Title: Superdensity Operators for Spacetime Quantum Mechanics,
Abstract: We introduce superdensity operators as a tool for analyzing quantum
information in spacetime. Superdensity operators encode spacetime correlation
functions in an operator framework, and support a natural generalization of
Hilbert space techniques and Dirac's transformation theory as traditionally
applied to standard density operators. Superdensity operators can be measured
experimentally, but accessing their full content requires novel procedures. We
demonstrate these statements on several examples. The superdensity formalism
suggests useful definitions of spacetime entropies and spacetime quantum
channels. For example, we show that the von Neumann entropy of a superdensity
operator is related to a quantum generalization of the Kolmogorov-Sinai
entropy, and compute this for a many-body system. We also suggest experimental
protocols for measuring spacetime entropies.