tailieunhanh - The Quantum Mechanics Solver 19

The Quantum Mechanics Solver 19 uniquely illustrates the application of quantum mechanical concepts to various fields of modern physics. It aims at encouraging the reader to apply quantum mechanics to research problems in fields such as molecular physics, condensed matter physics or laser physics. Advanced undergraduates and graduate students will find a rich and challenging source of material for further exploration. This book consists of a series of problems concerning present-day experimental or theoretical questions on quantum mechanics | 182 17 A Quantum Thermometer The mean excitation number is n 2 nPn E ne-nY En e-nY d n dy or 1 eY - 1 n b One can see on the above expression that n is a rapidly increasing function of the temperature. If the temperature is such that g 1 . kBT be or T K for this experiment the mean excitation number is of the order of e 1 1 . Below this temperature the occupation of the level nl 0 becomes predominant as can be seen on the curves of Fig. . The variation of lnpn as a function of n is linear h ln pn - - Const kB T The slope increases as the temperature decreases. The curves of Fig. 2 clearly show this linear variation. They correspond to ratios p1 p0 equal . temperatures of K 2 K 3 K and K respectively. c In order to measure a temperature with such a device one must use a statistical sample which is significantly populated in the level nl 1. It is experimentally difficult to go below a probability of 10 2 for the level nr 1 which corresponds to a temperature T K. d In order to improve the sensitivity of this thermometer one can Increase significantly the total time of measurement in order to detect occupation probabilities of the level nr 1 significantly less than 10 2 Reduce the value of the magnetic field B in order to reduce the cyclotron frequency wc and to increase for a given temperature the occupation probability of the level nl 1 . The data used in this chapter are extracted from the article of S. Peil and G. Gabrielse Observing the Quantum Limit of an Electron Cyclotron QND Measurements of Quantum Jumps between Fock States Physical Review Letters 83 p. 1287 1999 . Part III Complex Systems 18 Exact Results for the Three-Body Problem The three-body problem is a famous question of mechanics. Henri Poincare was the first to prove exact properties and this contributed to his celebrity. The purpose of this chapter is to derive some rigorous results for the three-body problem in quantum mechanics. Here we are .

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