Pre- and postselected quantum states: Density matrices, tomography, and Kraus operators
Ralph Silva, Yelena Guryanova, Nicolas Brunner, Noah Linden, Anthony James Short, Sandu Popescu · Physical Review A · 2014
We present a general formalism for characterizing 2-time quantum states, describing pre- and postselected quantum systems. The most general 2-time state is characterized by a ``density vector'' that is independent of measurements performed between the preparation and postselection. We provide a method for performing tomography of an unknown 2-time density vector. This procedure, which cannot be implemented by weak or projective measurements, brings new insight to the fundamental role played by Kraus operators in quantum measurements. Finally, after showing that general states and measurements are isomorphic, we show that any measurement on a 2-time state can be mapped to a measurement on a preselected bipartite state.