  The decoherence interpretation of quantum measurements is applied to Wigner's
friend experiments. A framework in which all the experimental outcomes arise
from unitary evolutions is proposed. Within it, a measurement is not completed
until an uncontrolled environment monitorizes the state composed by the system,
the apparatus and the observer. The (apparent) wave-function collapse and the
corresponding randomness result from tracing out this environment; it is thus
the ultimate responsible for the emergence of definite outcomes. Two main
effects arise from this fact. First, external interference measurements,
trademark of Wigner's friend experiments, modify the memory records of the
internal observers; this framework provides a univocal protocol to calculate
all these changes. Second, it can be used to build a consistent scenario for
the recenly proposed extended versions of the Wigner's friend experiment.
Regarding [D. Frauchiger and R. Renner, {\em Quantum theory cannot consistently
describe the use of itself}, Nat. Comm. {\bf 9}, 3711 (2018)], this framework
shows that the agents' claims become consistent if the changes in their
memories are properly taken into account. Furthermore, the particular setup
discussed in [C. Brukner, {\em A no-go theorem for observer-indepdendent
facts}, Entropy {\bf 20}, 350 (2018)] cannot be tested against the decoherence
framework, because it does not give rise to well-defined outcomes according to
this formalism. A variation of this setup, devised to fill this gap, makes it
possible to assign joint truth values to the observations made by all the
agents. This framework also narrows down the requisites for such experiments,
making them virtually impossible to apply to conscious (human) beings.
Notwithstanding, it also opens the door to future relizations on quantum
machines.
