Managing Multistate Quantum Entanglement

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Science  14 May 2010:
Vol. 328, Issue 5980, pp. 835-836
DOI: 10.1126/science.1190491

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Good experiments minimally perturb the sample under study, but in the quantum realm, measurement cannot be separated from the system. Before the measurement, all possible outcomes can form an entangled state; measurement then causes this entangled state to collapse to one outcome. In 1935, Schrödinger presented a thought experiment to show how strange this situation really is. A cat is hidden in a box along with a sample of radioactive nuclei. If a nuclear decay occurs, poison is released and the poor cat dies. However, until we look in the box, the state of the cat entangles both a live and a dead cat. Experiments with Schrödinger-cat states that entangle several particles continue to provide deep insights into the measurement process. One such state, called a NOON state, entangles a fixed number of photons N, all of which are in one of two possible states (if these were coins, they would be all heads or all tails). On page 879 of this issue, Afek et al. (1) achieved a record by making NOON states in an interferometer with five entangled photons. Their approach may have implications for other implementations of interferometry, such as gravity wave detectors.