 05187 G. L. Sewell
 On the Mathematical Structure of Quantum Measurement Theory
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May 26, 05

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Abstract. We show that the key problems of quantum measurement theory, namely the reduction of the wavepacket of a microsystem and the specification of its quantum state by a macroscopic measuring instrument, may be resolved within the traditional framework of the quantum mechanics of finite conservative systems. The argument is centred on the generic model of a microsystem, S, coupled to a finite measuring instrument, I, which itself is an Nparticle quantum system. The pointer positions of I correspond to the macrostates of this instrument, as represented by orthogonal eigenspaces of the Hilbert space of its pure states. These subspaces, or 'phase cells', are the simultaneous eigenspaces of a set of coarse grained intercommuting macroscopic observables, M, and, crucially, are of astronomically large dimensionalities, which increase exponentially with N. We formulate conditions on the conservative dynamics of the composite (S+I) under which it yields both a reduction of the wave packet describing the state of S and a ontoone correspondence, following the measurement, between the observed value of M and the resultant eigenstate of S; and we show that these conditions are fulfilled by the finite version of the ColemanHepp model.
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