In a recent post to his blog, David Deutsch, one of the founders of the field of quantum computing, claims that he now believes quantum computers are only years away. He says that while he previously believed quantum computing to be decades away, recent theoretical developments have changed his view. In particular, he cites the discovery of *cluster state* quantum computing as being a major theoretical breakthrough.

Cluster state quantum computing is a different way of thinking about quantum computing. In the standard model for quantum computing, we construct algorithms using circuits, much like in classical electronics or computer science. The cluster state model, on the other hand, is completely different. Instead of constructing a circuit, we prepare a large entangled state. Once this state is prepared, we simply perform a sequence of measurements on the state. The choice of measurements we perform implements the algorithm. This may seem rather uninteresting, but this model for quantum computation has numerous benefits over the standard circuit model. In particular, in many schemes, including linear optics quantum computing (my field of research), using the cluster state model massively reduces the physical requirements for building a quantum computer.

While I agree completely with Deutsch that cluster state quantum computing has been extremely positive, and represent a major theoretical development, I completely disagree with his view that, as a result, quantum computers are just around the corner. The reason for this is that while the cluster state model reduces the physical requirements to implement a quantum computer, it does not obviate the exquisite level of control which is required over microscopic systems. Quantum systems are extremely sensitive to noise, and the margins for error are extremely low. Unfortunately, in all of the major candidate systems for implementing quantum computing, achieving this level of control is still a major obstacle, and will likely remain so for quite a while to come.

Having said all this, I sincerely hope that I am wrong and Deutsch is right.