Propagation, coherent

The coherent propagation of star light through optical fibres over long distance has been mainly investigated at IRCOM and kilometric linkages are now possible. The two main points to be managed are ... 

While it is impossible to manipulate and track the details of the perturbations for truly macroscopic systems, the environment of isolated mesoscopic quantum systems can still be efficiently controlled. The present section focuses on one particular interaction between large molecules and the environment, namely on collisions between the coherently propagating molecules and various background gases [Homberger 2003 (a)]. 

The envelope of the beating maxima yields the time constant T = 30 ps. This value represents the decay time of NFID. Applying an analytic correction formula or fitting the theory of coherent propagation to the data (solid line) we obtain the de-... 

Figure Al.6.21. Bra and ket wavepacket dynamics which detennine the coherence overlap, (( ) ( ) ). Vertical arrows mark the transitions between electronic states and horizontal arrows indicate free propagation on the potential surface. Full curves are used for the ket wavepacket, while dashed curves indicate the bra wavepacket. (a) Stimulated emission, (b) Excited state (transient) absorption (from [41]).

Static defects scatter elastically the charge carriers. Electrons do not loose memory of the phase contained in their wave function and thus propagate through the sample in a coherent way. By contrast, electron-phonon or electron-electron collisions are inelastic and generally destroy the phase coherence. The resulting inelastic mean free path, Li , which is the distance that an electron travels between two inelastic collisions, is generally equal to the phase coherence length, the distance that an electron travels before its initial phase is destroyed ... 

At low temperatures, in a sample of very small dimensions, it may happen that the phase-coherence length in Eq.(3) becomes larger than the dimensions of the sample. In a perfect crystal, the electrons will propagate ballistically from one end of the sample and we are in a ballistic regime where the laws of conductivity discussed above no more apply. The propagation of an electron is then directly related to the quantum probability of transmission across the global potential of the sample. 

A useful concept in understanding interference is that of coherence. Consider Young s experiment again (this time with an arbitrary source S which need not be monochromatic illuminating the pinholes). The optical disturbance at point r and time t ean be written alternatively as a sum of the disturbances at the pinholes, with propagation faetors. 

It is usually assumed that the result of the propagation of electrons from the electron gun to the specimen is a plane wave. Partial coherence and/or convergent spherical illumination can then be accounted for by a partially coherent superposition of a set of plane waves. 

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