PHYSICS: Engineered Quantum Entanglement

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Science  31 Mar 2000:
Vol. 287, Issue 5462, pp. 2377b
DOI: 10.1126/science.287.5462.2377b

Entanglement of two or more quantum objects is a process whereby the quantum mechanical wavefunctions describing these individual objects are manipulated to create a new, interwoven state comprising a superposition of the individual wavefunctions. In this entangled system, all of the objects are correlated, and measuring the state of just one of them gives information on all of the others. Such a property is useful both for probing fundamental questions in quantum mechanics and for developing quantum computers. As the number of objects entangled increases, the superposed state becomes more complex, but exponentially more powerful.

Although entangled pairs and triplets have been demonstrated previously, the entanglement of larger systems has remained elusive. Sackett et al. have implemented a technique that, in principle, allows the controlled entanglement of many objects, and demonstrate four-particle entanglement—what, in essence, is a four-quantum-bit logic gate. Laser-cooling and optical pumping are used to position four beryllium ions in a line with all of them initially in the same spin-down state (the ground state). With illumination from a single laser pulse, the frequency and duration of which is selected carefully to minimize occupation of the intermediate states of mixed spin, all four ions are converted coherently to the spin-up state (the entangled state). Because there is a finite probability that the intermediate states are occupied and because of decoherence in the system when the spin information is lost, the scaling of this approach presently is limited to four particles, but these results illustrate the power of a technique that in the future may deliver many-body entanglement on demand.—ISO

Nature 404, 256 (2000).

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