Measuring the entangled Bell and GHZ aspects using a single-qubit   shuttle
  A complete, non-demolition procedure is established for measuring multi-qubit entangled states, such as the Bell-states and the GHZ-states, which is essential in certain processes of quantum communication, computation, and teleportation. No interaction between the individual parts of the entangled system, nor with any environment is required. A small probe (e.g. a single qubit) takes care of all interaction with the system, and is used repeatedly. The probe-qubit interaction is of the simplest form, and only this one type of interaction is required to perform a complete measurement. The process may be divided into elementary local operations and interactions, taking place sequentially as the probe visits each of the qubits. A shuttle mode is described, which may be repeated indefinitely. By the quantum Zeno effect, the entangled states can be maintained until released in a predictable state. This shuttle process is stable, and self-correcting, by virtue of the standard measurements performed repeatedly on the probe. 
Ulf Larsen