Quantum mechanics, information theory, and relativity theory are the basic foundations of theoretical
physics. The acquisition of information from a quantum system is the interface of classical
and quantum physics. Essential tools for its description are Kraus matrices and positive operator
valued measures (POVMs). Special relativity imposes severe restrictions on the transfer of information
between distant systems. Quantum entropy is not a Lorentz covariant concept. Lorentz
transformations of reduced density matrices for entangled systems may not be completely positive
maps. Quantum eld theory, which is necessary for a consistent description of interactions,
implies a fundamental trade-o between detector reliability and localizability. General relativity
produces new, counterintuitive eects, in particular when black holes (or more generally, event
horizons) are involved. Most of the current concepts in quantum information theory may then
require a reassessment.