Atomic Clocks

Atomic clocks are the most accurate time pieces in existence. They drive everything from the stock exchange to broadcast technology.

THE ATOMIC AGE: THE CAESIUM CLOCK (Cesium is the American English spelling)

How it works:

Cesium (Cs) is an Alkali metal with one electron in its outermost electron shell.

All atoms absorb and emit at a distinct frequency. In early experiments while physically burning elements a spectrometer showed that each element emitted unique concentrated narrow lines on the spectrum of light. This was caused by the valence electron jumping to quantified levels. We will not explain quantum physics here. The electron emits a photon whose color depends on the step of energy as that electron changes energy state. Electrons split and create double lines on the spectrum.

With cesium we take the two lines in the infrared spectrum and look closer (the D line). There is a split again between the spin of the nucleus and the spin of the electron. This is very complex and we won’t explain it here, so learn in greater detail here:

In the splitting of the frequencies in the D line of the cesium atom we will see 9.192631770 GHz, this is one second exactly, (or what humans have chosen to be precisely a second). Remember this goes back to the earlier part in this unit about Egyptions choosing to break the day into 24 parts, which led to minutes, and seconds later on.

So, we know that the second comes from measuring the frequency of the very fine transition of the D2 line ground state of the neutral cesium atom at rest. With this enormous precision we can do amazing things such as GPS, physics experiments and develop better technology all around!

Portable Atomic Clocks: 

In 1971 the first portable atomic clocks were used in the Hafele-Keating Experiment to test Einstein’s theory of relativity, both special relativity and general relativity. This clock was about the size of a toaster oven, before the clocks were the size of a large long table. Engineers at Hewitt Packert managed to make the microwave cavity smaller.  Below are the parts of the first portable atomic clock:

Key Parts of an HP 5061A 1968 Cesium Clock:

Cesium tube – cavity with 6 grams of cesium, heats and creates a beam
Magnet – changes outer electron spin state as the beam goes through it, sorts them
Microwave cavity – send the beam through this, and it’s fed by radiation from a quartz oscillator. If you hit the atom with the precise frequency, it will flip the atom.
Magnet – select the flipped atom again here, and see how many you get
Hot wire ionizer which gets rid of outer electron, charging it for later use
With a charged cesium atom you can now direct it with an electromagnet to form a electron spectrometer. The atoms eventually hit a plate and kicks off electrons, and this makes the output we can convert into time using some other parts. The absorption point of an atom is naturally occuring so it’s repeatable. Whether someone creates an atomic clock at high altitude, sea level or in space, it’s basically the same frequency.

Atomic Clocks Today:
We have achieved very portable atomic clocks with the CSAC (chip scale atomic clock). It uses a low power semiconductor laser as a light source. The current size of the world’s smallest atomic clock is 4×3.5×1 cm and 35 grams. This clock also uses Caesium-133.

Below is our short and easy explanation of the atomic clock from John Lowe of NIST (National Institute of Standards and Technology) :

on point of an atom is naturally occuring so it’s repeatable. Whether someone creates an atomic clock at high altitude, sea level or in space, it’s basically the same frequency.

We interviewed John Lowe from the National Institute of Standards and Technology about how the atomic clock works and a little about the development of the devices.

Next we will move on to test equipment which is based on time measurement.