1) Laser cooling uses lasers to cool atoms to very low temperatures, creating clouds of cold atoms. It involves using counter-propagating laser beams slightly detuned from the atomic resonance to create friction and cool atomic motion. 2) In 1985, an optical molasses was first observed, cooling atoms to below 1 mK. However, optical molasses have low density due to the lack of a restoring force. Jean Dalibard then proposed using polarized light and a magnetic field gradient to create a restoring force in a magneto-optical trap (MOT), greatly increasing atom density. 3) Now, researchers have demonstrated an efficient way to generate the laser beam configuration for a MOT or molasses using a

1. NATURE NANOTECHNOLOGY | VOL 8 | MAY 2013 | www.nature.com/naturenanotechnology 317
news & views
L
aser cooling has become an essential
tool in modern atomic physics.
By shining lasers at the centre of a
vacuum chamber with a low-pressure
vapour, clouds containing a large number
of cold atoms can be created (Fig. 1a).
The principle of laser cooling is shown in
Fig. 1b. Two counter-propagating laser
beams, which are slightly detuned from the
atomic resonance, create a friction force
that cools the atomic motion along the
direction of the lasers. Three pairs of beams
on three different axes create a so-called
optical molasses inside which atoms are
cooled from 300 K to below 1 mK in less
than a millisecond. Because of the absence
of any restoring force, the density of optical
molasses is limited to very small values,
typically 106
cm–3
.
An optical molasses was first observed
in 19851
and soon after, Jean Dalibard
proposed to slightly modify this apparatus,
he proposed to use circularly polarized
light with a magnetic field gradient to
take advantage of the internal energy
level structure of the atoms. The radiation
pressure force modified by the magnetic
gradient creates a restoring force that
concentrates the atoms around the zero
of the magnetic field (Fig. 1c). The first
experimental demonstration was realized
soon after and revolutionized the field2
.
The spectacular increase in density (~1010
to 1011
cm−3
) provided by this magneto-
optical trap (MOT) opened the way to other
cooling techniques down to the quantum
degenerate regime and to the observation
of Bose–Einstein condensation in 19953,4
.
These works were awarded two Nobel prizes,
the first in 1997 for the development of
laser cooling and the second in 2001 for the
achievement of Bose–Einstein condensation.
Nowadays, optical molasses and MOTs are
routinely used in hundreds of labs all over
the world, are commercially available, and
an experiment with an optical molasses
will soon be launched into space on the
International Space Station (ISS). Besides
their fundamental interest, laser-cooled
atoms have been revealed to be useful for
performing precision measurements of
various physical quantities such as time,
inertial forces and magnetic field. For
example, atomic fountain clocks, which are
the main contributors to the definition of
the coordinated universal time standard, use
optical molasses.
Now writing in Nature Nanotechnology,
Aidan Arnold from the University of
Strathclyde and other institutions in
the UK demonstrate an alternative and
efficient way to generate the laser-beam
arrangement required for a MOT or a
molasses5
. They have managed to replace
the usual six-beam configuration by a single
beam combined with a special diffraction
grating (Fig. 2). The incident beam is
normal to the grating and is diffracted
along three or four directions (depending
on the chip that is used) thus creating the
right combination of wave vectors to cool
atoms in all directions. Furthermore, the
incoming circular polarization of the beam
changes appropriately when it is diffracted
so that a MOT is obtained by adding a
magnetic field gradient. This is not the first
attempt to simplify and miniaturize atom
laser cooling, but this is the first time that
a nanofabricated device and a single beam
enables the trapping of a large number of
atoms (~108
), which is comparable to what
is obtained in a traditional set-up. The
reason for this success is that the design of
the grating is such that the trap volume, V,
which is given by the intersection volume of
the incident and diffracted beams, can reach
values of cm3
, enough to capture such a large
number of atoms. The authors compared
three different designs and verified that
the number of atoms trapped increases by
V1.2
, as has been established for a six-beam
MOT. The cooling performance of their
optical molasses also compares very well to a
traditional molasses.
This new technology is of great interest
for different reasons. First, it simplifies and
COLD ATOMS
Trapped by nanostructures
The design of specific nanostructured optical gratings allows the realization of efficient traps for a large number of
cold atoms.
Jérôme Estève
0 Restoring force
Friction force
a
b
c
z
Laser beams
Coils
Atoms
Energy levels
Energy levels
m = 0
m = 1
m = 0
m = −1
σ−
σ+
Figure 1 | Principle of laser cooling and magneto-optical trapping. a, Traditional six-beam configuration for
an optical molasses. Adding a magnetic field gradient, using for example two coils in an anti-Helmholtz
configuration, a restoring force appears that gathers the atoms around the zero of the magnetic field,
creating a so-called magneto-optical trap. b, Basic principle of laser cooling along one direction of an
optical molasses. The two beams are slightly red detuned from the atomic transition. In the frame of
an atom moving, for example, to the right, the frequency of the beam coming from the right is closer to
resonance than the frequency of the other beam because of the Doppler effect. Thus, the atom scatters
more photons from the right beam, which results in a friction force slowing down the atom. c, Taking
advantage of the internal energy level structure of the atom, the radiation pressure force of the beams in
the molasses can be spatially modulated by using circularly polarized light and the magnetic field gradient
created by the two black coils. The gradient shifts the atomic energy levels depending on their spin state.
Here, an atom at rest located at z > 0 scatters more photons from the beam coming from the right, which
creates a force pushing the atom towards z = 0.
© 2013 Macmillan Publishers Limited. All rights reserved

2. 318 NATURE NANOTECHNOLOGY | VOL 8 | MAY 2013 | www.nature.com/naturenanotechnology
news & views
miniaturizes the optical set-up required to
create a MOT, thus opening new ways to
integrate cold atoms into high-technology
apparatus, such as atomic clocks. The
authors’ work also indicate that the
diffraction grating improves the stability of
the MOT by ensuring a built-in alignment
of the different beams. This could greatly
benefit many experiments where atom
number stability is critical. Arnold and
colleagues mention that the same grating
can be used with far-detuned light to create
an optical lattice. These crystals of light
have been widely used in the cold-atom
community to mimic the behaviour of
electrons in a solid and to tackle condensed-
matter problems6
. Here again, the improved
stability of the lattice position could reveal
to be extremely useful. One could also
imagine tailoring complex optical potentials
with a reduced number of beams by using
the appropriate diffraction grating, opening
up new possibilities to simulate complex
quantum systems with cold atoms. ❐
Jérôme Estève is at the Laboratoire Kastler Brossel,
Département de Physique Ecole Normale Supérieure,
24 rue Lhomond, F-75231 Paris Cedex 05, France.
e-mail: esteve@lkb.ens.fr
References
1. Chu, S., Hollberg, L., Bjorkholm, J., Cable, A. Ashkin, A. Phys.
Rev. Lett. 55, 48–51 (1985).
2. Raab, E., Prentiss, M., Cable, A., Chu, S. Pritchard, D. Phys. Rev.
Lett. 59, 2631–2634 (1987).
3. Anderson, M. H., Ensher, J. R., Matthews, M. R., Wieman, C. E.
Cornell, E. A. Science 269, 198–201 (1995).
4. Davis, K. et al. Phys. Rev. Lett. 75, 3969–3973 (1995).
5. Nshii, C. C. et al. Nature Nanotech. 8, 321–324 (2013).
6. Bloch, I., Dalibard, J. Zwerger, W. Rev. Mod. Phys.
80, 885–964 (2008).
Input beam
Quarter
waveplate
Grating
Chip
Fibre
Lens
MOT
Vacuum cell
Figure 2 | Laser cooling with nanostructured diffraction gratings5
. With the set-up shown here,
Arnold and co-workers have obtained a laser-beam configuration similar to the one shown in Fig. 1, which
allowed them to trap and cool Rb atoms close to the grating surface5
. This is the first time that a MOT
containing a large number of atoms has been produced using such an integrated set-up with one single
beam and a nanofabricated device.
M
agnetic vortices form naturally
in simple magnetic nanodisks
and have a high structural and
thermal stability. They can have four
different configurations due to the various
possible combinations of the clockwise or
anticlockwise circulation of the magnetization
and the up or down orientation of the vortex
core, which is magnetized perpendicular to
the disk. This feature makes magnetic vortices
appealing for future memory devices because
they could store two units of information in
a single nanodisk. However, reliable ways
to switch between the four states are still
required, although switching of magnetic
vortex cores has previously been achieved by
applying suitable electrical or magnetic field
pulses1–3
. Writing in Nature Nanotechnology,
Vojtěch Uhlíř and colleagues at the University
of California, San Diego, Brno University
of Technology and Lawrence Berkeley
National Laboratory have now shown that
the circulation direction of magnetization of
vortices can be quickly and reliably switched
with a single magnetic field pulse4
.
The main drawback of conventional
random access memories (RAMs) is their
volatility, which means that the stored
data has to be constantly refreshed and
significant amounts of power consumed,
even when no operation is performed. To
overcome this issue, non-volatile magnetic
RAMs (MRAMs) have been proposed as
a low-power alternative to conventional
semiconductor memory devices. In
MRAMs5
, information is read by monitoring
variations in their magnetoresistivity due
to changes in the relative alignment of the
magnetization in a ‘free’ magnetic layer
(which carries a unit of information) with
respect to a ‘fixed’ magnetic reference
layer. Ideally, the free layer should possess
perfect magnetic bistability for the sample to
always be in one of the two possible states,
representing a logic ‘1’ or ‘0’. However,
MRAMs have problems with scalability
due to the stability and selectivity of the
magnetization state of the magnetic layers.
Furthermore, functional memories require
small switching fields, a uniform and fast
switching process, and good thermal stability.
Soft magnetic materials, such as
permalloy (a nickel–iron magnetic alloy),
are useful for MRAMs because they can
MAGNETIC NANOSTRUCTURES
Vortex states à la carte
The circulation direction of magnetization in magnetic vortices created in skewed nanodisks can be reversed using
nanosecond field pulses.
Riccardo Hertel
© 2013 Macmillan Publishers Limited. All rights reserved