Number 722, March 3, 2005
by Phil Schewe and Ben Stein
240 Electrons Set in Motion
A soccerball-shaped carbon-60 molecule, possessing a mobile team of
up to about 240 valence electrons holding the structure together, is
sort of halfway between being a molecule and a solid. To explore how
all those electrons can move as an ensemble, a team of scientists working
at the Advanced Light Source synchrotron radiation lab in Berkeley,
turned the C-60 molecules into a beam (by first ionizing them) and then
shot ultraviolet photons at them. When a photon absorbed, the energy
can be converted into a collective movement of the electrons referred
to as a plasmon.
Previously a 20-electron-volt “surface plasmon” was
observed: the absorption of the UV energy resulted in a systematic oscillation
of the ensemble of electrons visualized as a thin sphere of electric
charge. Now a new experiment has found evidence of a second resonance
at an energy of 40 eV. This second type of collective excitation is
considered a “volume plasmon” since the shape of the collective electron
ensemble is thought to be oscillating with respect to the center of
the molecule (see figure at http://www.aip.org/png/2005/230.htm).
The collaboration consists of physicists from the University of Nevada,
Reno (Ronald Phaneuf, 775-784-6818, phaneuf@unr.edu), Lawrence Berkeley
National Lab, Justus-Liebig-University (Giessen, Germany), and the Max
Planck Institute (Dresden). (Scullyet al., Physical Review Letters, 18 February 2005)
First Evidence For Entanglement of Three Macroscopic Objects
First evidence for entanglement of three macroscopic objects has been
seen in a superconducting circuit built at the University of Maryland.
By examining an electrical circuit operating at temperatures near absolute
zero, the researchers have found new evidence that the laws of quantum
mechanics apply not just to microscopic particles such as atoms and
electrons, but also to large electronic devices called superconducting
quantum bits (qubits).
While researchers have previously created superconducting
qubits, and other groups have entangled two macroscopic objects (Update
558), this research is the first to observe the quantum interaction
of three macroscopic components: a niobium inductor-capacitor (LC) circuit
plus a pair of Josephson junctions, each a sandwich of two superconductors
separated by an insulator. Remarkably, all three macroscopic devices
seem to act, when cold enough, like huge atoms. The LC circuit coupled
the Josephson junctions in such a way as to transfer quantized oscillations
of current in one junction to the other junction. The LC circuit was
more than a simple connector; its condition depended upon the two Josephson
junctions in a way defined by the laws of quantum mechanics.
The researchers
obtained evidence of the entanglement indirectly, through the use of
microwave pulses that probed the Josephson junctions; future experiments
will seek to directly control the junctions and obtain evidence more
directly. Superconducting circuits such as this one provide a promising
route towards a practical quantum computer, which would require the
entanglement of many qubits.
Scaling up superconducting devices to many-qubit
systems should be possible once single superconducting qubits are perfected,
according to team member Frederick Strauch, (now at NIST, 301-975-5159,
Frederick.Strauch@nist.gov). The challenge will be to fabricate sufficiently
high-quality circuits so that the superconducting qubits achieve the
very low noise levels necessary for quantum computing. (Xuet al., Physical Review Letters, 21 January 2005)
X-Ray Thunderbolt
Scientists have long suspected that lightning might generate x rays.
However, until recently the observation of such x-rays has remained
elusive, largely owing to the unpredictable nature of lightning. In
the last few years a series of experiments by Joseph Dwyer and his colleagues
at the Florida Institute of Technology and the University of Florida
has shown that lightning indeed emits large bursts of x rays with energies
up to about 250 keV (about twice that of a chest x ray).
These x rays
are mostly produced not by the bright return strokes, but by the leaders
that precede the stroke, as they propagate from the cloud to the ground.
Now, Dwyer and his colleagues have discovered that these bursts of x
rays are produced at the precise moment that the lightning steps forward
along its jagged path. For unknown reasons, lightning does not travel
to the ground in a continuous manner, but instead traverses the distance
in a series of discrete steps.
It is this stepping process that gives
lightning its jagged, sometimes forked, appearance, and Dwyer has now
shown that this same stepping process also makes x rays. The x rays
are likely produced by strong electric fields that accelerate electrons
to close to the speed of light. These so-called runaway electrons collide
with air producing bremsstrahlung ("braking radiation" in German) x-rays.
Dwyer says that higher energy gamma rays are also emitted sometimes,
but that these seem to come from the thunderstorm cloud itself and not
from the lightning stroke. (Dwyeret al., Geophysical Review Letters, 15 January 2005.)
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