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Ion-Photon Entanglement Demonstrated

Photonics Spectra
Apr 2004
Researchers at the University of Michigan in Ann Arbor, have demonstrated quantum entanglement between a single trapped cadmium ion and a photon emitted by the ion. The work, which was published in the March 11 issue of Nature, points to potential applications not only in scalable quantum information processing schemes involving multiple and remotely located entangled ions, but also in quantum communication and cryptography.

In the experiment, the researchers used a polarized 214.5-nm laser pulse to initialize a single cadmium ion held in a radio-frequency trap to a particular hyperfine ground state. A second pulse then excited the ion to a higher state, from which it decayed and emitted a photon. After a microwave rotation pulse prepared the ion for measurement, a third laser pulse was used to detect the internal state of the atom.

To observe and quantify entanglement, the researchers compared the polarization of the emitted photon and the state of the ion. Over approximately 1000 trials, the system demonstrated an entanglement fidelity of at least 0.87.


GLOSSARY
photon
A quantum of electromagnetic energy of a single mode; i.e., a single wavelength, direction and polarization. As a unit of energy, each photon equals hn, h being Planck's constant and n, the frequency of the propagating electromagnetic wave. The momentum of the photon in the direction of propagation is hn/c, c being the speed of light.
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