Fivephoton entanglement achieved by physicists at Hefei National Laboratory for Physical Sciences at the Microscalethe(HFNL), University of Science and Technology in China (USTC), contributs to one of top stories in physics in 2004 slected by the American Institute of Physics (AIP).
Entanglement is perhaps the weirdest of all aspects of quantum behavior. If several particles are entangled, this means that they participate in a single quantum state which can be in several unique states at the same time. Furthermore, the measurable properties of the particles, such as their spins, will be correlated, even if subsequently the particles are located at great distances from each other and the properties measured separately. Previously the greatest degree of full quantum entanglement came in experiments involving four particles.
The USTC researchers entangle two pairs of photons, and then entangle these with yet another single photon. (Zhao et al., Nature, 1 July 2004.) The progress from four to five entangled particles is significant since apparently the handling of quantum information (such as in a quantum computer) with a builtin error correction process would require the manipulation of five entangled particle engineered to serve as qubits (see, for example, Laflamme et al., Physical Review Letters, 1 July 1996.)
Experimental demonstration of fivephoton entanglement and opendestination teleportation
Nature 430, 5458 (1 July 2004)  doi:10.1038/nature02643; Received 12 February 2004; Accepted 10 May 2004
Quantummechanical entanglement of three or four particles has been achieved experimentally, and has been used to demonstrate the extreme contradiction between quantum mechanics and local realism. However, the realization of fiveparticle entanglement remains an experimental challenge. The ability to manipulate the entanglement of five or more particles is required for universal quantum error correction. Another key process in distributed quantum information processing, similar to encoding and decoding, is a teleportation protocol that we term 'opendestination' teleportation. An unknown quantum state of a single particle is teleported onto a superposition of N particles; at a later stage, this teleported state can be read out (for further applications) at any of the N particles, by a projection measurement on the remaining particles. Here we report a proofofprinciple demonstration of fivephoton entanglement and opendestination teleportation (for N = 3). In the experiment, we use two entangled photon pairs to generate a fourphoton entangled state, which is then combined with a singlephoton state. Our experimental methods can be used for investigations of measurementbased quantum computation and multiparty quantum communication.
Full text Download PDF
