Abstract
We define a quantum entropy conditioned on postselection which has the von Neumann entropy of pure states as a special case. This conditional entropy can take negative values, which is consistent with part of a quantum system containing less information than the whole, which can be in a pure state. The definition is based on generalized density operators for postselected ensembles. The corresponding density operators are consistent with the quantum generalization of classical conditional probabilities following Dirac's formalism of quasiprobability distributions.
Original language  English 

Article number  022116 
Number of pages  4 
Journal  Physical Review A: Atomic, Molecular and Optical Physics 
Volume  90 
Issue number  2 
Early online date  25 Aug 2014 
DOIs  
Publication status  Published  Aug 2014 
Keywords
 quantph
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Profiles

Dr Roman C V Schubert
 Probability, Analysis and Dynamics
 School of Mathematics  Lecturer in Mathematics
 Mathematical Physics
 Pure Mathematics
Person: Academic , Member