Super-resolving phase measurements with a multiphoton entangled state

Interference phenomena are ubiquitous in physics, often forming the basis of demanding measurements. Examples include Ramsey interferometry in atomic spectroscopy, X-ray diffraction in crystallography and optical interferometry in gravitational-wave studies(1,2). It has been known for some time that the quantum property of entanglement can be exploited to perform super-sensitive measurements, for example in optical interferometry or atomic spectroscopy(3-7). The idea has been demonstrated for an entangled state of two photons(8), but for larger numbers of particles it is difficult to create the necessary multiparticle entangled states(9-11). Here we demonstrate experimentally a technique for producing a maximally entangled three-photon state from initially non-entangled photons. The method can in principle be applied to generate states of arbitrary photon number, giving arbitrarily large improvement in measurement resolution(12-15). The method of state construction requires nonunitary operations, which we performusing post-selected linear-optics techniques similar to those used for linear-optics quantum computing(16-20).