  A quantum computer promises efficient processing of certain computational
tasks that are intractable with classical computer technology. While basic
principles of a quantum computer have been demonstrated in the laboratory,
scalability of these systems to a large number of qubits, essential for
practical applications such as the Shor algorithm, represents a formidable
challenge. Most of the current experiments are designed to implement sequences
of highly controlled interactions between selected particles (qubits), thereby
following models of a quantum computer as a (sequential) network of quantum
logic gates. Here we propose a different model of a scalable quantum computer.
In our model, the entire resource for the quantum computation is provided
initially in form of a specific entangled state (a so-called cluster state) of
a large number of qubits. Information is then written onto the cluster,
processed, and read out form the cluster by one-particle measurements only. The
entangled state of the cluster thus serves as a universal substrate for any
quantum computation. Cluster states can be created efficiently in any system
with a quantum Ising-type interaction (at very low temperatures) between
two-state particles in a lattice configuration.
