Most of us have heard about atomic clocks: Clocks that can
measure time with stunning precision. The most precise of these can
measure time with a precision of 1 second per 200 million years. In this
writeup, we will see how an atomic clock works, and why it is relevant
to have accurate timekeeping
The name atomic clock is a bit strange: all matter is composed of
atoms. So, what makes the atoms in an atomic clock special? The
answer is that an atomic clock looks at the behavior of single atoms. The
behavior of single atoms is best described with quantum mechanics, and has
some interesting properties that we can use for keeping time.
An atom can exist in many different energy states. Each of
these energy states corresponds to a slightly different internal energy
and slightly different internal structure. Now, the crux is that these
states are discrete: an atom might have state 1 or state 2 or state 3, but
never something like state 1.7. Such a change in state is called a
transition.
Atoms normally reside in a state close to the lowest energy state (the
ground state). In order to get the atom to a higher energy state, we
need to supply energy. We can do this by supplying electromagnetic
radiation. Electromagnetic radiation can be thought of to consist of
photons that each carry a discrete packet of energy. Now,
transitions between various states are most efficient when the
energy of the photon matches that of the transition. This well-known fact
is the principle behind the atomic clock.
As an example, we'll take a look at the most accurate type of atomic
clock available: the cesium fountain clock. In this clock, a cloud of
ultra-cool cesium gas is produced in a vacuum chamber. It is important the
gas is cool: temperature fluctuations would add extra bits of energy,
ruining accuracy. The cesium now is cooled even further using laser
cooling. Then, the cloud of vapor is given a little kick up by one of the
lasers. It flies up, through a microwave source. This microwave source is
capable of generating a very precise frequency - 9,192,631,770 Hertz. At
this frequency, the absorption of energy by the cesium is optimal. It then
flies up a little further, and, under the influence of gravity, falls
back. It then picks up energy a second time. We now have a cloud of cesium
that is partly in the lower state and partly in the higher state.
By shooting a laser on the cloud, we can induce fluorescence. This
means that the cesium atoms will produce light, but light of a
different color than the light of the laser. By choosing the right
laser, we can make sure that only the cesium atoms that have been
flipped by the microwaves fluoresce. By tuning the microwaves so that
the output of the fluorescence is optimal, we can find the exact frequency
belonging to the transition. What is left now is counting.
The second is defined as 9,192,631,770 cycles of radiation
coming from this particular transition of cesium. Put differently, count
to 9,192,631,770, and you have exactly one second. By tuning the
microwave to the cesium, all we have to do is count cycles in the
microwave. Of course, the actual operation of an atomic clock is a lot
more complicated, especially the tuning-but this covers the basics
Atomic clocks offer a very precise way of measuring time - in fact, they are so precise the very standard of time is defined using them. The
fundamental principle behind their operation is the fact that atoms can
undergo an energy transition if they are hit with precisely the right kind
of radiation. The frequency of this radiation can then be measured, and
from this, time can be measured.