Spin state in the propagation of quantum relativistic particles along classical trajectories

We address the propagation of the spin along classical trajectories for a 1/2-spin particle obeying the Dirac equation with scalar potentials. Focusing on classical trajectories as the exact propagation of wave-function discontinuities we find an explicit spin-transport law for the case of the Dirac oscillator. In the general case we examine the spin propagation along classical trajectories emerging as an approximation of the quantum dynamics via the mechanical analog of the optical eikonal asymptotic approach. Throughout we establish as many parallels as possible with the equivalent situation for the electromagnetic field.Comment: 8 pages, 5 figure

SU(2)-invariant depolarization of quantum states of light

We develop an SU(2)-invariant approach to the depolarization of quantum systems as the effect of random unitary SU(2) transformations. From it we derive an SU(2)-invariant Markovian master equation. This is applied to several quantum states examining whether nonclassical states are more sensible to depolarization than the classical ones. Furthermore, we show that this depolarization model provides a nontrivial generalization of depolarization channels to states of arbitrary dimension.Comment: RevTex4 file, color figures, published versio

Nonlinear Michelson interferometer for improved quantum metrology

We examine quantum detection via a Michelson interferometer embedded in a gas with Kerr nonlinearity. This nonlinear interferometer is illuminated by pulses of classical light. This strategy combines the robustness against practical imperfections of classical light with the improvement provided by nonlinear processes. Regarding ultimate quantum limits, we stress that, as a difference with linear schemes, the nonlinearity introduces pulse duration as a new variable into play along with the energy resources.Comment: 5 pages, 1 figure. Close to published versio