  We propose a Heisenberg-limited quantum interferometer whose input is twin
optical beams from which one or more photons have been indistinguishably
subtracted. Such an interferometer can yield Heisenberg-limited performance
while at the same time giving a direct fringe reading, unlike for the twin-beam
input of the Holland-Burnett interferometer. We propose a feasible experimental
realization using a photon-number correlated source, such as non-degenerate
parametric down-conversion, and perform realistic analyses of performance in
the presence of loss and detector inefficiency.
