  As primitives for entanglement generation, controlled phase gates take a
central role in quantum computing. Especially in ideas realizing instances of
quantum computation in linear optical gate arrays a closer look can be
rewarding. In such architectures, all effective non-linearities are induced by
measurements: Hence the probability of success is a crucial parameter of such
quantum gates. In this note, we discuss this question for controlled phase
gates that implement an arbitrary phase with one and two control qubits. Within
the class of post-selected gates in dual-rail encoding with vacuum ancillas we
identify the optimal success probabilities. We construct networks that allow
for an implementation by means of todays experimental capabilities in detail.
The methods employed here appear specifically useful with the advent of
integrated linear optical circuits, providing stable interferometers on
monolithic structures.
