On Thursday I attended a seminar by Simon Webster from the Ion trap QC group in Oxford. I didn’t realise that ion trap QC was so advanced. Having spent so much time in the happy world of LSI Josephson logic, I had a prehistoric picture of an ion trap being a large metallic cavity surrounded by huge electrodes similar to plasma confinement systems. But no, it can be done to micrometer precision on chip with ions trapped in tiny channels. You can shuttle individual ions, or little chains of them around the chip, allow them to interact and evolve to perform the computation, and then move them elsewhere, or read them out.
Alas, it is still only possible to manipulate a couple of qubits at the moment, as is the case with most QC realizations. The qubits themselves are formed by manipulating transitions between energy levels of the ions. In this case, Ca+ ions. Entanglement can occur for example between the ion and the photon emitted during a relaxation from an excited state. Therefore one advantage of Ion trap QC is that it is natively good at handling static (ionic) and flying (photonic) qubits with the same technology, and quantum information can therefore be transferred over long distances and on/off chip quite easily.
Exciting stuff. I’ve still got my money on Josephson junctions, but competition in experimental QC is healthy 🙂