How Does an Atomic Clock Keep Track of Time?

Have you ever wondered how do you know exactly what time it is?

I know, you looked at your watch, but when you first bought it?

From the news or the Internet, right? And where do they get it from?

NIST-F1 is a cesium fountain atomic clock that serves as the United States' primary time and frequency standard and it's the "master clock" of all clocks.

Contrary to popular beliefs, atomic clocks are not radioactive. They do not use the atomic decay; actually, they have an oscillating mass and a spring, just like ordinary clocks.

Unlike ordinary clocks, they do not fall behind or pick up the pace. They are the most accurate time keepers of all, better than the rotation of the Earth and the movement of the stars.

The most precise atomic clock on the planet falls behind one second every 70 million years! If it were built just before the extinction of the dinosaurs, it would still be right on time.

By the way, do you know the definition of a "second"?

Nope, it's not the sixtieth part of a minute.

Since 1967, the International System of Units (SI) has defined the second as the duration of 9 192 631 770 cycles of the radiation which corresponds to the transition between two energy levels of the ground state of the cesium-133 atom. Try saying that out loud!

Today's best atomic frequency standards (or clocks) are based on absorption spectroscopy of cold atoms in atomic fountains.

Atoms have characteristic oscillation frequencies. Perhaps the most familiar frequency is the orange glow from the sodium in table salt if it is sprinkled on a flame. An atom will have many frequencies, some at radio wavelength, some in the visible spectrum, and some in between the two. Cesium 133 is the element most commonly chosen for atomic clocks.

To create a clock, cesium is first heated so that atoms boil off and pass down a tube maintained at a high vacuum. First they pass through a magnetic field that selects atoms of the right energy state; then they pass through an intense microwave field. The frequency of the microwave energy sweeps backward and forward within a narrow range of frequencies, so that at some point in each cycle it crosses the frequency of exactly 9,192,631,770 Hertz (Hz, or cycles per second). The range of the microwave generator is already close to this exact frequency, as it comes from an accurate crystal oscillator. When a cesium atom receives microwave energy at exactly the right frequency, it changes its energy state.

At the far end of the tube, another magnetic field separates out the atoms that have changed their energy state if the microwave field was at exactly the correct frequency. A detector at the end of the tube gives an output proportional to the number of cesium atoms striking it, and therefore, peaks in output when the microwave frequency is exactly correct. This peak is then used to make the slight correction necessary to bring the crystal oscillator and hence the microwave field exactly on frequency. This locked frequency is then divided by 9 192 631 770 to give the familiar one pulse per second required by the real world.

Atomic clocks are the basis of the GPS navigation system. The GPS master clocks are Atomic clocks at the ground stations, and each of the GPS satellites has an on-board atomic clock.

So, there you have it, once again: the proof that time and space are closely linked together.