  An open quantum system in steady state $\hat\rho_{ss}$ can be represented by
a weighted ensemble of pure states $\hat\rho_{ss}=\sum_{k}\wp_{k}\ket{\psi_k}
\bra{\psi_k}$ in infinitely many ways. A physically realizable (PR) ensemble is
one for which some continuous measurement of the environment will collapse the
system into a pure state $\ket{\psi(t)}$, stochastically evolving such that the
proportion of time for which $\ket{\psi(t)} = \ket{\psi_{k}}$ equals $\wp_{k}$.
Some, but not all, ensembles are PR. This constitutes the preferred ensemble
fact, with the PR ensembles being the preferred ensembles. We present the
necessary and sufficient conditions for a given ensemble to be PR, and
illustrate the method by showing that the coherent state ensemble is not PR for
an atom laser.
