ATOMIC CLOCKS

By Charles Truett

The first atomic clock came into existence in 1949. The U.S. National Bureau of standards was the home of this prototype of modern atomic clocks. The first accurate atomic clock was designed in 1955. This clock was based upon the transition of the caesium-133 atom. The discovery of this technology was the basis if the internationally agreed upon second. The United States has an official clock, an atomic clock, that is responsible for the accuracy of time throughout the country and is working in conjunction with clocks around the world to ensure that time is kept in unison across the globe.

Today we can find atomic clocks for our homes, offices, and electronic devices. These highly popular clocks can be purchased from your local retailers or from reputable online merchants who specialize in bringing you only the best, most precise timepieces around. Various styles and designs will make it easy for you to choose the atomic clock that best suits your needs. The atomic clocks used by government entities are by far the most accurate, but personal atomic clocks are highly reliable and you will be certain that you always know exactly what time it is at any given moment.

When shopping for an atomic clock, keep in mind that versions available to the general public are not suitable for scientific calculations, but instead a personal asset and a symbol of precise decisiveness. You can find an atomic clock in any price range and in many styles. An atomic clock makes a great conversation piece and will be a stylish addition to your home or office. Taste, style, and precision are what you will find when you begin your search for the perfect atomic clock. Great deals and numerous choices are a click away.

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Atomic Clock
By Wikipedia
An Atomic Clock is a type of clock that uses an atomic resonance frequency standard to feed its counter. Early atomic clocks were masers with attached equipment. Today's best atomic frequency standards (or clocks) are based on absorption spectroscopy of cold atoms in atomic fountains. National standards agencies maintain an accuracy of 10-9 seconds per day (approximately 1 part in 1014), and a precision equal to the frequency of the radio transmitter pumping the maser. The clocks maintain a continuous and stable time scale, International Atomic Time (TAI). For civil time, another time scale is disseminated, Coordinated Universal Time (UTC). UTC is derived from TAI, but synchronized with the passing of day and night based on astronomical observations.

History
The first atomic clock was built in 1949 at the U.S. National Bureau of Standards (NBS). The first accurate atomic clock, a cesium standard based on a certain transition of the cesium-133 atom, was built by Louis Essen in 1955 at the National Physical Laboratory in the UK. This led to the internationally agreed definition of the second being based on atomic time.

For decades, scientific-instrument companies, such as Hewlett-Packard, have been making cesium-fountain clocks for entities like NIST and USNO, at prices rivalling those of cars.

In August 2004, NIST scientists demonstrated a chip-scaled atomic clock. According to the researchers, the clock was believed to be one hundredth the size of any other. It was also claimed that it requires just 75 mW, making it suitable for battery-driven applications. This device could conceivably become a consumer product. It will presumably be much smaller, much less power-thirsty, and much cheaper to make than the traditional cesium-fountain clocks used by NIST and USNO as reference clocks. However, it is uncertain whether it will ever become a consumer product.

How they work
Frequency reference masers use glowing chambers of ionized gas, often cesium because that is the element used in the official international definition of the second.

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. This definition makes the cesium oscillator (often called an atomic clock) the primary standard for time and frequency measurements (see cesium standard). Other physical quantities, like the volt and metre, rely on the definition of the second as part of their own definitions. [1]

The core of the atomic clock is a tuneable microwave cavity containing the gas. In a hydrogen maser clock the gas emits microwaves (mases) on a hyperfine transition, the field in the cavity oscillates, and the cavity is tuned for maximum microwave amplitude. Alternatively, in a cesium or rubidium clock, the gas absorbs microwaves and the cavity contains an electronic amplifier to make it oscillate. For both types the atoms in the gas are prepared in one electronic state prior to filling them into the cavity. For the second type the electronic state of leaking atoms is detected and the cavity is tuned for a maximum of detected state changes.

This adjustment process is where most of the work and complexity of the clock lies. The adjustment tries to correct for unwanted side-effects, such as frequencies from other electron transitions, temperature changes, and the "spreading" in frequencies caused by ensemble effects. One way of doing this is to sweep the microwave oscillator's frequency across a narrow range to generate a modulated signal at the detector. The detector's signal can then be demodulated to apply feedback to control long-term drift in the radio frequency. In this way, the quantum-mechanical properties of the atomic transition frequency of the cesium can be used to tune the microwave oscillator to the same frequency, except for a small amount of experimental error. When a clock is first turned on, it takes a while for the oscillator to stabilize.

In practice, the feedback and monitoring mechanism is much more complex than described above.
Historical accuracy of atomic clocks from NIST.
Historical accuracy of atomic clocks from NIST.

A number of other atomic clock schemes are in use for other purposes. Rubidium standard clocks are prized for their low cost, small size (commercial standards are as small as 400 cm³), and short term stability. They are used in many commercial, portable and aerospace applications. Hydrogen masers (often manufactured in Russia) have superior short term stability to other standards, but lower long term accuracy.

Often, one standard is used to fix another. For example, some commercial applications use a Rubidium standard slaved to a GPS receiver. This achieves excellent short term accuracy, with long term accuracy equal to (and traceable to) the U.S. national time standards.

The lifetime of a standard is an important practical issue. Modern Rubidium standard tubes last more than ten years, and can cost as little as US$50. Cesium reference tubes suitable for national standards currently last about seven years and cost about US$35,000. The long-term stability of hydrogen maser standards decreases because of changes in the cavity's properties over time.

Modern clocks use magneto-optical traps to cool the atoms for boosted precision