The presence of a single electron's spin has been detected by a magnetic
resonance force microscope (MRFM), a device which brings together two
exquisite sensing technologies---magnetic resonance imaging (MRI) and
atomic force microscopy (AFM).
The ultimate goal of MRFM is to map the interior of a material sample,
such as a complicated semiconductor structure or a bio-molecule, at
atomic-scale resolution. To do this the MRFM uses a very frail cantilever,
85 microns long and 150 nm thick, with a tiny magnetic tip, plus a nearby
radio-frequency coil to create a bowl-shaped resonance zone.
Any magnetic particle---such as a single electron or even the nucleus
of a hydrogen atom (a proton)---that comes into the zone can interact
magnetically with the cantilever, whose oscillation frequency is altered
in a detectable way by the presence of the spin. Spin is a quantum parameter;
a particle with spin will undergo interactions with other magnetic objects.
Classically speaking, a particle with spin will behave like a tiny bar
An MRFM scan differs from an MRI scan in that "scanning" in MRI uses
very sophisticated techniques by which a signal is obtained from all
different 3D regions (voxels) simultaneously. MRFM, by contrast, is
more of a point-wise scan, followed by an image reconstruction procedure.
MRFM isn't just a form of microscopy (telling you where the molecules
are) but in the case of nuclear spins, is also a form of spectroscopy
that can, in principle, identify certain chemical elements, at least
those whose nuclei are magnetic.
Since MRFM made its debut more than a decade ago the sensitivity of
the device has improved by a factor of ten million, but it can't yet
detect single nuclei. The intrinsic magnetic strength (the "magnetic
moment") of a single nucleus is just too weak, about 650 times weaker
than an electron's magnetic moment.
To detect individual nuclear spins and thereby achieve spatial resolution
at the atomic scale, a further improvement in sensitivity by a factor
of about a thousand will be necessary. Currently, conventional MRI forms
images from nuclear spins, but needs a trillion or more to get a strong
Now, for the first time, an MRFM has mustered sufficient sensitivity
to detect the spin of a single electron amid a sample where most of
the electrons in the atoms are paired up (and thus rendered nonmagnetic).
In the 15 July issue of Nature, Dan Rugar and his colleagues at IBM
Almaden (San Jose) report on an MRFM device which uses a slender cantilever
operating at a temperature of 1.6 K.
The precision of the setup and the chilly conditions permit single
electrons in a silicon dioxide sample to be located. The associated
spatial resolution, at least in one of the three dimensions, is a mere
25 nm. (A few months ago an MRFM result with something like a million-electron-spin
resolution was reported; see Update
In terms of imaging sharpness the new IBM device is about 40 times
better than the best conventional MRI available. Not only is MRFM a
potentially splendid imaging device, but it may also play a part in
future quantum information devices owing to MRFM's ability to manipulate
and read the quantum state of individual spins. (See IBM's