Fun_People Archive
16 Feb
Exciting Happenings in the Quantum Realms

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From: Peter Langston <psl>
Date: Tue, 16 Feb 99 15:20:32 -0800
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Subject: Exciting Happenings in the Quantum Realms

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Excerpted-from: PHYSICS NEWS UPDATE #414
    The American Institute of Physics Bulletin of Physics News
  Number 414 February 11, 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,
harald.weinfurter@uibk.ac.at, 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 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 Feb.)

© 1999 Peter Langston