What is an "Atomic" Clock?

The watches we wear on our wrists or the clocks we put on our walls are not atomic clocks. There are not all that many true atomic clocks - and they tend to be rather large and expensive. So, unless it looks something like the picture alongside from the National Institute for Standards and Technology (NIST), it is not a state-of-the-art atomic clock.

We will talk about the device that we tend to call an atomic clock, but first we will look at what an atomic clock really is. According to the U.S. Naval Observatory,

A "cesium(-beam) atomic clock" (or "cesium-beam frequency standard") is a device that uses as a reference the exact frequency of the microwave spectral line emitted by atoms of the metallic element cesium, in particular its isotope of atomic weight 133 ("Cs-133"). The integral of frequency is time, so this frequency, 9,192,631,770 hertz (Hz = cycles/second), provides the fundamental unit of time, which may thus be measured by cesium clocks.

Well, perhaps we need to start from the beginning. . .

What is "atomic" about an atomic clock?

All modern clocks have some type of internal mechanism that repeats itself in a regular fashion to count out the passage of time. In many contemporary clocks, it is an oscillating crystal of quartz that keeps the beat of passing time in a precise and periodic way. For an "atomic" clock, the oscillator consists of the very atoms that make up its core. And, unlike other types of oscillators, the period is not set by human construction but comes from the quantum mechanical nature of the atom itself!

Quantum mechanics of the atom tells us that the electrons in atoms can only exist in certain, stable orbits produced by the electric interaction of each electron with the nucleus of the atom. Movement of electrons between these possible orbits is responsible for the colors of light we see produced by specific elements, like the red glow of an actual neon sign.

But, there are other electric and magnetic interactions within an atom, and they occur with other energies and wavelengths that do not form visible light. The hyperfine interaction involves the electric charge of the electron and the magnetic field of a spinning nucleus. The energies of the hyperfine interaction occur in the radio and microwave region. This process is used as the basis of "atomic" clocks, through a mechanism called atomic beam magnetic resonance. (I. I. Rabi received the 1944 Nobel Prize in Physics for his work on this phenomenon.) For each energy or wavelength, there is also a frequency associated that can act as a time base. Once you define how many oscillations there are to a second, you're all set to go!

Why don't we just use visible light, why this microwave hyperfine thing?

Resonant visible light transitions are very useful to us, they are the basis of visible lasers like laser pointers. But, the frequency of visible light is very high, much higher than microwaves. That makes it hard to count accurately, and accuracy is essential to our atomic clock. We do have a lot of experience in dealing with microwave frequencies, however, since they are widely used in telecommunications. So, we know how to make good and accurate electronic circuits to work at microwave rates.

More fundamentally, the physics of the hyperfine magnetic interaction makes us prefer it. Quantum mechanics puts in a fundamental limitation to the time (or frequency) width of an atomic event through something called the Heisenberg uncertainty relations. The larger the energy spread an atomic event has, the larger its frequency spread. The energy spread of the hyperfine transition is very narrow compared to a visible light transition, making its frequency spread - or frequency accuracy - much better.

Even here, our choice for type of atom and its "environment" can strongly influence our eventual clock accuracy. The first atomic clock was built using microwave resonance in ammonia molecules for the frequency standard. The "environment" keeps the molecules far apart from each other (in a high vacuum) so that they do not disturb each other and alter the frequency spread. But, there is no way to overcome the influence of atoms on each other within a single molecule. This early atomic clock was only as accurate as a conventional quartz clock today.

Cesium atoms were then chosen because the outer electron of a cesium atom is essentially undisturbed by the presence of any other electron in the cesium atom. For the chemistry-inclined, all the inner electrons are paired into complete shells - making them very stable and inert. When cesium atoms are then kept in a very good vacuum, the outer electron of each atom acts in an essentially ideal way - it is as good as it can get!

In the current state-of-the-art cesium fountain clock at NIST, the accuracy is given as 1 part in 10¹⁵ - which would correspond to a possible error of 1 second in about 30 million years.

In 1967, the 13th General Conference on Weights and Measures first defined the International System (SI) unit of time, the second, in terms of atomic time rather than the motion of the Earth. Specifically, a second was defined as the duration of 9,192,631,770 cycles of microwave light absorbed or emitted by the hyperfine transition of cesium-133 atoms in their ground state, undisturbed by external fields.

The Nitty Gritty: How Does It Work?

The Basic Atomic Clock:

• Cesium atoms are sprayed in high vacuum from the source to filter A
• Filter A allows only one type of atom (Cs-133 in its ground state) to enter the microwave cavity
• Microwaves at just the right frequency cause a hyperfine quantum change (excitation) in the atoms
• Filter B allows only changed atoms to reach the detector and be counted
• The control mechanism uses the detector signal to adjust the microwave frequency until it sees the most changed atoms
• This frequency, the atoms' natural hyperfine transition frequency, is counted to determine the length of a second

So, in operation an atomic clock:

1. Magnetically filters atoms to select only those in one hyperfine quantum state
2. Sends the beam of selected atoms through microwaves oscillating at the hyperfine transition frequency
3. Measures only how many changed atoms come out the other end
4. Tunes the microwave generator to the exact frequency causing the maximum number of hyperfine transitions (which is the atom's natural excitation frequency) and
5. Uses that signal to calculate that every 9,192,631,770 oscillations (in the case of Cs-133) represent one second

OK. Now we know what an "atomic" clock is. What is the thing we wear as a watch or put on the wall?

What is a radio-controlled atomic clock?

The atomic clocks we've been talking about may be the most accurate timekeepers on the planet, but they aren't very useful unless their information (time signals) are widely communicated. So the U.S. government has mandated for its uses that the official standard time is to be maintained by the Time and Frequency Division of NIST and by the U.S. Naval Observatory. These time signals are broadcast by radio and over the internet through these official (primary) timekeepers and relayed through a series of secondary timekeepers.

Since you are reading this online right now, you can visit the Official U.S. Time web site and choose a U.S. time zone when you get there. For a more global view, try the World Time Server and pick your location.

At several broadcast frequencies, NIST operated radio transmitters located near Fort Collins, Colorado spread the time signal for all to use. As long as you are within about 2000 miles of their location, you can pick up and interpret the time information. These broadcasts are critically important for the function of the Global Positioning Satellite system (GPS), among other navigational tools.

Antenna field for NIST stations WWV, WWVB, and WWVH.

Since 1962, WWVB has been broadcasting this time signal at an operating frequency of 60 kHz, well below the standard AM radio station frequencies in the U.S. The time code is synchronized with the 60 kHz carrier frequency, and contains information for the year, day of year, hour, minute, second, and flags to indicate the status of Daylight Saving Time, leap years, and leap seconds. By tuning in to this signal, all the information you could want about the current time can be obtained.

Finally, we know the secret of those amazingly accurate timepieces, the radio-controlled atomic clock. Using a miniature radio receiver tuned to WWVB and built in to each radio controlled clock or watch, these marvels are programmed to try to receive the WWVB broadcast, usually once per day. Many of them can also be manually told to "listen" for the time code. When it is successfully received and interpreted, the time code is used to reset the displayed time - date, daylight saving time, and all. Then, these timepieces use their own, internal quartz oscillators to keep a good level of accuracy until the next time code comes in.

Of course, not all locations are equally suited to receiving the WWVB signal. Generally, it is easiest to pick up the time code at night - just the way reception of distant AM stations improves at night for regular radio (there is less solar interference with the ionosphere). But, you need to be within the effective range of the broadcast which is about 2000 miles. The WWVB signal blankets most of the continental U.S. and Canada, even reaching parts of South America at times. But it will not reach Alaska or Hawaii.

There can also be natural or man-made obstructions to the radio signal. Deep valleys tend to have more trouble receiving the broadcast. And modern buildings with steel reinforced construction can act as radio barriers (known as Faraday cages), blocking the WWVB signal just the way they do standard radio reception. Still, a bit of planning can help to solve the difficulty. Decide in advance where you will leave your radio controlled clock during the night, when it will try to pick up the time code. Try for a high location in the building if you are in a valley, and set it near a large window if you are in a steel building. Keep it away from other sources of broad band radio interference, like televisions or computers. You just might succeed!