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Physical Review Letters

Spontaneous polarization of particles

Ulrich Schmidt, G. Baum, D. Dubbers

Published May 31, 1993• 6 citations
Publisher

Abstract

Unpolarized particles with spin may spontaneously polarize during their flight through a resonant cavity, due to their interaction with the cavity's quantized radiation field. This effect, of purely electromagnetic origin, could, if not recognized as such, be mistaken for a parity violating weak interaction effect. For a better understanding of this effect we have simulated it, as well as the related quantum field effects of collapse, revival, and hidden coherence, with a beam of polarized neutrons and a set of classical magnetic fields.

DOI
10.1103/physrevlett.70.3396

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Physical Review•Sep 1, 1924•74 citations
Unpolarized particles with spin may spontaneously polarize during their flight through a resonant cavity, due to their interaction with the cavity's quantized radiation field. This effect, of purely electromagnetic origin, could, if not recognized as such, be mistaken for a parity violating weak interaction effect. For a better understanding of this effect we have simulated it, as well as the related quantum field effects of collapse, revival, and hidden coherence, with a beam of polarized neutrons and a set of classical magnetic fields.
For jg equals j yields je equals j - 1 and jg equals j yields je equals j optical transitions (where jg, je are the total angular momenta of ground (g) and excited (e) states) previously unknown exact solution of quantum self-consistent problem on resonant atom-electromagnetic field interaction is found. This solution describes arbitrary set of field modes having same uniform elliptical polarization and atoms being in noninteracting with field coherent population trapping (CPT)-state. Some peculiarities of this spontaneous CPT-effect are investigated. We define the geometrical conditions (the relative orientation of light and static fields) for the existence of spontaneous CPT-effect in atoms + quantized field system in the presence of external static magnetic and electric fields.
We analyzed spin polarization dynamics in a two-dimensional system of spin 1/2 charged particles with spin-orbit interaction in perpendicular magnetic field in the presence of external noise. It was shown that spin polarization reveals quantum oscillations, collapses, and revivals. The hierarchy of time scales corresponding to quantum oscillations, collapses, and revivals was identified and analyzed. It was demonstrated that variation of external magnetic field and noise intensity results in the significant changing of spin fluctuations spectra, which modifies from the triplet structure to the well resolved fine structure.
A vapor of paramagnetic atoms, under conditions in which electronic spin exchange takes place faster than collisional spin relaxation, should acquire a spontaneous spin polarization when illuminated by unpolarized light tuned to excite only certain ground-state atomic hyperfine levels. In magnetic fields above a certain size, the atomic polarization has two stable states, either parallel or antiparallel to the field. Experiments to demonstrate this effect and explore possible applications may be feasible by the use of alkali-metal vapors.
We study the dynamics of molecular nanomagnets through a fully quantum mechanical model describing high-spin and high-anisotropy magnetic molecules subjected to a time-dependent magnetic field along the quantization axis, which continuously inverts the population of spin states. Crystals of molecular nanomagnets placed inside a resonant cavity interact with a quantized electromagnetic field. Relaxation of excited states takes place by means of spin-photon interaction, allowing stimulated emission of radiation and creating a maser effect.
Effect of a magnetic field on the polarization of resonance radiation of mercury and of sodium.---(1) Mercury $\ensuremath{\lambda}2536$. A beam of plane polarized monochromatic light was focussed on the bulb containing the vapor at low pressure (0\ifmmode^\circ\else\textdegree\fi{}C), and the polarization of the resonance radiation was determined photographically with the aid of a quartz wedge and a double image quartz prism. If the exciting beam is going east with the electric vector vertical, when the earth's field is carefully neutralized the polarization is 90 per cent in any horizontal direction and zero vertically; a field of only 2 gauss directed north reduces the polarization in its direction to nearly zero (the decrease with increasing field being exponential), changes the polarization east and also up to 60 percent, and increases the intensity directed upwards threefold. Similar effects are produced by fields in other directions. The polarization with zero field does not approach 100 per cent at low pressures, since 90 per cent was found also with the vapor at -50\ifmmode^\circ\else\textdegree\fi{}C, when the radiation first appeared. It was thought that the high value might be due to an orientation of the resonating atom by the field of the exciting light, but a beam of concentrated sunlight produced no effect. As found by Malinowski, a strong field of 10,000 gauss merely increased the intensity of the radiation about 10 per cent. This added light was found to be unpolarized. (2) Sodium DDD line. In the case of sodium vapor the tube was heated to 185\ifmmode^\circ\else\textdegree\fi{}C, the observations were visual, and as a source of unreversed light, the glow on the surface of a sodium glass vacuum tube carrying a discharge, was used. The effects observed are similar to those for mercury, but differ for some directions; fields forty times stronger were required, and the polarization in zero field was only 6 per cent, probably due to traces of hydrogen. This was increased by the field in some directions to 30 per cent.Effect of magnetic field on fluorescent light of iodine vapor and on white scattered light from mercury and ether vapors.---The polarization was found not to be affected.