"I think what makes this picture particularly interesting to people is
that you can see the surrounding apparatus," Nadlinger told Live
Science. "And I think people are also surprised by how big the
atom looks here. … I hope I'm not undoing 100 years of science
education with this photo — atoms actually are unbelievably
small!"
To be clear, Nadlinger said, the purple speck at
the center of this photo is not the true size of
the strontium atom itself; it's the light from an
array of surrounding lasers being re-emitted by
the atom. When bathed in a specific
wavelength of blue light, strontium creates a
glow hundreds of times wider than the radius of
the atom itself (which is about a quarter of a
nanometer, or 2.5x10 to the -7 meters,
Nadlinger said). This glow would be barely
perceptible with the naked eye but becomes
apparent with a little camera manipulation.
"The apparent size you see in the picture is
what we'd call optical aberration," Nadlinger
said. "The lens we're seeing it through is not
perfect — also it's slightly out of focus and
slightly overexposed. You could compare it to
looking at the stars in the night sky, which
appear bright but are actually much, much
smaller than the size they seem to be, just
because our eyes (or the camera) don't have
enough resolution to process them."
So, seeing a single atom with the naked eye is
impossible. Trapping one in a lab, however, is
a little more doable.
To catch an ion by the toe
To make a single atom camera-ready like this,
researchers first need to turn it into an ion: an
atom with an unequal number of protons and
electrons, giving it a positive or negative net
charge.
David Nadlinger
"We can only ever trap charged particles,"
Nadlinger said. "So, we take a stream of neutral
strontium atoms, which come from an oven,
and shine lasers at them to selectively photo-
ionize them. This way, we can create single
ions."
When placed in an ion-trap apparatus, single
atoms are held in place by four blade-shaped
electrodes like those seen above and below the
strontium speck in Nadlinger's photo (two
additional electrodes are out of view). These
electrodes create a current that keeps the atom
fixed on the vertical axis; the two needle-
shaped cylinders on either side of the atom
keep it trapped horizontally.
As the currents from these electrodes interact,
they create what is called a rotating saddle
potential . "You can see videos online where
people literally take a saddle and rotate it and
put a ball on it; because of the rotation, the ball
actually stays in the center of the saddle. So
that's what these electrodes do to confine the
ion," Nadlinger said.
Once an atom is confined, an array of lasers
hits the atom, which scatters light in all
directions; in Nadlinger's photo, you can see
traces of the blue laser throughout the
background. Using this system, researchers
can potentially trap strings of hundreds of ions
between the little electrodes, resulting in some
stunning imagery.
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