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Experimental entanglement of four particles


Quantum mechanics allows for many-particle wavefunctions that cannot be factorized into a product of single-particle wavefunctions, even when the constituent particles are entirely distinct. Such ‘entangled’ states explicitly demonstrate the non-local character of quantum theory1, having potential applications in high-precision spectroscopy2, quantum communication, cryptography and computation3. In general, the more particles that can be entangled, the more clearly nonclassical effects are exhibited4,5—and the more useful the states are for quantum applications. Here we implement a recently proposed entanglement technique6 to generate entangled states of two and four trapped ions. Coupling between the ions is provided through their collective motional degrees of freedom, but actual motional excitation is minimized. Entanglement is achieved using a single laser pulse, and the method can in principle be applied to any number of ions.

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Figure 1: Entanglement scheme for two particles.
Figure 2: Determination of ρ(↑↓).


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We thank J. Bollinger and E. A. Cornell for comments on the manuscript. This work was supported by the US National Security Agency, Army Research Office and Office of Naval Research.

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Correspondence to C. Monroe.

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Sackett, C., Kielpinski, D., King, B. et al. Experimental entanglement of four particles. Nature 404, 256–259 (2000).

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