonetheless, a closer look at the principles and formal body of this theory shows that, beyond longstanding prejudices, there is still room enough for alternative tools. This is the case, for example, of Bohmian mechanics. As it is discussed here, there is nothing contradictory or wrong with this hydrodynamical representation, which enhances the dynamical role of the quantum phase to the detriment (to some extent) of the probability density. The possibility to describe the evolution of quantum systems in terms of trajectories or streamlines is just a direct consequence of the fact that Bohmian mechanics (quantum hydrodynamics) is just a way to recast quantum mechanics in the more general language of the theory of characteristics. Misconceptions concerning Bohmian mechanics typically come from the fact that many times it is taken out of context and considered as an alternative theory to quantum mechanics, which is not the case. On the contrary, an appropriate contextualization shows that Bohmian mechanics constitutes a serious and useful representation of quantum mechanics, at the same level as any other quantum picture, such as Schrödinger's, Heisenberg's, Dirac's, or Feynman's, for instance. To illustrate its versatility, two phenomena will be briefly considered, namely dissipation and light interference. © Published under licence by IOP Publishing Ltd.

Valdes A.,Institute Fisica Fundamental IFF CSIC | Kroes G.-J.,Leiden University
Journal of Physical Chemistry C | Year: 2012

We report 10-dimensional quantum dynamic calculations of the translation-rotation energy levels of two hydrogen molecules inside the cages of the structure H clathrate hydrate. For the small and medium cages, positive ground state energies are reported supporting their single occupancy, while for the large cage negative energy levels are calculated and some low-lying states are characterized for two para-H 2, ortho-H 2, para-D 2, ortho-D 2, and HD molecules inside the cage. The calculations are performed using the block improved relaxation method implemented in the multiconfiguration time-dependent Hartree (MCTDH) code. © 2012 American Chemical Society.

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