This story is a supplement to the feature "Quantum Computing with Ions" which was printed in the August 2008 issue of Scientific American.

An alternative approach to trapped-ion computing is to link the ions using the photons they emit. Two remotely located trapped ions (purple), each isolated in a vacuum tube (photograph left), are excited with laser pulses and emit photons into optical fibers. The frequencies of the photons depend on the magnetic orientation of the ions; a photon emitted by an ion in a 50–50 superposition state—half up, half down—would be in a superposition of frequencies (half red and half blue in this example). If the photons from the two ions are in the same state, the beam splitter will shunt both to one photodetector. But if the photons are in different states, they will travel to separate detectors. Once that happens, the ions are entangled, because researchers cannot tell which ion has emitted which photon.


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Researchers are excited about the photon-linking approach because it offers a relatively easy way to connect large numbers of ions. Laser beams could be directed at an array of trapped ions, and the emitted photons could travel to a bank of beam splitters. A charge-coupled device (CCD) camera could easily detect when any two ions become entangled, and each entanglement would increase the processing power of the trapped-ion computer.