  Precision measurements are important across all fields of science. In
particular, optical phase measurements can be used to measure distance,
position, displacement, acceleration and optical path length. Quantum
entanglement enables higher precision than would otherwise be possible. We
demonstrate an optical phase measurement with an entangled four photon
interference visibility greater than the threshold to beat the standard quantum
limit--the limit attainable without entanglement. These results open the way
for new high-precision measurement applications.
