Experimental quantum-enhanced estimation of a lossy phase shift

A paradigm for quantum-enhanced precision metrology is found in optical interferometry(1), which is capable of sensing diverse physical quantities through measurement of a phase shift. When standard light sources are used, the precision of the phase determination is limited by shot noise, the origin of which can be traced to the random manner in which individual photons emerge from the interferometer. Quantum entanglement provides a means to exceed this limit(2-6) with the celebrated example of N00N states(7-10), which saturate the ultimate Heisenberg limit on precision(11), but are extremely fragile to losses(12-14). In contrast, we present experimental evidence that appropriately engineered quantum states(15) outperform both standard and N00N states in the precision of phase estimation when losses are present. This shows that the quantum enhancement of metrology is possible even when decoherence is present, and that the strategy for realizing the enhancement is quite distinct from protecting quantum information encoded in light(16,17).