  We study the use of detection devices in entanglement-based state
preparation. In particular we consider optical detection devices such as
single-photon sensitivity detectors, single-photon resolution detectors and
detector cascades (with an emphasis on the performance of realistic detectors).
We develop an extensive theory for the use of these devices. In
entanglement-based state preparation we perform measurements on subsystems, and
we therefore need precise bounds on the distinguishability of these
measurements (this is fundamentally different from, e.g., tomography, where an
ensemble of identical states is used to determine probability distributions,
etc.). To this end, we introduce the confidence of preparation, which may also
be used to quantify the performance of detection devices in entanglement-based
preparation. We give a general expression for detector cascades of arbitrary
size for the detection up to two photons. We show that, contrary to the general
belief, cascading does not give a practical advantage over detectors with
single-photon resolution in entanglement-based state preparation.
