Number 414, February 12, 1999 by Phillip F. Schewe and Ben Stein
THE FIRST ENTANGLEMENT OF THREE PHOTONS has been experimentally demonstrated by researchers at the University of Innsbruck (contact Harald Weinfurter, firstname.lastname@example.org, 011-43-512-507-6316). Individually, an entangled particle has properties (such as momentum) that are indeterminate and undefined until the particle is measured or otherwise disturbed. Measuring one entangled particle, however, defines its properties and seems to influence the properties of its partner or partners instantaneously, even if they are light years apart. In the present experiment, sending individual photons through a special crystal sometimes converted a photon into two pairs of entangled photons. After detecting a "trigger" photon, and interfering two of the three others in a beamsplitter, it became impossible to determine which photon came from which entangled pair. As a result, the respective properties of the three remaining photons were indeterminate, which is one way of saying that they were entangled (the first such observation for three physically separated particles). The researchers deduced that this entangled state is the long-coveted GHZ state proposed by physicists Daniel Greenberger, Michael Horne, and Anton Zeilinger in the late 1980s. In addition to facilitating more advanced forms of quantum cryptography, the GHZ state will help provide a nonstatistical test of the foundations of quantum mechanics. Albert Einstein, troubled by some implications of quantum science, believed that any rational description of nature is incomplete unless it is both a local and realistic theory: "realism" refers to the idea that a particle has properties that exist even before they are measured, and "locality" means that measuring one particle cannot affect the properties of another, physically separated particle faster than the speed of light. But quantum mechanics states that realism, locality--or both--must be violated. Previous experiments have provided highly convincing evidence against local realism, but these "Bell's inequalities" tests (see Update 399) require the measurement of many pairs of entangled photons to build up a body of statistical evidence against the idea. In contrast, studying a single set of properties in the GHZ particles (not yet reported) could verify the predictions of quantum mechanics while contradicting those of local realism. (Bouwmeester et al., Physical Review Letters, 15 February 1999)
A NEW ONLINE EXHIBIT DEVOTED TO WERNER HEISENBERG traces the birth of quantum mechanics, the wartime effort to build a German atom bomb, and other episodes from a remarkable life. Prepared by leading Heisenberg biographer David Cassidy, the exhibit is available now on the website www.aip.org/history/heisenberg of the Center for History of Physics, the premier clearinghouse of physics-related archived papers, photos, and taped interviews (3000 hours' worth). Located at the American Institute of Physics in College Park, Maryland, the Center houses the Niels Bohr Library (email@example.com, 301-209- 3184), strong in books from the mid-19th century to the present, and the Emilio Segre Visual Archives (containing 25,000 items). The Center itself possesses several valuable collections of papers and provides support to other institutions in their efforts to archive the papers of important physicists (contact Spencer Weart or Joe Anderson at firstname.lastname@example.org). The Center has a prominent place on the Internet (www.aip.org/history), where it maintains the International Catalog of Sources for History of Physics and Allied Sciences. In addition to the Heisenberg site, the Center website is also home to two other widely popular exhibits, one devoted to Albert Einstein and one to JJ Thomson's discovery of the electron. Soon an exhibit devoted to Andrei Sakharov will also be available.