atomic clock

Most people cannot afford to install the equipment necessary to detect the sub-atomic level activity of cesium, so installing an atomic clock in your house is most likely out of the question. Fortunately, the National Institute for Standards and Technology lets you use theirs, in case you need to know the time accurate to within one millionth of a second per year. And in today's fast-paced world of hectic schedules and non-stop stress, who doesn't?

Radio controlled atomic clocks look just like regular clocks, but have an AM radio receiver and a small microchip in them to receive and translate a radio signal broadcast from the Boulder, Colorado atomic clock. The NIST broadcasts the time over the AM radio station WWVB, which, when conditions are right, reaches most places in North America. If you live far from Colorado, you will probably only receive a good signal in clear weather at night. Broadcast power at the time of this writing is 50kW.

Because the broadcast distance varies dramatically with weather conditions, upper atmospheric conditions, and interference from other radio signals, radio controlled clocks cannot rely completely on WWVB to keep time. For this reason, they also have a regular quartz crystal clock that can keep perfectly accurate time for a few days if it can't find a good signal (defined at 100 microvolts per meter, although some clocks are more sensitive). In some parts of the North America, the clock may only update every couple of days or so. Quartz crystal clocks can keep time accurate to within three seconds per year, so a few days without synchronizing with the atomic clock will not cause any noticeable discrepancy (less than 1/100 of a second per day).

The time code embedded in WWVB's 60kHz signal contains the year, date, hour, minute, second, and flags that indicate the status of Daylight Saving Time, leap years, and leap seconds. With all this information, most radio controlled clocks have the ability to automatically update themselves for daylight saving time, and adding the day and date is trivial. There should also be an option to disable daylight saving time if you live in Arizona, Indiana, or Hawaii.

Radio controlled clocks can look like virtually any regular clock, due to the small size of the electronics needed to receive and process the time signal. The one I have is a 13" diameter analog face wall clock powered by a single AA battery. There are also digital displays, desktop models, calendar designs, and dozens more in a wide range of prices, anywhere from $30US on up. There are even wristwatch models!

To set up a radio controlled clock, install the battery or plug it in and set the time to approximately correct (it can't set the hour for you because it doesn't know which time zone you are in). There will usually be a button to push which tells the clock to start synchronizing with the atomic clock's broadcast. My model indicated the first synchronization had not been done yet by "double stepping" the second hand, it would wait two seconds between ticks and jump two seconds ahead instead of one. On this model, I could check the quality of the received signal by pushing the button again, it would beep for 30 seconds to indicate the signal quality. If I got a beep every second, the signal was good. My signal wasn't very good. Because I live in Minnesota (far from Colorado) and the weather was bad that day, it was not able to receive its first good signal until sometime in the middle of the night. When I checked on it the next day it was keeping perfect time and behaving normally again.

Some tips to help you get a better signal include facing the back of the clock towards Colorado, keeping the clock at least three feet away from sources of electromagnetic interference such as televisions, computers, and microwave ovens, and try not to install the clock on the other side of a metal wall or metal siding from Colorado.

For Europeans, Redalien says atomic clock signals are also available from Rugby (UK) and Frankfurt (Germany). The Rugby broadcast covers the UK and western Europe at 60kHz, while the Frankfurt broadcast covers northern and central Europe at 77.5kHz. I do not know if North American radio controlled clocks are compatible with the UK broadcast.

Sources:
Coverage area: http://www.boulder.nist.gov/timefreq/stations/wwvbcoverage.htm
General info: http://www.boulder.nist.gov/
Chicago Lighthouse Atomic Commercial Clock instruction manual

Update (11/1/04): My radio controlled atomic clock has indeed automatically adjusted itself for daylight saving time.

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.