PW_mar13_sample_issue

Frontiers
In brief
Diamond downsizes MRI and NMR
Magnetic-resonance-imaging technology
has been shrunk to the nanoscale by two
independent teams of researchers in Germany
and the US, so that molecular samples just a
few cubic nanometres in volume can now be
detected and imaged at room temperature. Both
groups used nitrogen-vacancy (NV) defects in
diamonds as magnetic-field sensors to probe
such minute samples. NV defects occur when
two neighbouring carbon atoms in diamonds
are replaced by a nitrogen atom and an empty
lattice site. NV sites are capable of detecting the
very weak oscillatory magnetic fields that come
from the spins of protons in a sample. Apart
from being able to resolve a single atom at room
temperature, the technique could be used as a
polarizing agent for traditional NMR and could
also help the nanotechnology community image
tiny devices (Science 339 557; 339 561).
‘Just add water’ for hydrogen on demand
Silicon nanoparticles could be used to produce
hydrogen almost instantly, as they react with
water, according to researchers in the US. The
reaction does not require any heat, light or
electricity and the hydrogen generated could
be used to power small fuel cells. In essence,
the technique recovers some of the energy that
goes into refining the silicon and producing the
nanoparticles in the first place. Thanks to their
high surface-to-volume ratio, the nanoparticles
should naturally generate hydrogen much more
quickly than bulk silicon. The advantage of
silicon is that it is abundant on our planet, has
a high energy density and does not release any
carbon dioxide when it reacts with water. The
researchers have already successfully tested
their technique in a small fuel cell that they used
to power a fan (Nano Lett. 10.1021/nl304680w).
Stored photons interact in atom cloud
Physicists in the UK have come up with a
new way of storing a handful of photons in an
ultracold atomic gas, in which strong interactions
between neighbouring photons can be switched
on and off using microwaves. Once stored, the
photons can be made to interact strongly, before
being released again. An important feature of
the technique is that it uses microwaves, which
are also used to control some types of stationary
qubit. The team believes that the technique
could be used to create optical logic gates in
which single photons could be processed one at
a time. The method could also prove useful for
connecting quantum-computing devices based
on different technologies (arXiv:1207.6007v3).
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4
A new method to produce indistinguishable
and coherent electrons has been developed
by scientists in France. They have
used it to make a small, electron-emitting
chip that can produce two single electrons
emitted from different sources that are in
the same quantum state. This is a key step
in developing electron-based quantum-
information-processing techniques.
Electrons are fermions and so must obey
Pauli’s exclusion principle, which prevents
identical fermions from occupying the
same state, and so leads to anticorrelations
or “antibunching”. Erwann Bocquillon and
Gwendal Fève at the Ecole Normale Supérieure
in Paris and Lyon, along with colleagues
from the Laboratory for Photonics
and Nanostructures, Paris, wanted to see if
indistinguishable electrons could be generated
by independent sources. But as there
are many electrons in any system, and they
all interfere with each other and with the
environment, making coherent electron
beams is difficult.
The researchers’ electron-emitting chip
physicsworld.com
Emitting indistinguishable electrons
emitter 1
beam splitter
Neutrons on demand
emitter 2
Coherent beams A chip off the semiconductor dot.
A new compact high-flux source of energetic
neutrons has been built by physicists
in Germany and the US. The laser-based
device has the potential to be cheaper and
more convenient than the large neutron
facilities currently used by scientists and
could be housed in university laboratories.
Built by Markus Roth of the Technische
Universität Darmstadt and colleagues at
Los Alamos and Sandia national laboratories,
the device builds on previous research
carried out at Los Alamos in 2006, which
used computer simulations to show that
an intense laser beam can penetrate a thin
solid target, producing the necessary highenergy
neutron flux. Roth’s team directed
extremely powerful and well-defined
pulses from the Los Alamos TRIDENT
laser onto a 400 nm-thick plastic target
D Darson
(pictured) was built using a “very clean”
micron-sized bulk-semiconductor sample
in which the electrons propagate in straight
lines for several microns in 2D before being
scattered, limiting their interactions. A
strong magnetic field is then used to further
restrict the movement of the electrons
to only 1D so that single electrons may be
guided to each of the emitters. By applying a
voltage pulse to metallic electrodes deposited
on top of the emitters, the researchers
trigger the emission of a single electron to
an electronic beamsplitter that is made up
of two input and two output arms. Fève says
that their sample is capable of emitting billions
of single electrons per second – one
electron per nanosecond.
“The two sources are perfectly synchronized
such that both particles arrive
simultaneously on the splitter and perfect
antibunching occurs, meaning the two
electrons always exit in different outputs,”
explains Fève. That means that the two
electrons, generated by the two identical,
synchronized emitters would arrive simultaneously
at the two input arms of the splitter
and would always emerge in two distinct
outputs, obeying Pauli’s principle.
But Fève is quick to point out that while
the team did achieve a high degree of
indistinguishability, some minimal environmental
interaction did occur. The
team is looking at making its sample even
smaller so that the electrons travel even
shorter distances, while keeping in mind
the effects of temperature at such sizes
(Science 10.1126/science.1232572).
doped with deuterium atoms that was positioned
5 mm in front of a secondary target
made from beryllium.
Even though the pulses delivered less
than a quarter of the energy employed in
previous experiments, they produced neutrons
that were nearly 10 times as energetic
– up to 150 MeV – and nearly 10 times as
numerous. The group took the first radiographs
using this beam by placing a series of
tungsten, steel and plastic objects between
the neutron source and a scintillating fibre
array that was linked to a CCD camera.
Roth says that although his group’s device
produces fewer neutrons than reactors or
accelerators do, it packs the neutrons into
pulses – each lasting just 10 –8 s. This makes
it suitable for applications that need high
temporal resolution. Roth claims that, once
commercialized, the entire device would fit
on a lab bench and that only the target would
need shielding (Phys. Rev. Lett. 110 044802).
Physics World March 2013