We describe the fabrication of an atom mirror by etching of a common hard
drive, and we report the observation of specular retroreflection of 11 uk
cesium atoms using this mirror. The atoms were trapped and cooled above the
hard drive using the mirror magneto-optical trap technique, and upon release,
two full bounces were detected. The hard drive atom mirror will be a useful
tool for both atom optics and quantum computation.Comment: 4 pages, 4 figure

Lassailly, Yves

Lee, Chungsok

Lev, Benjamin

Mabuchi, Hideo

Scherer, Axel

English

arXiv

Benjamin Lev

Yves Lassailly

Chungsok Lee

Axel Scherer

Hideo Mabuchi

Crossref

Atom mirror etched from a hard drive

We describe the fabrication of an atom mirror by etching of a common hard drive, and we report the observation of specular retroreflection of 11 muK cesium atoms using this mirror. The atoms were trapped and cooled above the hard drive using the mirror magneto-optical trap technique, and upon release, two full bounces were detected. The hard drive atom mirror will be a useful tool for both atom optics and quantum computation

Caltech Authors - Main

APPLIED PHYSICS LETTERS VOLUME 83, NUMBER 2 14 JULY 2003Atom mirror etched from a hard drive
Benjamin Lev,a) Yves Lassailly,b) Chungsok Lee, Axel Scherer, and Hideo Mabuchi
California Institute of Technology, Pasadena, California 91125
~Received 1 April 2003; accepted 16 May 2003!
We describe the fabrication of an atom mirror by etching of a common hard drive, and we report the
observation of specular retroreflection of 11 mK cesium atoms using this mirror. The atoms were
trapped and cooled above the hard drive using the mirror magneto-optical trap technique, and upon
release, two full bounces were detected. The hard drive atom mirror will be a useful tool for both
atom optics and quantum computation. © 2003 American Institute of Physics.
@DOI: 10.1063/1.1592305#Laser cooling and trapping techniques have made pos-
sible the preparation of extremely cold samples of atoms.
Atom optics employs elements such as mirrors, lenses, grat-
ings, and beam splitters to manipulate these cold atoms in a
fashion similar to the familiar photon optics. The advent of
the Bose–Einstein condensation of neutral atoms has en-
hanced the importance of developing atom optical elements.
In particular, atom mirrors—surfaces that reflect atoms—
play a crucial role in the field of atom optics, and it is of keen
interest to develop mirrors that are simple to fabricate yet
highly specular. In this letter we demonstrate a straightfor-
ward technique to produce large area, high resolution
permanent-magnetic structures on flat, rigid, and inexpensive
substrates.
Several types of atom mirrors have been fabricated using
evanescent light fields,1 dynamic magnetic fields,2 and static
magnetic fields.3 Evanescent mirrors repulse atoms from a
prism surface using a potential created by a blue-detuned
light field. Although magnetic mirrors cannot generally be
modulated as easily as evanescent mirrors, they do offer
many advantages: passive operation, compactness ~no laser
access is needed!, and much larger repulsive areas.
Magnetic mirrors employ a sheet of alternating current
or magnetization to create an exponentially increasing poten-
tial near the mirror surface.4 To lowest order, this potential is
proportional to B0e2ky. The surface field, B0 , sets the maxi-
mum atom energy that can be reflected, and the spatial pe-
riod of the current or magnetization, a52p/k , determines
the amount of time the atoms interact with the mirror. The
magnetic mirror approximates a perfectly flat mirror as B0
increases and a decreases. For example, if B051 kG and a
51 mm, a cesium atom in the 62S1/2 F54, mF54 state will
be reflected when dropped from a height of 0.4 m, and will
only interact with the mirror for 5 ms if dropped from 2 cm.
Mirrors made from serpentine patterns of wires can pro-
duce time-dependent reflection potentials. However, they
have not been fabricated with periods smaller than 10 mm,
and the power dissipated by the small wires requires cooling
by liquid nitrogen and pulsed operation.5 Sinusoidal magne-
tization of audio-tape, floppy disks, and videotape can pro-
a!Electronic mail: benlev@caltech.edu
b!Present address: Laboratoire PHMC/Ecole Polytechnique, 91128 Palaiseau
cedex, France.3950003-6951/2003/83(2)/395/3/$20.00
Downloaded 18 Dec 2005 to 131.215.240.9. Redistribution subject toduce magnetic mirrors with magnetization periods down to
12 mm.3 Mirrors made from millimeter-sized arrays of per-
manent magnets have been demonstrated, as have mirrors
produced by 1–4 mm periodic structures fabricated by sput-
tering ferromagnetic material onto a grooved substrate pat-
terned by electron-beam lithography.6
We recently fabricated a magnetic mirror by etching a
common hard drive, and we have used this mirror to retrore-
flect a cold cloud of 106 cesium atoms. Hard drives offer
several advantages for making and using atom mirrors. The
common hard drive provides a large surface area of thin
magnetic film whose surface is specifically designed to be
very flat, smooth, and rigid. Furthermore, the film’s remnant
magnetic field and coercivity can be as large as 7 and 3 kG,
respectively.7 An atom mirror could in principle be fabricated
with a 2 mm periodicity over the entire surface of the hard
drive. Old or discarded hard drives may be used: an Apple
hard drive from the mid-1990’s was used for the experiment
presented here.
We fabricate the mirror by etching 2 mm wide,
;100 nm deep trenches into a 1 cm2 section of the surface
of the hard drive. These 100 nm trenches extend past the
magnetic layer to form a periodic array of 1 mm wide, 30 nm
thick, and 1 cm long stripes of cobalt alloy ~see Fig. 1!. The
cobalt alloy is granular, which enhances the coercivity and
allows us to magnetize the material in plane and parallel to
the short axis of the magnetic strips. The typical grain size is
20–50 nm,7 and we expect the magnetization to be uniform
for our much larger features. We do not know the exact ma-
terials and thicknesses of the layers of the proprietary hard
drive. However, etching ;100 nm is sufficient to remove the
magnetic layer.
Standard photolithography is used to create the etch
FIG. 1. A cross section of the etched hard drive. The magnetization is
in-plane. See Ref. 7 for a description of the hard drive layers.© 2003 American Institute of Physics
 AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp
396 Appl. Phys. Lett., Vol. 83, No. 2, 14 July 2003 Lev et al.mask. After cutting the hard drive into 2 – 3 cm2 sections,
positive photoresist ~TSMR-8900, Tokyo Ohka Kogyo Co.!
is spun onto the cleaned hard drive surface for 40 s at 4200
rpm. A 15 s exposure followed by 65 s in the developer
~NMD-W 2.38%! maps the photomask lines into the resist.
The sample is ion milled with argon in a inductively coupled
plasma ~ICP! system. We etch for 8 minutes at a forward
power of 100 W, ICP power of 400 W, and an argon flow of
40 sccm. The remaining photoresist is removed with acetone
and, if necessary, a soft swab. To erase the hard drive’s bits
and magnetize it as a mirror, we insert the hard drive section
into the field of an 8 kG electromagnet whose field is parallel
to the surface and perpendicular to the magnetic stripes.
The magnetic field from the etched hard drive, with in-
plane magnetization, M 0 , parallel to the short axis of the
magnetic stipes, is analogous to a periodic sheet of alternat-
ing in-plane magnetization 1M 0/2 and 2M 0/2. In the infi-
nite array limit, the magnetic field above the surface is
B25B1
2e22ky12B1B3 cos~2kx !e24ky1B3
2e29ky1 . . . ,
~1!
where B15m0M 0(12e2kb)/p , B35m0M 0(12e23kb)/3p ,
and b530 nm is the thickness of the magnetic layer. The
field has no components in the z direction, and rotates with a
period equal to a in the x – y plane. Cesium atoms in the F
54, mF54 state, which has the largest weak-field seeking
magnetic moment, would have to be dropped from a height
of 25 cm to penetrate to a height at which the second term in
the expansion is equal the first, so to a good approximation
the field may be written as
B’B1e2ky1B3e23ky cos~2kx !. ~2!
For our hard drive mirror, B1 is equal to 2–4 kG depending
on the specific cobalt alloy. When a51 mm, the ratio of the
first harmonic term to the purely exponential term for a ce-
sium atom dropped from a height of 2 mm ~20 mm! is 1
31026 (131023) at the turning point y50.8 mm (y
50.4 mm).
The etched hard drive used for the experiment has a
’3 mm and c’1 mm resulting in a ratio of magnetic layer
to gap that is approximately 1:2. Figures 2~a! and 2~b! show
20 mm wide atomic force microscopy ~AFM! and MFM
scans of the hard drive surface. The trenches in the AFM
scan are dark, and the light to dark variation of magnetic
strips shows the north and south poles of the magnetization.
Figure 2~c! shows a 20 mm cross section of the MFM scan:
peaks represent the north and south poles. To describe the
field above our etched hard drive, Eq. ~2! can be modified to
account for the deviation from a 1:1 width ratio by multiply-
FIG. 2. 20 mm wide ~a! AFM scan, ~b! MFM scan, and ~c! MFM cross-
section of the etched hard drive surface.Downloaded 18 Dec 2005 to 131.215.240.9. Redistribution subject toing B1 by sin(pc/a) and B3 by sin(3pc/a). In our device, the
ratio of c/a’1/3 decreases the B1 term by 0.9, but causes
the corrugation term to nearly vanish.
The atom mirror is placed facing upwards in a vacuum
chamber pumped to 531029 Torr. In contrast to the experi-
ments that use a standard magneto-optical trap ~MOT! to trap
and cool the atoms ;2 cm above the mirror, we use the
mirror MOT technique to collect the atoms 1.5–4 mm above
the surface.8 A MOT requires the zero of a magnetic quad-
rupole field to be centered at the intersection of six circularly
polarized laser beams coming from all cardinal directions. To
satisfy this configuration near the hard drive surface, two 1
cm diameter beams of opposite circular polarization reflect at
45° from the 1 cm2 etched region. A retroreflected beam is
positioned perpendicular to the 45° beams and grazes the
surface of the hard drive. Aligning the axis of the quadrupole
field with one of the 45° beams completes the mirror MOT
configuration. The trapping lasers, each with an intensity of
4 mW/cm2 and 1 cm wide, are detuned by 10 MHz from
cesium’s F54, F855 cycling transition. A repumping beam
tuned to the F53, F854 transition is superimposed onto
both the grazing beam and a 45° beam. The atoms are loaded
from a thermal vapor.
In previous experiments using a perfectly reflecting gold
mirror, we have been able to trap 23106 cesium atoms in a
mirror MOT and cool them to 3 mK. One might expect trap-
ping and cooling to be much less effective with the etched
hard drive due to its poor qualities as an optical mirror: the
reflectivity is only ;50%, it is a good optical grating, and
the magneto-optical Kerr effect degrades the circularity of
the reflected 45° beams. Nevertheless, we have been able to
collect 13106 atoms and subdoppler cool them to 11 mK.
Achieving this low temperature is crucial because the atoms
released directly from the mirror MOT, at a temperature of
;120 mK, expand too quickly and become too diffuse to
detect by the time they reach the hard drive surface.
The poor optical reflectivity of the mirror does slightly
complicate the subdoppler cooling procedure; however, with
careful zeroing of the magnetic field it is still possible to
achieve polarization-gradient cooling to 11 mK in a ~down-
wards! moving reference frame. The atoms are optically
pumped into the F54, mF54 Zeeman substate just before
being dropped, and we apply a 100 mG bias field parallel to
the magnetic stripes in order to maintain alignment of the
atomic spins while they are falling/bouncing.
We have been able to detect two full bounces of the
atoms from the hard drive atom mirror. Figures 3~a! and 3~b!
show data from five runs of the experiment. The top panel
shows the mean position of the atoms above the hard drive
surface as a function of time. Superimposed is a curve de-
picting the expected trajectory of a particle falling under
gravity and bouncing from a hard wall. The slope of a line fit
to the lateral expansion of the falling atom cloud provides a
measure of the atoms’ rms velocity. A nonspecular mirror
would heat and diffusely scatter the reflected atoms as they
bounce, resulting in a sharp increase of the cloud expansion
rate. We made a linear fit to prereflection (t,15 ms) data in
each of the data sets, and deviation from this line, postreflec-
tion, would be evidence of nonspecularly. The dashed seg-
ment demarcates the region of unfitted data, and we do not
 AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp
397Appl. Phys. Lett., Vol. 83, No. 2, 14 July 2003 Lev et al.see any increase or offset of the residuals in this postreflec-
tion region: to within the experimental resolution, we do not
detect any deviation from specular reflection.
We have realized a specular atom mirror built by etching
a common hard drive. Magnetization periodicity of 3 mm has
been achieved, and we believe it would be straightforward to
reduce this to 2 mm with photolithography and to ;1 mm
using a large area electron-beam writer. The hard drive atom
mirror is compact, passive, relatively simple to fabricate, and
possesses a large remanent magnetic field. Moreover, it has
several desirable properties for applications beyond simple
reflection of atoms. The hard drive’s large coercivity should
allow one to use wires fabricated directly on its surface to
augment the mirror’s ability to manipulate atoms. Likewise,
FIG. 3. Panel ~a! shows the mean height of the atoms above the hard drive
surface during the first bounce. Panel ~b! shows the residuals from a linear
fit to the width of the expanding atom cloud.Downloaded 18 Dec 2005 to 131.215.240.9. Redistribution subject toelectric pads could be printed on the surface. These pads
would allow state-independent forces to act in concert with
the state-dependent forces from the mirror’s magnetic field to
perform quantum logic gates necessary for quantum
computation.9 The mirror can trap cold atom gases in two
dimensions, and can act as an adjustable grating when used
in conjunction with a magnetic bias field.10,11 Large area mir-
rors can be fabricated, and it seems possible that these mir-
rors could be useful for guiding or confining cold neutrons.12
As hard drive platters are expected to have good surface
flatness and substrate rigidity, it may be possible to create
two-dimensional waveguides and other devices by holding
an opposing pair of atom mirrors a few microns apart.
The authors thank the Roukes group and Marko Loncar.
This work was supported by the Multidisciplinary University
Research Initiative program under Grant No. DAAD19-00-
1-0374.
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An atom mirror etched from a hard drive

An atom mirror etched from a hard drive

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