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Pablo L. Saldanha

Publications and source records attributed to Pablo L. Saldanha.

At least 37 records · Page 2Linked to original sources
arXiv2019

Effective electrostatic attraction between electrons due to quantum interference

We show how, with the use of quantum interference, we can violate, in some sense, the rule that charges of equal sign always repel each other. By considering two electrons that propagate parallel to each other in a Mach-Zehnder interferometer, we show that the quantum superposition of the electrostatic repulsion when the electrons propagate in the same path with the absence of interaction when they propagate in opposite paths may result in an effective attraction between them, when we post-select by which port each electron leaves the interferometer. We also discuss an experimental setup that could be used to verify such an effect.

quant-ph↗
arXiv2018

Quantum Interference of Force

We show that a quantum particle subjected to a positive force in one path of a Mach-Zehnder interferometer and a null force in the other path may receive a negative average momentum transfer when it leaves the interferometer by a particular exit. In this scenario, an ensemble of particles may receive an average momentum in the opposite direction of the applied force due to quantum interference, a behavior with no classical analogue. We discuss some experimental schemes that could verify the effect with current technology, with electrons or neutrons in Mach-Zehnder interferometers in free space and with atoms from a Bose-Einstein condensate.

quant-ph↗
arXiv2018

Hidden momentum and the Abraham-Minkowski debate

We use an extended version of electrodynamics, which admits the existence of magnetic charges and currents, to discuss how different models for electric and magnetic dipoles do or do not carry hidden momentum under the influence of external electromagnetic fields. Based on that, we discuss how the models adopted for the electric and magnetic dipoles from the particles that compose a material medium influence the expression for the electromagnetic part of the light momentum in the medium. We show that Abraham expression is compatible with electric dipoles formed by electric charges and magnetic dipoles formed by magnetic charges, while Minkowski expression is compatible with electric dipoles formed by magnetic currents and magnetic dipoles formed by electric currents. The expression $\varepsilon_0\mathbf{E}\times\mathbf{B}$, on the other hand, is shown to be compatible with electric dipoles formed by electric charges and magnetic dipoles formed by electric currents, which are much more natural models. So this expression has an interesting interpretation in the Abraham-Minkowski debate about the momentum of light in a medium: It is the expression compatible with the nonexistence of magnetic charges. We also provide a simple justification of why Abraham and Minkowski momenta can be associated with the kinetic and canonical momentum of light, respectively.

physics.class-ph↗
arXiv2017

Reply to Comment on "Wigner rotations and an apparent paradox in relativistic quantum information"

As Lanzagorta and Crowder have shown in their Comment [Phys. Rev. A 96, 026101 (2017)], the linear application of the Wigner rotations to the quantum state of two massive relativistic particles does not entail the instantaneous transmission of information as we concluded in our paper [Phys. Rev. A 87, 042102 (2013)]. But in the new version of our paper in arXiv [arXiv:1303.4367v2] we show (and solve) another paradox that is generated by the linear application of the Wigner rotations to a system with a single relativistic particle. So the conclusion of our paper that we cannot in general linearly apply the Wigner rotations to the quantum state of a relativistic particle without considering the appropriate physical interpretation is still valid, although the paradox presented in that paper is inappropriate.

quant-ph↗
arXiv2017

Wigner rotations and an apparent paradox in relativistic quantum information

It is shown that a general model for particle detection in combination with a linear application of the Wigner rotations, which correspond to momentum-dependent changes of the particle spin under Lorentz transformations, to the state of a massive relativistic particle in a superposition of two counter-propagating momentum states leads to a paradox. The paradoxical behavior is that the probability of finding the particle at different positions would depend on the reference frame. A solution to the paradox is given when the physical construction of the corresponding state is taken into account, suggesting that we cannot in general linearly apply the Wigner rotations to a quantum state without considering the appropriate physical interpretation.

quant-ph↗
arXiv2016

Alternative expression for the electromagnetic Lagrangian

We reintroduce an alternative expression for the Lagrangian density that governs the interaction of a charged particle with external electromagnetic fields, proposed by Livens about one century ago. This Lagrangian is written in terms of the local superposition of the particle fields with the applied electromagnetic fields, not in terms of the particle charge and of the electromagnetic potentials as is usual. Here we show that the total Lagrangian for a set of charged particles assumes a simple elegant form with the alternative formulation, giving an aesthetic support for it. We also show that the alternative Lagrangian is equivalent to the traditional one in their domain of validity and that it provides an interesting description of the Aharonov-Bohm effect.

quant-ph↗
arXiv2016

Photon reflection by a quantum mirror: a wave function approach

We derive from first principles the momentum exchange between a photon and a quantum mirror upon reflection, by considering the boundary conditions imposed by the mirror surface on the photon wave equation. We show that the system generally ends up in an entangled state, unless the mirror position uncertainty is much smaller than the photon wavelength, when the mirror behaves classically. Our treatment leads us directly to the conclusion that the photon momentum has the known value hk/2π. This implies that when the mirror is immersed in a dielectric medium the photon radiation pressure is proportional to the medium refractive index n. Our work thus contributes to the longstanding Abraham-Minkowski debate about the momentum of light in a medium. We interpret the result by associating the Minkowski momentum (which is proportional to n) with the canonical momentum of light, which appears naturally in quantum formulations.

quant-ph↗
arXiv2015

Hidden momentum in a hydrogen atom and the Lorentz force law

By using perturbation theory, we show that a hydrogen atom with magnetic moment due to the orbital angular momentum of the electron has "hidden momentum" in the presence of an external electric field. This means that the atomic electronic cloud has a nonzero linear momentum in its center-of-mass rest frame due to a relativistic effect. This is completely analogous to the hidden momentum that a classical current loop has in the presence of an external electric field. We discuss that this effect is essential for the validity of the Lorentz force law in quantum systems. We also connect our results to the long-standing Abraham-Minkowski debate about the momentum of light in material media.

quant-ph↗
arXiv2015

'Quantum Cheshire Cat' as Simple Quantum Interference

In a recent work, Aharonov et al. suggested that a photon could be separated from its polarization in an experiment involving pre- and post-selection [New J. Phys 15, 113015 (2013)]. They named the effect 'quantum Cheshire Cat', in a reference to the cat that is separated from its grin in the novel Alice's Adventures in Wonderland. Following these ideas, Denkmayr et al. performed a neutron interferometric experiment and interpreted the results suggesting that neutrons were separated from their spin [Nat. Commun. 5, 4492 (2014)]. Here we show that these results can be interpreted as simple quantum interference, with no separation between the quantum particle and its internal degree of freedom. We thus hope to clarify the phenomenon with this work, by removing these apparent paradoxes.

quant-ph↗
arXiv2014

Single-Photon Superradiance in Cold Atoms

The interaction of an ensemble of atoms with common vacuum modes may lead to an enhanced emission into these modes. This phenomenon, known as superradiance, highlights the coherent nature of spontaneous emission, resulting in macroscopic entangled states in mundane situations. The complexity of the typical observations of superradiance, however, masks its quantum nature, allowing alternative classical interpretations. Here we stress how this picture changed with the implementation ten years ago of a new process for single-photon generation from atomic ensembles. We present then the last piece of evidence for the superradiant nature of such process, reporting the observation of an accelerated emission of the photon with a rate that may be tuned by controllably changing the number of atoms in the ensemble. Such investigation paves the way to a new, bottom-up approach to the study of superradiance.

quant-ph↗
arXiv2014

Interpreting a nested Mach-Zehnder interferometer with classical optics

In an recent work with the title "Asking Photons Where They Have Been", Danan et al. experimentally demonstrate an intriguing behavior of photons in an interferometer [Phys. Rev. Lett. 111, 240402 (2013)]. In their words: "The photons tell us that they have been in the parts of the interferometer through which they could not pass." They interpret the results using the two-state vector formalism of quantum theory and say that, although an explanation of the experimental results in terms of classical electromagnetic waves in the interferometer is possible (and they provide a partial description), it is not so intuitive. Here we present a more detailed classical description of their experimental results, showing that it is actually intuitive. The same description is valid for the quantum wave function of the photons propagating in the interferometer. In particular, we show that it is essential that the wave propagates through all parts of the interferometer to describe the experimental results. We hope that our work helps to give a deeper understanding of these interesting experimental results.

quant-ph↗
arXiv2013

Quantum analysis of the direct measurement of light waves

In a beautiful experiment performed about a decade ago, Goulielmakis et al. made a direct measurement of the electric field of light waves [E. Goulielmakis et al., Science 305, 1267-1269 (2004)]. However, they used a laser source to produce the light field, whose quantum state has a null expectation value for the electric field operator, so how was it possible to measure this electric field? Here we present a quantum treatment for the f:2f interferometer used to calibrate the carrier-envelope phase of the light pulses in the experiment. We show how the special nonlinear features of the f:2f interferometer can change the quantum state of the electromagnetic field inside the laser cavity to a state with a definite oscillating electric field, explaining how the "classical" electromagnetic field emerges in the experiment. We discuss that this experiment was, to our knowledge, the first demonstration of an absolute coherent superposition of different photon number states in the optical regime.

quant-ph↗
arXiv2013

Dynamics of the reading process of a quantum memory

The mechanism of extraction of information stored in a quantum memory is studied here in detail. We consider memories containing a single excitation of a collective atomic state, which is mapped into a single photon during the reading process. A theory is developed for the wavepacket of the extracted photon, leading to a simple analytical expression depending on the key parameters of the problem, like detuning and intensity of the read field and the number of atoms in the atomic ensemble. This theory is then compared to a large set of experimental situations and a satisfactory quantitative agreement is obtained. In this way, we are able to systematically study the saturation and spectrum of the reading process, as well as clarify the role of superradiance in the system.

quant-ph↗
arXiv2013

Do entangled photons induce two-photon two-atom transitions more efficiently than other states of light ?

We study in this paper the efficiency of different two-photon states of light to induce the simultaneous excitation of two atoms of different kinds when the sum of the energies of the two photons matches the sum of the energies of the two atomic transitions, while no photons are resonant with each individual transition. We find that entangled two-photon states produced by an atomic cascade are indeed capable of enhancing by a large factor the simultaneous excitation probability as compared to uncorrelated photons, as predicted some years ago by Muthukrishnan et al, but that several non-entangled, separable, correlated states, produced either by an atomic cascade or parametric down conversion, or even appropriate combinations of coher- ent states, have comparable efficiencies. We show that the key ingredient for the increase of simultaneous excitation probability is the presence of strong frequency anti-correlation and not time correlation nor time-frequency entanglement.

quant-ph↗
arXiv2012

Energy and momentum entanglement in parametric downconversion

We present a simple treatment for the phenomenon of parametric downconversion considering the coherent scattering of one pump photon into a photon pair by a nonlinear crystal. The energy and momentum entanglement of the quantum state of the generated twin photons are seen as a consequence of the fundamental indistinguishability of the time and the position in which the photon pair is created inside the crystal. We also discuss some consequences of the system entanglement.

quant-ph↗
arXiv2012

Spin quantum correlations of relativistic particles

We show that a pair of massive relativistic spin-1/2 particles prepared in a maximally entangled spin state in general is not capable of maximally violating the Clauser-Horne-Shimony-Holt (CHSH) version of Bell's inequalities without a post-selection of the particles momenta, representing a major difference in relation to non-relativistic systems. This occurs because the quantization axis of the measurements performed on each particle depends on the particle velocity, such that it is not possible to define a reduced density matrix for the particles spin. We also show that the amount of violation of the CHSH inequality depends on the reference frame, and that in some frames the inequality may not be violated.

quant-ph↗
arXiv2012

Physical interpretation of the Wigner rotations and its implications for relativistic quantum information

We present a new treatment for the spin of a massive relativistic particle in the context of quantum information based on a physical interpretation of the Wigner rotations, obtaining different results in relation to the previous works. We are lead to the conclusions that it is not possible to define a reduced density matrix for the particle spin and that the Pauli-Lubanski (or similar) spin operators are not suitable to describe measurements where spin couples to an electromagnetic field in the measuring apparatus. These conclusions contradict the assumptions made by most of the previous papers on the subject. We also propose an experimental test of our formulation.

quant-ph↗