Number 546, July 5, 2001 by Phil Schewe,
James Riordon, and Ben Stein
Watching Optical Tornadoes Reverse Their Spin
From water whirlpools to meteorological tornadoes to superfluid Bose-Einstein
condensates, vortices abound in nature. But it is difficult, if not
impossible, to reverse the direction of rotation in a vortex without
first destroying it.
Now, a Barcelona-Arizona collaboration (Gabriel Molina-Terriza, Technical
University of Catalonia, Spain, Molina@tsc.upc.es)
has observed in detail for the first time a reversal in the spin of
an optical vortex, a specially prepared light beam with a central dark
core.
Studying the reversal of spin in this relatively simple type of vortex
may provide powerful insights into other vortices and whether they too
can reverse direction. Around the dark eye of an optical tornado, the
energy carried by the light beam flows like a spiral staircase, in a
clockwise or counterclockwise direction. Researchers in the last decade
have built devices to reverse the spin of an optical vortex, but they
have not observed what happens during the reversal.
Now, the researchers employ a trick both to reverse and observe the
optical vortex: they pass it through a cylindrical lens. As the vortex
travels beyond the lens, its once-spherical core elongates like putty
until it is a vanishingly thin line. As the vortex moves farther beyond
the lens, the core eventually compresses itself into an ellipse but
the energy around it spins in an opposite direction (see figures at
Physics News Graphics).
These optical maelstroms can potentially carry several channels of
quantum data for such applications as quantum entanglement and teleportation,
and they can serve as optical tweezers for holding and rotating microscopic
objects. They can also shed light on vortex behavior in Bose-Einstein
condensates, since both optical and BEC vortices are described by similar
equations. The researchers' observations with light suggest that BECs
with weakly interacting atoms may have vortices whose spins constantly
reverse direction. (Molina-Terriza et al., Phys. Rev. Lett.,
9 July 2001; text at Physics
News Select.)
Chains of individual gold atoms exhibit bonds that are about twice
as strong as bonds between atoms in bulk gold, according to the quantum
theory. Now researchers have directly measured the strength of gold
chains, as well as other mechanical properties, by stretching strings
of the precious atoms between two scanning tunneling microscope (STM)
probes (also see Update
455).
In a collaboration between researchers from the Universidad Autónoma
in Madrid and the Technical University of Denmark in Lyngby, Gabino
Rubio-Bollinger, Karsten Jacobsen (kwj@fysik.dtu.dk,
011-4545-253186), and colleagues drew chains of up to seven atoms out
of gold electrodes. By monitoring the conductance of the chains and
the tension between the STM probes, the researchers could observe chain
growth as atoms popped out of the electrodes at one end or the other
and joined the atomic strand, until the chains finally broke under the
strain (see figure at Physics
News Graphics).
In addition to confirming theoretical predictions of bond strengths,
the study shows that the atomic chains have electrical conductance very
close to the smallest value permitted by quantum mechanics, and that
the side-to-side chain stiffness is strongly affected by the atomic
arrangement at the locations where they are anchored to the electrodes.
While it's not yet clear that gold atom chains will have any practical
use, the study is an example of engineering analysis on the very smallest
scale. As technologies at tiny dimensions progress toward practical
micro- and nano-sized devices, such mechanical tests will become crucial
to developing minuscule structures consisting of small numbers of molecules
or atoms. (G. Rubio-Bollinger et al, Physical Review Letters,
9 July 2001; text at Physics
News Select.)
The Microwave Anisotropy Probe (MAP) telescope has been successfully
launched in the direction of its Lagrangian-point orbit where, poised
1.5 million km from Earth, its solar array will also block light from
Sun, Moon, and Earth from entering the sensitive detector designed to
give the best map yet of the cosmic microwave background by the end
of its 2-year mission (background
article: Science,
22 June).
Enlarge text
Shrink text
Print
Digg this
Del.icio.us
Furl