OK. I confess. I had a problem with quantum computation. Much as I tried to follow the rapid developments in the field, my brain just seemed to max out, to refuse to contemplate the possibilities.

I don't think it was just the conceptual difficulty of qubits, superpositions, and entanglement, of quantum logic operations and computational algorithms. There was also the question, should quantum information processing systems be realized, what will they be used for? Secure communications for protecting bank transactions for sure, but is there anything else?

But mostly I stumbled over the huge challenge of realizing scalable, manufacturable, quantum computation systems. Will it really be possible to control single qubits in the solid state, to couple them in quantum logic gates, and all at room temperature with quantum states hanging around long enough to do anything with?

I have now been persuaded that the answer is yes, thanks to presentations at the recent American Physical Society March Meeting in New Orleans. It's all down to the unique properties of that girl's best friend, diamond.

Many of the advantages of diamond for quantum computation result from one particular type of impurity, explained David D. Awschalom of the University of California, Santa Barbara. A nitrogen-vacancy (N-V) center consists of a N atom in place of a C atom on the diamond lattice next to a vacancy where another C atom would be. An electron at an N-V site can act as a qubit, with a spin state that can be polarized using visible light. Furthermore, one of the N-V spin states fluoresces more brightly than the other, giving optical readout for free: spin state ‘1’ is bright and state ‘0’ is dim. Ronald Hanson of Delft University of Technology in The Netherlands has demonstrated that the spin states of individual N-V centers can indeed be initialized and read out optically. In between, radiofrequency radiation can be used to flip the spins up and down between ‘0’ and ‘1’ states and create superposition states. Crucially, this is all possible at room temperature, where spin states are also remarkably stable with coherence times of up to 1 ms.

To carry out operations, individual spins will have to be coupled. The splitting of spin states at N-V sites is sensitive to the state of neighboring N impurities, which could allow a logic gate to be developed. But in future computers, it will also be necessary to couple distant qubits using photons. Charles Santori at Hewlett-Packard Laboratories and Steven Prawer of the University of Melbourne, Australia are each working on photonic diamond microstructures to get strong coupling between spins and a confined photon field.

There is much to be done to put this all together. But I have been convinced of the promise of diamond-based quantum computing and I am excited about the prospects.

Read full text on ScienceDirect

DOI: 10.1016/S1369-7021(08)70068-3