1. Department of Physics, University of Toronto, 60 St George Street, Toronto, Ontario M5S 1A7, Canada

Correspondence to: M. W. Mitchell Correspondence and requests for materials should be addressed to M.W.M. (Email: mitchell@physics.utoronto.ca).

Abstract

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, 4, 5, 6, 7}. The idea has been demonstrated for an entangled state of two photons⁸, but for larger numbers of particles it is difficult to create the necessary multiparticle entangled states^{9, 10, 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, 13, 14, 15}. The method of state construction requires non-unitary operations, which we perform using post-selected linear-optics techniques similar to those used for linear-optics quantum computing^{16, 17, 18, 19, 20}.

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