  We extend the input-output formalism to study the behavior of uncoupled
discrete modes (bosonic cavity modes and fermionic qubits) when they decay to
the same Markovian continuum. When the continuum interacts with only a single
mode, this decay is irreversible. However, when multiple modes decay to the
same Markovian continuum they develop correlations and decay collectively. In
the input-output formalism these correlations manifest in additional terms in
the quantum Langevin equation. For two modes this collective decay can
dramatically extend the lifetimes of both modes (Dicke subradiance) and, within
the single-mode subsystem, induces non-Markovian memory effects including
energy back flow.
