Abstract
States of a quantum system may be influenced by an external intervention. FollowingHaag andKastler, such a transformation of states is called an operation, and is called pure if it transforms pure states into pure states. Operations are discussed here under the assumption that they are caused by interactions with another system (apparatus), described by aS matrix. Pure operations are then shown to correspond, with one exception, to operatorsA with norm smaller than one. The Hermitean operatorsF=A*A represent quantum effects as defined axiomatically byLudwig. The particular case of local operations in quantum field theory is also investigated.
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Supported in part by the Deutsche Forschungsgemeinschaft.
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Hellwig, K.E., Kraus, K. Pure operations and measurements. Commun.Math. Phys. 11, 214–220 (1969). https://doi.org/10.1007/BF01645807
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DOI: https://doi.org/10.1007/BF01645807