  We explore the advantages offered by twin light beams produced in parametric
down-conversion for precision measurement. The symmetry of these bipartite
quantum states, even under losses, suggests that monitoring correlations
between the divergent beams permits a high-precision inference of any
symmetry-breaking effect, e.g. fiber birefringence. We show that the quantity
of entanglement is not the key feature for such an instrument. In a lossless
setting, scaling of precision at the ultimate `Heisenberg' limit is possible
with photon counting alone. Even as photon losses approach 100% the precision
is shot-noise limited, and we identify the crossover point between quantum and
classical precision as a function of detected flux. The predicted
hypersensitivity is demonstrated with a Bayesian simulation.
