  We consider an interferometer powered by laser light (a coherent state) into
one input port and ask the following question: what is the best state to inject
into the second input port, given a constraint on the mean number of photons
this state can carry, in order to optimize the interferometer's phase
sensitivity? This question is the practical question for high-sensitivity
interferometry. We answer the question by considering the quantum
Cram{\'e}r-Rao bound for such a setup. The answer is squeezed vacuum.
