IEEE Spectrum December, 2017 - 37

In research now published at Physical Review Letters,
we show that such a mobile clock is possible. We hope to
make one soon.

he core of an atomic clock is a vacuum chamber
containing a thin cloud of vaporized metal, usually cesium. Atoms in the vapor resonate at a precise frequency, meaning that their electrons will accept
energy only from photons having just the right amount
of it. If those photons have a little too much or too little
energy-that is, if their frequency is a little too high or too
low-the absorption falls off markedly. This is the key feature of an atomic clock.
Here's how it works. An electrical oscillator creates a
microwave frequency very close to the energy level of
the atom we are using for our clock. If the oscillator deviates slightly from the correct frequency, the absorption
changes, the change is detected by a laser, and the laser's
signal is used to tune the oscillator. This feedback loop
corrects the oscillator's imperfections.
Unlike the pendulum of a clock or the mechanical mechanism of a watch, atoms do not suffer from manufacturing
error or wear; with proper isolation from the environment, their resonant frequency is set by the laws of physics. Achieving the necessary level of isolation in practice
means that the best atomic clocks take up entire rooms.
Commercial atomic clocks are usually the size of suitcases.
In 2004, in a tour de force of microfabrication, scientists
at the National Institute of Standards and Technology managed to shrink this entire setup into a stack of components
a few millimeters high. Such "chip-scale" atomic clocks are
now available commercially and are used in niche applications, such as military communications and underwater
navigation. But this miniaturization comes at a price-and
not just in manufacturing cost: Because so many of the
atoms interact with the chamber walls, the tiny size of the
vacuum chamber can lead to small changes in the clock frequency, while the heater that generates the vapor constitutes a hefty power drain for a portable device. It'll be a long
time before this technology makes it into your cellphone.
Fortunately, there is an alternative, proposed in 2008
by Andrew Briggs and Arzhang Ardavan at the University
of Oxford, in England. Instead of using a chamber to trap
atoms, this method makes use of nature's own trap: an
endohedral fullerene.
Endohedral fullerenes, or endofullerenes, are remarkable molecules that almost seem to defy ordinary laws of
chemical bonding. The outside is a fullerene (named after
Buckminster Fuller, champion of the geodesic dome), a
hollow ball of atoms that can function as a container. An
atom or a smaller molecule can fit inside without bonding
to the shell and is thus protected from its environment-

ILLUSTRATION BY

Emily Cooper

even though an atomic cage filled in this way has properties similar to those of an empty fullerene.
Without doubt the most extraordinary of any such
endohedral fullerene is N@C60-a nitrogen atom inside a
-60-carbon fullerene cage, which resembles a soccer ball.
It's as though the nitrogen atom floats inside the fullerene,
retaining its atomic characteristics. Noble gas elements,
such as helium and neon, have also been incorporated in
C60. These are very inert species, unlike nitrogen, which is
one of the most reactive elements known. And it turns out
that nitrogen is key for making an accurate atomic clock.
N@C60 is a molecule that, given the reactivity of nitrogen, should not exist. Nevertheless, there are several ways
to synthesize N@C60. Extreme conditions are needed for
all of these methods, because pushing the nitrogen atom
through the carbon cage is thermodynamically unfavorable-the chemical equivalent of pushing water uphill.
Once the molecule is formed, however, the fullerene cage
isolates and stabilizes the nitrogen atom, so the products
synthesized can be collected and stored.
At our laboratory at the University of Oxford, we make
these caged molecules with what's called ion implantation.
Heat is used to vaporize fullerenes in a vacuum chamber,
where they waft onto a surface. This process gradually
builds up a C60 film on that surface.
While the C60 film is growing, nitrogen ions are shot
onto the surface of the film. Some of those nitrogen ions
get trapped in the growing C60 film, forming the desired
molecule. The yield, however, is very low: For every molecule of N@C60, there will be about 10,000 molecules of
nitrogenless C60.

FILLED FULLERENE: Sixty carbon
atoms form a spherical cage, called a fullerene,
that can trap a nitrogen molecule. This
arrangement protects the nitrogen [blue] from
magnetic interference, allowing it to resonate
and so serve as the basis for an atomic clock.

SPECTRUM.IEEE.ORG | NORTH AMERICAN | DEC 2017 | 37

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Table of Contents for the Digital Edition of IEEE Spectrum December, 2017