Are Quantum Computing Models Realistic?

The commonly used circuit model of quantum computing leaves out the problems of state preparation (how to get the individual particles into a specific state while ensuring that the particles remain entangled), particle statistics (indistinguishability of quantum particles belonging to the same coherent state), and error correction (current techniques cannot correct all small errors). The initial state in the circuit model computation is a mixture whereas useful quantum computation requires a coherent state. We review some limitations of the circuit model and speculate on the question if a hierarchy of quantum-type computing models exists.