Decoherence by a spin thermal bath: Role of the spin-spin interactions and initial state of the bath

We study the decoherence of two coupled spins that interact with a spin-bath environment. It is shown that the connectivity and the coupling strength between the spins in the environment are of crucial importance for the decoherence of the central system. For the anisotropic spin-bath, changing the connectivity or coupling strenghts changes the decoherence of the central system from Gaussian to exponential decay law. The initial state of the environment is shown to affect the decoherence process in a qualitatively significant manner.Comment: submitted to PR

Photons uncertainty solves Einstein-Podolsky-Rosen paradox

Einstein, Podolsky and Rosen (EPR) pointed out that the quantum-mechanical description of "physical reality" implied an unphysical, instantaneous action between distant measurements. To avoid such an action at a distance, EPR concluded that Quantum Mechanics had to be incomplete. However, its extensions involving additional "hidden variables", allowing for the recovery of determinism and locality, have been disproved experimentally (Bell's theorem). Here, I present an opposite solution of the paradox based on the greater indeterminism of the modern Quantum Field Theory (QFT) description of Particle Physics, that prevents the preparation of any state having a definite number of particles. The resulting uncertainty in photons radiation has interesting consequences in Quantum Information Theory (e.g. cryptography and teleportation). Moreover, since it allows for less elements of EPR physical reality than the old non-relativistic Quantum Mechanics, QFT satisfies the EPR condition of completeness without the need of hidden variables. The residual physical reality does never violate locality, thus the unique objective proof of "quantum nonlocality" is removed in an interpretation-independent way. On the other hand, the supposed nonlocality of the EPR correlations turns out to be a problem of the interpretation of the theory. If we do not rely on hidden variables or new physics beyond QFT, the unique viable interpretation is a minimal statistical one, that preserves locality and Lorentz symmetry.Comment: Published version, with updated referenc

Hardy's argument and successive spin-s measurements

We consider a hidden-variable theoretic description of successive measurements of non commuting spin observables on a input spin-s state. In this scenario, the hidden-variable theory leads to a Hardy-type argument that quantum predictions violate it. We show that the maximum probability of success of Hardy's argument in quantum theory is $(\frac{1}{2})^{4s}$, which is more than in the spatial case.Comment: 7 page

Spin and charge pumping in magnetic tunnel junctions with precessing magnetization: A nonequilibrium Green function approach

We study spin and charge currents pumped by precessing magnetization of a single ferromagnetic layer within F|I|N or F|I|F (F-ferromagnet; I-insulator; N-normal-metal) multilayers of nanoscale thickness attached to two normal metal electrodes with no applied bias voltage between them. Both simple one-dimensional model, consisting of a single precessing spin and a potential barrier as the "sample," and realistic three-dimensional devices are investigated. In the rotating reference frame, where the magnetization appears to be static, these junctions are mapped onto a four-terminal dc circuit whose effectively half-metallic ferromagnetic electrodes are biased by the frequency $\hbar \omega/e$ of microwave radiation driving magnetization precession at the ferromagnetic resonance (FMR) conditions. We show that pumped spin current in F|I|F junctions, diminished behind the tunnel barrier and increased in the opposite direction, is filtered into charge current by the second $F$ layer to generate dc pumping voltage of the order of $\sim 1$ $\mu$V (at FMR frequency $\sim 10$ GHz) in an open circuit. In F|I|N devices, several orders of magnitude smaller charge current and the corresponding dc voltage appear concomitantly with the pumped spin current due to barrier induced asymmetry in the transmission coefficients connecting the four electrodes in the rotating frame picture of pumping.Comment: 8 pages, 5 figure

The Quantum Mechanics of Hyperion

This paper is motivated by the suggestion [W. Zurek, Physica Scripta, T76, 186 (1998)] that the chaotic tumbling of the satellite Hyperion would become non-classical within 20 years, but for the effects of environmental decoherence. The dynamics of quantum and classical probability distributions are compared for a satellite rotating perpendicular to its orbital plane, driven by the gravitational gradient. The model is studied with and without environmental decoherence. Without decoherence, the maximum quantum-classical (QC) differences in its average angular momentum scale as hbar^{2/3} for chaotic states, and as hbar^2 for non-chaotic states, leading to negligible QC differences for a macroscopic object like Hyperion. The quantum probability distributions do not approach their classical limit smoothly, having an extremely fine oscillatory structure superimposed on the smooth classical background. For a macroscopic object, this oscillatory structure is too fine to be resolved by any realistic measurement. Either a small amount of smoothing (due to the finite resolution of the apparatus) or a very small amount of environmental decoherence is sufficient ensure the classical limit. Under decoherence, the QC differences in the probability distributions scale as (hbar^2/D)^{1/6}, where D is the momentum diffusion parameter. We conclude that decoherence is not essential to explain the classical behavior of macroscopic bodies.Comment: 17 pages, 24 figure

Entanglement for all quantum states

It is shown that a state that is factorizable in the Hilbert space corresponding to some choice of degrees of freedom, becomes entangled for a different choice of degrees of freedom. Therefore, entanglement is not a special case but is ubiquitous in quantum systems. Simple examples are calculated and a general proof is provided. The physical relevance of the change of tensor product structure is mentioned.Comment: 9 page

Quantum oscillations and black hole ringing

We show that strongly coupled field theories with holographic gravity duals at finite charge density and low temperatures can undergo de Haas - van Alphen quantum oscillations as a function of an external magnetic field. Exhibiting this effect requires computation of the one loop contribution of charged bulk fermions to the free energy. The one loop calculation is performed using a formula expressing determinants in black hole backgrounds as sums over quasinormal modes. At zero temperature, the periodic nonanalyticities in the magnetic susceptibility as a function of the inverse magnetic field depend on the low energy scaling behavior of fermionic operators in the field theory, and are found to be softer than in weakly coupled theories. We also obtain numerical and WKB results for the quasinormal modes of charged bosons in dyonic black hole backgrounds, finding evidence for nontrivial periodic behavior as a function of the magnetic field.Comment: 1+53 pages. 9 figures. v2: important changes to sections 3.4 - 3.6. contribution of branch cut poles include

Entropy production and equilibration in Yang-Mills quantum mechanics

The Husimi distribution provides for a coarse grained representation of the phase space distribution of a quantum system, which may be used to track the growth of entropy of the system. We present a general and systematic method of solving the Husimi equation of motion for an isolated quantum system, and we construct a coarse grained Hamiltonian whose expectation value is exactly conserved. As an application, we numerically solve the Husimi equation of motion for two-dimensional Yang-Mills quantum mechanics (the x-y model) and calculate the time evolution of the coarse grained entropy of a highly excited state. We show that the coarse grained entropy saturates to a value that coincides with the microcanonical entropy corresponding to the energy of the system.Comment: 23 pages, 23 figure

Quantum mechanics and elements of reality inferred from joint measurements

The Einstein-Podolsky-Rosen argument on quantum mechanics incompleteness is formulated in terms of elements of reality inferred from joint (as opposed to alternative) measurements, in two examples involving entangled states of three spin-1/2 particles. The same states allow us to obtain proofs of the incompatibility between quantum mechanics and elements of reality.Comment: LaTeX, 12 page

Invalidity of Classes of Approximated Hall Effect Calculations

In this comment, I point out a number of approximated derivations for the effective equation of motion, now been applied to d-wave superconductors by Kopnin and Volovik are invalid. The major error in those approximated derivations is the inappropriate use of the relaxation time approximation in force-force correlation functions, or in force balance equations, or in similar variations. This approximation is wrong and unnecessary.Comment: final version, minor changes, to appear in Phys. Rev. Let