Spin coherence and Humpty-Dumpty. III. The effects of observation.
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In recent work a Stern-Gerlach interferometer (SGI) was considered in which a polarized beam of spin-(1/2 particles is split by a Stern-Gerlach apparatus into two partial beams, and then subsequent Stern-Gerlach deflecting magnets are used to reconstitute these two beams into one. In these studies it was shown that when such a coherent polarized beam passed through a SGI, some spin coherence is inevitably lost. In this regard, folk wisdom concerning irreversibility provides something of a guide to the present problem, since we all know that when Humpty-Dumpty had his great fall nobody could put him together again. In the present paper we consider the fate of our spin-(1/2 Humpty-Dumpty when a detector is present that is sensitive to the passage of particles along one trajectory, but not the other. It is not surprising that coherence is destroyed as soon as one is able to tell along which path the atom traveled. However, there seems to be no general agreement about the mechanism of coherence loss. Our conclusion is that the loss of coherence in measurements on quantum systems can always be traced to the dynamics of correlations between the measuring apparatus and the system being observed. 1989 The American Physical Society.
