Number 754, November 16, 2005
by Phil Schewe and Ben Stein
Hyper-Entangled Photon Pairs
Physicists at the University of
Illinois at Urbana-Champaign have demonstrated for the first time the entanglement of
two objects not merely in one aspect of their quantum natures, such
as spin, but in a multitude of ways.
Entanglement is the quantum
affinity between or among particles (such as atoms or photons) in
which the measurement of some property for one particle
automatically and instantaneously determines the corresponding
property of the other particle.
Take the case of two photons
entangled with respect to polarization, the orientation of the
electric field associated with the photon.
The photons, until detected, have no spin orientation; this is the
principle of quantum indeterminacy. Indeed, both photons are said
to be in a superposition of arbitrary -- but
parallel -- polarization states. Consequently, each photon has a
50 percent likelihood of being measured to have any polarization
state -- e.g., +45 or -45 degrees. If now one photon's polarization
is measured to be +45, then its entangled twin will surely also be
polarized along +45, owing to the way the photons are made in this
setup.
One of the chief hopes of entanglement research is to
exploit the superposition idea and the entanglement idea for
performing unusually fast quantum computation.
In the Illinois experiment, two photons, produced in a
"down-conversion" process whereby one photon enters an optical
crystal and sunders into two lesser-energy correlated daughter
photons, are entangled not just in terms of polarization, but also
in a number of other ways: energy, momentum, and orbital angular
momentum (see PNU 721).
Actually, the photon pair can be produced in either of two crystals,
and the uncertainty in the production details of the individual
photons is what provides the ability to attain entanglement in all
degrees of freedom.
Is it better to entangle two particles in ten
ways or ten particles in two ways? They're probably equivalent,
says Paul Kwiat, leader of the Illinois group, but for the purpose
of quantum computing or communication it might be of some advantage
if multiple quantum bits (or qubits) of information can be encoded
in a single pair of entangled particles. Kwiat (217-333-9116,
kwiat@uiuc.edu) says that his lab detects a record two million
entangled photon pairs per second with ample determination of
numerous properties, allowing a complete characterization of the
entanglement produced.
Human singers
send their voice into the supporting medium of air. Whales send
their songs into ocean water. One particular song, a sort of
fluttering echo, or "boing," sound first heard by human listeners in
the North Pacific Ocean in the 1950s (and recorded by US Navy
submarines) baffled scientists. Where was it coming from? Only now
have the sounds been identified as coming from minke whales.
Shannon Rankin and Jay Barlow, scientists at the National Marine
Fisheries Service in La Jolla, California, have gathered hydrophone
data in the body of ocean between Mexico and Hawaii and combined
this with visual sightings of the marine mammals. Not only has the
source been traced to minke whales, but the songs seem to be
somewhat different on either side of a certain longitude.
To the
east, the boing sound is issued at a frequency of about 92 Hz and an
average duration of 3.6 seconds. The west boing, by contrast,
consists of a 135-Hz vocalization with a duration of about 2.6
seconds. The acoustic trace is both frequency modulated (FM) and
amplitude modulated (AM).
Scientists at the Ruhr-Universität Bochum in
Germany have performed high-precision, ultracold chemical studies of
nitrogen oxide (NO) molecules by inserting them into droplets of
liquid helium (see figure).
NO,
Science magazine's "molecule of the year" for 1992, is important
because of its role in atmospheric chemistry and in signal
transduction in biology. A radical is a molecular entity
(sometimes charged and sometimes neutral) which enters into chemical
reactions as a unit. To sharpen our understanding of this important
molecule and its reactions, it would be desirable to cool it down,
the better to observe its complex spectra of quantum levels
corresponding to various vibrational and rotational states.
In the
new experiment, liquid helium is shot from a cold nozzle into
vacuum. The resultant balls, each containing about 3,000 atoms, are
allowed to fall into a pipe where NO molecules are lurking. The NO
is totally enveloped and, within its superfluid-helium cocoon at a
temperature of about 0.4 Kelvin, it spins freely. The helium acts
provides a cold environment but does not interact chemically with
the NO molecules. Because of this a high-resolution infrared
spectrum of NO in fluids could be recorded for the first time.
NO
has been observed before in the gas phase, but never before has such
a high resolution spectrum be seen in the helium environment.
Haeften et al.,
Physical Review Letters, 18 November 2005
Contact Martina Havenith,
martina.havenith@ruhr-uni-bochum.de
The Havenith lab's Web site
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