  Entanglement are the non-local correlations permitted by quantum theory,
believed to play a fundamental role in a quantum computer. We have investigated
these correlations in a number of theoretical models for condensed matter
systems. Such systems are likely candidates for quantum computing, and
experimentally feasible for instance as superconducting qubits. At quantum
critical points the ground state of these systems is very complicated, and the
entanglement is usually larger than at non-critical points. This entanglement
can be used to identify the critical points through what we denote the
entanglement signature, even for very small systems. From another perspective,
it seems that the entanglement is an essential tool to find an unknown ground
state, since this gives rise to a simple decomposition of the state.
