In the quest to a build quantum processor, we are still at the point of investigating several different systems. You see, no “holy grail” system as been found yet. There is always a small caveat. Example: nuclear magnetic resonance is tremendously good at controlling the state of the qubits and has low decoherence (the nasty tendency of quantum systems to lose their “quantumness”), but its scalability is jeopardized by the low polarization of the nuclear spins. Another promising system, electron spins in quantum dots, has a strong scalability potential, but work still needs to be done to contain decoherence (though major improvements have been made). Also, photons do not decohere much, but entanglement is challenging to generate (but again, new discoveries are made frequently). For uninitiated readers, quantum entanglement is the ability of multiple quantum systems to share very strong correlations, but that would be the subject of another post entirely…
Trapped ions are contenders — along with the quantum dots, photons, superconducting nanocircuits and few others — as the bases of a scalable system. Many interesting and significant results have been demonstrated in the past few years, including the experimental realization of quantum error correction, quantum simulation and steady improvement in the control of qubits.
One of the biggest challenges is to create and keep large entangled states. Decoherence really doesn’t like entanglement, and conspires to destroy it whenever possible. In 2005, the trapped ion group in Austria led by Dr. Rainer Blatt demonstrated the generation of entanglement in an 8-qubit ion trap system — a world record at the time and quite an achievement.
Recently, while reading Physical Review Letters, I was impressed to see that they have cranked up that number to 14 entangled qubits! To learn the details of the experiment, visit PRL 106 130506 (2011). Although I’m exited by this result, I should comment that it is still a little too early to conclude that we now have a 14-qubit quantum processor. You see, while they are able to create specific entangled states, you must have the ability to produce any entangled state but in order to perform universal quantum computation in the circuit model.
Nonetheless, this kind of result can be paramount in the study of entanglement decoherence in larger systems (which they also did in their article) and the type of entangled state they created is at the heart of building the next generation of measurement and sensing tools.