arXiv subjects

Pablo L. Saldanha

Publications and source records attributed to Pablo L. Saldanha.

At least 19 recordsLinked to original sources

arXiv2026

Repulsive Gravitational Force as a Witness of the Quantum Nature of Gravity

We show that a single spatially superposed 'source' mass acting on a 'probe' matter wavepacket can reveal the quantum nature of the gravitational field. For this we use a specific state preparation and measurement of the superposed source mass, including a postselection, which altogether results in a repulsive gravitational force on the probe particle. A classical gravitational field can never lead to repulsion, as the effect requires quantum interference of two distinct states of gravity. The eventual observation of such an effect would be a violation of Einstein's theory of general relativity, where gravity is always attractive. We also present a calculation in the Heisenberg picture under the formalism of weak values that illustrates how repulsion is achieved. Finally, we estimate the range of parameters (masses and the spatio-temporal extent of interference) for which the experiment is feasible.

quant-ph↗

arXiv2025

Topology of the Aharonov-Bohm effect in different reference frames

Recent works showed that the Aharonov-Bohm (AB) phase difference for a quantum charged particle can be written in terms of electric and magnetic fluxes in a spacetime surface whose boundaries are the possible particle worldlines in the interferometer, relative to the possible paths. After presenting this result in a more detailed way, reinforcing its topological nature, we study the magnetic and electric versions of the AB effect in different inertial reference frames. We find a particular reference frame for a magnetic AB effect where the magnetic flux has a null contribution for the AB phase difference, which is entirely due to an electric flux. Also, we find a particular reference frame for an electric AB effect where the electric flux has a null contribution for the AB phase difference, which is entirely due to a magnetic flux. In this sense, the nomenclatures 'magnetic AB effect' and 'electric AB effect' lose their meaning. We have electromagnetic AB effects.

quant-ph↗

arXiv2025

Topological schemes in spacetime for the electrodynamic Aharonov-Bohm effect

We consider different schemes for the electrodynamic Aharonov-Bohm (AB) effect introduced in Ref. [Phys. Rev. A $\mathbf{108}$, 062218 (2023)], exploring the phenomenon to enhance the understanding of its topological nature in spacetime. In the treated examples, the electric current in a solenoid varies in time, changing its internal magnetic field and producing an external electric field, while a quantum charged particle is in a superposition state inside two Faraday cages in an interferometer. The Faraday cages cancel the electric field at their interiors, such that the particle is always subjected to null electromagnetic fields. We discuss how the AB phase difference depends on the topology of the electric and magnetic fields in spacetime in the different treated situations. In particular, we discuss interesting results when a conducting wire connects the two Faraday cages, with the AB phase depending on the wire position. We also show an amplification of the AB phase when the wire makes several turns around the solenoid, which could enable an experimental verification of the effect.

quant-ph↗

arXiv2025

High entanglement regimes in the Weisskopf-Wigner theory for spontaneous decay

In this work we review the Weisskopf-Wigner formalism for spontaneous emission considering the spatial modes of light as well as external atomic degrees of freedom which we introduce in the theory by modeling the atom as a wavepacket in momentum space with a given initial uncertainty. We perform a purity calculation in order to quantify the entanglement encoded in the momentum variables of the atom-photon system. Our purity calculations reveal two high entanglement regimes depending on the initial atomic momentum uncertainty: $1)$ the Recoil entanglement regime (which arises in the small momentum uncertainty region) where recoil effects dominate the mechanisms that originate entanglement, and $2)$ the Doppler entanglement regime (in the large momentum uncertainty region) where homogeneous Doppler shifts in the emitted photon's frequency play the fundamental part in the build up of quantum correlations in the system. Physical considerations are made to explain the nature of each entanglement regime as well as provide their respective thresholds.

quant-ph↗

arXiv2024

Quantum Interference of Force with Entangled Photons

In this work we experimentally demonstrate the quantum interference of force effect using pairs of entangled photons. Although photons are massless particles, they have linear momentum, and our experiments show that the quantum superposition of a positive momentum transfer with a null momentum transfer may result in a negative momentum transfer to an ensemble of quantum particles (photons), a behavior with no classical analogue. The momentum transfer to each photon is defined by the result of a polarization measurement performed in a second photon, initially entangled with it.

quant-ph↗

arXiv2024

Gauge invariance of the Aharonov-Bohm effect in a quantum electrodynamics framework

The gauge invariance of the Aharonov-Bohm (AB) effect with a quantum treatment for the electromagnetic field is demonstrated. We provide an exact solution for the electromagnetic ground energy due to the interaction of the quantum electromagnetic field with the classical charges and currents that act as sources of the potentials in a classical description, in the Lorenz gauge. Then, we use first-order perturbation theory to compute an extra change on the electromagnetic ground energy due to the presence of a quantum charged particle with known wave function in the system. This energy in general depends on the quantum particle path in an interferometer, what results in an AB phase difference between the paths. The gauge invariance of this AB phase difference is then shown for the magnetic, electric, and the recently proposed electrodynamic versions of the AB effect. However, the AB phase difference could depend on the gauge for nonclosed paths, what reinforces the view that it only can be measured in closed paths.

quant-ph↗

arXiv2024

Association between quantum paradoxes based on weak values and a realistic interpretation of quantum measurements

Many quantum paradoxes based on a realistic view of weak values were discussed in the last decades. They lead to astonishing conclusions such as the measurement of a spin component of a spin-1/2 particle resulting in $100\hbar$, the separation of a photon from its polarization, and the possibility of having 3 particles in 2 boxes without any 2 particles being in the same box, among others. Here we show that the realistic view of the weak values present in these (and other) works is equivalent to a realistic (and highly controversial) view of quantum measurements, where a measurement reveals the underlying reality of the measured quantity. We discuss that all quantum paradoxes based on weak values simply disappear if we deny these realistic views of quantum measurements and weak values. Our work thus aims to demonstrate the strong assumptions and the corresponding problems present in the interpretation of these quantum paradoxes.

quant-ph↗

arXiv2023

Electrodynamic Aharonov-Bohm effect

We propose an electrodynamic Aharonov-Bohm (AB) scheme where a nonzero AB phase difference appears even if the interferometer paths do not enclose a magnetic flux and are subjected to negligible scalar potential differences during the propagation of the quantum charged particle. In the proposal, the current in a solenoid outside the interferometer varies in time while the quantum particle is in a superposition state inside two Faraday cages, such that it is always subjected to negligible electromagnetic fields. At first glance, this result could challenge the topological nature of the AB effect. However, by considering the topology of the electromagnetic field configuration and the possible particle trajectories in spacetime, we demonstrate the topological nature of this situation.

quant-ph↗

arXiv2023

Describing two-mode squeezed-light experiments without two-mode entanglement or squeezing

In a recent work [Phys. Rev. A \textbf{102}, 053723 (2020)] we have shown that experiments that produce and characterize single-mode light squeezing can be explained in a way where no single-mode squeezed light state is produced in the setup. Here we apply the same ideas to demonstrate that experiments that produce and characterize two-mode light squeezing can also be explained without the production of two-mode squeezed light states. In particular, we show that there is no entanglement between the signal and idler ``twin beam'' modes. This fact may be surprising, since this setup is frequently used to implement entangled-based quantum information protocols such as quantum teleportation. Our work brings an alternative view of the phenomenon. We generalize the Luis and Sánchez-Soto's two-mode relative phase distribution [Phys. Rev. A \textbf{53}, 495 (1996)] to treat four modes, showing that a general physical explanation for the noise reduction in the experiments is a better definition of a phase relation among the four involved optical modes: Signal, idler, and two local oscillators.

quant-ph↗

arXiv2023

Recoil momentum of an atom absorbing light in a gaseous medium and the Abraham-Minkowski debate

We discuss a fundamental question regarding the Abraham-Minkowski debate about the momentum of light in a medium: If an atom in a gas absorbs a photon, what is the momentum transferred to it? We consider a classical model for the internal degrees of freedom of the absorbing atom, computing the absorbed energy and momentum using the Lorentz force law due to the microscopic electromagnetic fields. Each non-absorbing atom from the gas is treated as a dielectric sphere, with the set of atoms forming a linear, dielectric, non-magnetic, and non-absorbing medium with a refractive index $n$ close to one. Our numerical results indicate that if the atoms are classically localized, the average absorbed momentum increases with $n$, but is smaller than Minkowski's momentum $np_0$, $p_0$ being the photon momentum in vacuum. However, experiments performed with Bose-Einstein condensates [Phys. Rev. Lett. $\mathbf{94}$, 170403 (2005)] are consistent with the atom absorbing Minkowski's momentum. We argue that there is no contradiction between these results since, in a Bose-Einstein condensate, the atoms are in a quantum state spatially superposed in a relatively large volume, forming a ``continuous'' medium. In this sense, the experimental verification of an atomic momentum recoil compatible with Minkowski's momentum would be a quantum signature of the medium state.

physics.optics↗

arXiv2022

Relative Phase Distribution and the Precision of Optical Phase Sensing in Quantum Metrology

One of the quantum metrology goals is to improve the precision in the measurement of a small optical phase introduced in one optical mode in an interferometer, i.e., phase sensing. In this paper, we obtain the relative phase distribution introduced by Luis and Sánchez-Soto (LSS) [Phys. Rev. A $\mathbf{53}$, 495 (1996)] for several two-mode pure quantum light states useful in quantum metrology. We show that, within the numerical precision of our calculations, the Fisher information obtained from the LSS relative phase distribution is equal to the quantum Fisher information for the considered states (the average difference for the tested states is smaller than 0.1%). Our results indicate that the LSS relative phase distribution can be used to predict the minimum uncertainty possible in the process of phase sensing in quantum metrology, since this uncertainty depends on the quantum Fisher information, at least for pure states.

quant-ph↗

arXiv2021

Aharonov-Casher and shielded Aharonov-Bohm effects with a quantum electromagnetic field

We use a covariant formalism that is capable of describing the electric and magnetic versions of the Aharonov-Bohm effect, as well as the Aharonov-Casher effect, through local interactions of charges and currents with the quantum electromagnetic field. By considering that only local interactions of a quantum particle with the quantum field can affect its behavior, we show that the magnetic Aharonov-Bohm effect must be present even if the solenoid generating the magnetic field is shielded by a perfect conductor, as experimentally demonstrated.

quant-ph↗

arXiv2021

Apparent quantum paradoxes as simple interference: Quantum violation of the pigeonhole principle and exchange of properties between quantum particles

It was recently argued that the pigeonhole principle, which states that if three pigeons are put into two pigeonholes then at least one pigeonhole must contain more than one pigeon, is violated in quantum systems [Y. Aharonov et al., PNAS 113, 532 (2016)]. An experimental verification of this effect was recently reported [M.-C. Chen et al., PNAS 116, 1549 (2019)]. In another recent experimental work, it was argued that two entities were observed to exchange properties without meeting each other [Z.-H. Liu et al., Nat. Commun. 11, 3006 (2020)]. Here we describe all these proposals and experiments as simple quantum interference effects, where no such dramatic conclusions appear. Besides demystifying some of the conclusions of the cited works, we also present physical insights for some interesting behaviors present in these treatments. For instance, we associate the anomalous particles behaviors in the quantum pigeonhole effect to a quantum interference of force.

quant-ph↗

arXiv2021

Local description of the Aharonov-Bohm effect with a quantum electromagnetic field

In the seminal works from Santos and Gozalo [Europhys. Lett. $\mathbf{45}$, 418 (1999)] and Marletto and Vedral [Phys. Rev. Lett. $\mathbf{125}$, 040401 (2020)], it is shown how the Aharonov-Bohm effect can be described as the result of an exchange of virtual photons between the solenoid and the quantum charged particle along its propagation through the interferometer, where both the particle and the solenoid interact locally with the quantum electromagnetic field. This interaction results in a local and gauge-independent phase generation for the particle propagation in each path of the interferometer. Here we improve the cited treatments by using the quantum electrodynamics formalism in the Lorentz gauge, with a manifestly gauge-independent Hamiltonian for the interaction and the presence of virtual longitudinal photons. Only with this more complete and gauge-independent treatment it is possible to justify the acquired phases for interferometers with arbitrary geometries, and this is an advantage of our treatment. We also extend the results to the electric version of the Aharonov-Bohm effect. Finally, we propose an experiment that could test the locality of the Aharonov-Bohm phase generation.

quant-ph↗

arXiv2020

Hidden momentum in continuous media and the Abraham-Minkowski debate

We perform a detailed study of the connection between hidden momentum and the Abraham-Minkowski debate about the electromagnetic momentum density in material media. The results of a previous work on the subject [P. L. Saldanha and J. S. Oliveira Filho, Phys. Rev. A 95, 043804 (2017)] are extended to the continuous medium limit, where some subtleties arise. We consider a polarized and magnetized continuous medium with applied electric and magnetic static fields, where the medium polarization can be due to either an electric charge density or a hypothetical magnetic current density and the medium magnetization can be due to either an electric current density or a hypothetical magnetic charge density. Each model leads to a different expression for the system material hidden momentum and for the electromagnetic momentum density in the medium. We show that the main results of the cited reference are sustained in the continuous medium limit: Abraham momentum is compatible with a model for the medium where the polarization is due to electric charges and the magnetization is due to magnetic charges, Minkowski momentum is compatible to a model where the polarization is due to magnetic currents and the magnetization is due to electric currents, and the expression $\varepsilon_0\mathbf{E}\times\mathbf{B}$ is compatible with a model where the polarization is due to electric charges and the magnetization is due to electric currents, which is the natural model. These results are illustrated with the example of a uniformly polarized and magnetized sphere.

physics.optics↗

arXiv2020

Describing squeezed-light experiments without squeezed-light states

Coherent states are normally used to describe the state of a laser field in experiments that generate and detect squeezed states of light. Nevertheless, since the laser field absolute phase is unknown, its quantum state can be described by a statistical mixture of coherent states with random phases, which is equivalent to a statistical mixture of Fock states. Here we describe single-mode squeezed vacuum experiments using this mixed quantum state for the laser field. Representing the laser state in the Fock basis, we predict the usual experimental results without using the squeezing concept in the analysis and concluding that no squeezed state is generated in the experiments. We provide a general physical explanation for the noise reduction in the experiments in terms of a better definition of the relative phase between the signal and local oscillator fields. This explanation is valid in any description of the laser field (in terms of coherent or Fock states), thus providing a deeper understanding of the phenomenon.

quant-ph↗

arXiv2020

Inconsistency of a realistic interpretation of quantum measurements: A simple example

We use a simple example to illustrate why it is not possible to consider that a measurement reveals an underlying objective reality of a property of a quantum system, that continues the same after the measurement is performed. This kind of incompatibility between realism and quantum mechanics is theoretically demonstrated with an example where sequential spin measurements are performed on a spin-2 quantum particle. We discuss the relation of this result with other works that investigate the concept of reality in quantum mechanics.

quant-ph↗

arXiv2019

Comment on "Nonlocal Coherent Perfect Absorption"

We show that, contrary to what is claimed in Ref. [J. Jeffers, Phys. Rev. Lett. 123, 143602 (2019)], there is no nonlocal coherent absorption by the two lossy beam splitters considered in detail in the treatment of the setup presented in this reference. What happens is a local coherent absorption by the other two lossy beam splitters of the setup.

quant-ph↗