the PDF file at the bottom of this NIST page:

The clock starts with an oven containing a chunk of cesium. Atoms from the oven stream out in an atomic beam, but the atoms are initially distributed over many states, when clock operation demands that they be in a single well-specified state. In older atomic clocks, and the clocks on the GPS satellites, the specific state of interest is selected out by using magnets to deflect the other states out of the beam, so they don't enter the area where they are exposed to the light.

The physics behind this deflection is the same as the Stern-Gerlach experiment, done by Otto Stern and Walter Gerlach in 1921. Stern and Gerlach sent a beam of silver atoms between the poles of a tapered magnet, so that the magnetic field was stronger on one side than the other. The beam of atoms split into two beams, which they detected by letting them fall on a photographic plate.

As is typical of pioneering experiments, Stern and Gerlach actually measured something other than what they thought they were measuring-- they thought they were just seeing an effect cause by an electron orbiting the nucleus of a silver atom, with atoms whose electrons orbited clockwise experiencing a different force than atoms whose electrons orbited counter-clockwise. but the planetary type model behind that picture doesn't work. Instead, what they discovered was the intrinsic spin of the electron, which has its own angular momentum as if it were a spinning ball of charge. That angular momentum can take on one of two possible values, and in the case of silver, leads to a force on an atom between Stern and Gerlach's magnets that is either up or down, depending on the particular electron.

Stern and Gerlach were fortunate that they were using silver, which only has two states. Other atoms have more complicated level structures, and split into more than two beams, but the essential physics is the same. This splitting is the basis for the state selection in an atomic clock-- the cesium atomic beam passes through a Stern-Gerlach magnet, and then a pinhole blocks everything but a single state that happens to be bent onto exactly the right trajectory.

After the cesium atoms are exposed to the light source, they enter a second set of magnets, which again splits the atomic beam, with the atoms that have changed states being sent one way, and atoms that have not changed states sent another way. Measuring the fraction that changed tells you how close the frequency of the light is to the frequency the cesium wants to absorb, and allows you to correct the frequency to keep it right on target. That, in turn, provides an absolute reference for the GPS satellite to use in determining the time, which it encodes into the signal that it broadcasts to your GPS receiver.

So, while satellites are the reall

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