But Pryde and his coworkers in Australia have demonstrated a way of reaching the Heisenberg limit of measurement precision without needing these elusive states: by looking at photons traversing an interferometer's arms one at a time. The key is to avoid making measurements that determine which arm the photon is in, until the beams are recombined at the end. This allows the shot noise to be more or less smoothed away.
Another very clever experiment. Way to go, people!