  We prove that the K-theory of an exact quasicategory can be computed via a
higher categorical variant of the Q construction. This construction yields a
quasicategory whose weak homotopy type is a delooping of the K-theory space. We
show that the direct sum endows this homotopy type with the structure of a
infinite loop space, which agrees with the canonical one. Finally, we prove a
proto-devissage result, which gives a necessary and sufficient condition for a
"nilimmersion" of stable quasicategories to be a K-theory equivalence. In
particular, we prove that a well-known conjecture of Ausoni and Rognes is
equivalent to the weak contractibility of a particular quasicategory.
