From: LARRY KLAES (ljk4_at_msn.com)
Date: Thu Apr 08 2004 - 11:19:41 PDT
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From: physnews_at_aip.org<mailto:physnews_at_aip.org>
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Sent: Thursday, April 08, 2004 1:56 PM
Subject: Physics News Update 680
PHYSICS NEWS UPDATE
The American Institute of Physics Bulletin of Physics News
Number 680 April 8, 2004 by Phillip F. Schewe, Ben Stein
MRI WITH 80-NM RESOLUTION, far better than for the best medical
ENTANGLEMENT BETWEEN A PHOTON AND A TRAPPED ATOM has been directly
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scans, has been achieved with a device that combines atomic force
microscope (AFM) and nuclear magnetic resonance (NMR; also known as
magnetic resonance imaging, or MRI) technology. In the hybrid
methodology called magnetic resonance force microscopy (MRFM), a
tiny magnetized particle is attached to a cantilever which is then
brought near a sample which surrounded by a coil that emits radio
waves. When a tiny magnetic domain in the sample feels just the
right amount of magnetic field from the nearby coil and magnetic
particle it will vigorously interact with them resonantly. (The
tiny volume being probed is referred to as a voxel, and the
sample-coil-particle combination is equivalent to the setup in a
standard MRI machine for imaging, say, a tumor.) The
sample-particle resonant interaction causes the cantilever to
oscillate (the particle on the cantilever is like a man bouncing
resonantly, higher and higher, on a diving board). The oscillating
cantilever, monitored with a laser beam, is then scanned from place
to place, filling out a two-dimensional and then a three-dimensional
map of the resonant interaction. (The scanned, oscillating
cantilever plus laser readout is the AFM part of the setup.) The
goal is not to help surgeons (the best medical MRI has a spatial
resolution of about a tenth of a millimeter) but to be able to scan
and image small objects---especially particles of biological
importance, such as viruses and proteins---with atomic-scale
resolution. In other words, you would like to increase the
sensitivity so as to map the presence of single spins. The voxel in
this case would be shrunk to less that than 1 nm.
A new experiment at the University of Washington is far from
reaching this goal, but researchers have improved sensitivity by a
factor of almost 10,000 from the time of the earliest MRFM imaging
papers in 1996. (For a report from 1997, see
http://www.aip.org/enews/physnews/1997/split/pnu313-1.htm
The higher sensitivity in general comes by shrink the apparatus and
cooling things (currently, to 80 K) as much as possible, the better
to read out the oscillations and position the sample with greater
accuracy. The Washington voxel of 80 nm---how big is it? One of
the team members, John Sidles (206-543-3690, s
idles_at_u.washington.edu<mailto:idles_at_u.washington.edu>) says that about a million of these voxels
could fit inside a typical blood cell. (Chao, Dougherty, Garbini,
Sidles, Review of Scientific Instruments, May 2004; website,
courses.washington.edu/goodall/MRFM ) Other groups are working in
this area and are attempting to marshal the requisite equipment
needed for single-spin imaging. According to Joseph Shih-hui Chao,
one of the authors, this would include millikelvin temperatures,
30-nm-sized magnetic particles, sub-nm positioning accuracy, and
yet softer cantilevers.
observed for the first time, offering a method for establishing
links between quantum memories over appreciable distances.
Entanglement--a sort of arranged marriage between two or more
particles--has usually been directly measured between species of the
same kind, such as all photons or all atoms. In recent experiments,
however, University of Michigan researchers achieve inter-species
entanglement by trapping a cadmium ion with electric fields. They
put the trapped cadmium's outer electron into an excited
(high-energy) state. The atom immediately decays to one of two
ground (low-energy) states--let's call them A and B--while emitting
a photon. State A represents the case in which the spin of the
atom's outer electron is lined up with the spin of the atom's
nucleus; B represents the case in which the electron's spin is
opposite to that of the nucleus. The photon's polarization--the
direction of its electric field--correlates with the resulting
ground state of the atom. In other words, if the atom decays to
state A, the photon's electric field rotates clockwise, and if it
decays to state B, counterclockwise.
Because each path is equally likely, quantum mechanics forces us to
consider both decay routes as occurring at the same time. So once
the atom decays, both it and the photon essentially carry out both
possibilities--each enters a "superposition" of two states.
Meanwhile, their properties remain interdependent--or
correlated--with each another. As a result, the atom and photon are
in an entangled superposition. While the individual participants
are in fuzzy, unresolved states, the terms of their marriage are
perfectly defined. However, measuring the photon--the act of
observing it--forces the photon to make a commitment. Upon
measurement it must assume one polarization state or
another--clockwise or counterclockwise. And this in turn forces the
atom to collapse into state A (if the polarization is clockwise) or
state B (if polarization is counterclockwise). One could conduct
powerful logic operations based on these interdependencies. This
cross-species entanglement technique has shortcomings--researchers
cannot actively create an entangled state but must wait for it to
occur by detecting the photon, so the entanglement is immediately
destroyed and efficiency is not high. However, if two remotely
located trapped atoms simultaneously decay in the same way as
reported in this experiment, and the two emitted photons are jointly
detected after interfering on a beamsplitter, then the two atoms
become entangled and available for subsequent use for long-distance
quantum computing and quantum communication. (Blinov et al., Nature,
11 March 2004; contact Chris Monroe, crmonroe_at_umich.edu<mailto:crmonroe_at_umich.edu>)
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