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Atomic clock - How they work

Atomic clock - How they work: Encyclopedia II - Atomic clock - How they work

Frequency reference masers use glowing chambers of ionized gas, most often caesium, because caesium 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 9,192,631,770 cycles of the radiation which corresponds to the transition between two energy levels of the ground state of the Caesium-133 atom. This definition makes the caesium oscillator (often called an atomic clock) the primary standard for time and frequency measurements (see caesium standard). Other physical quantities, like the volt and metre, rely on the definition o ...

Atomic clock: Encyclopedia II - Atomic clock - How they work

Atomic clock - How they work

Frequency reference masers use glowing chambers of ionized gas, most often caesium, because caesium 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 9,192,631,770 cycles of the radiation which corresponds to the transition between two energy levels of the ground state of the Caesium-133 atom. This definition makes the caesium oscillator (often called an atomic clock) the primary standard for time and frequency measurements (see caesium standard). Other physical quantities, like the volt and metre, rely on the definition of the second as part of their own definitions.

The core of the atomic clock is a microwave cavity containing the ionized gas, a tunable microwave radio oscillator, and a feedback loop which is used to adjust the oscillator to the exact frequency of the absorption characteristic defined by the behavior of the individual atoms.

The microwave transmitter fills the chamber with a standing wave of radio waves. When the radio frequency matches the hyperfine transition frequency of caesium, the caesium atoms absorb the radio waves and emit light. The radio waves make the electrons move farther from their nuclei. When the electrons are attracted back closer by the opposite charge of the nucleus, the electrons wiggle before they settle down in their new location. This moving charge causes the light, which is a wave of alternating electricity and magnetism.

A photocell looks at the light. When the light gets dimmer because the frequency of the excitation has drifted from the true resonance frequency, electronics between the photocell and radio transmitter adjusts the frequency of the radio transmitter.

This adjustment process is where most of the work and complexity of the clock lies. The adjustment tries to eliminate unwanted side-effects, such as frequencies from other electron transitions, distortions in quantum fields and temperature effects in the mechanisms. For example, the radio wave's frequency could be deliberately cycled sinusoidally up and down to generate a modulated signal at the photocell. The photocell's signal can then be demodulated to apply feedback to control long-term drift in the radio frequency. In this way, the ultra-precise quantum-mechanical properties of the atomic transition frequency of the caesium can be used to tune the microwave oscillator to the same frequency (except for a small amount of experimental error). In practice, the feedback and monitoring mechanism is much more complex than described above. When a clock is first turned on, it takes a while for it to settle down before it can be trusted.

A counter counts the waves made by the radio transmitter. A computer reads the counter, and does math to convert the number to something that looks like a digital clock, or a radio wave that is transmitted. Of course, the real clock is the original mechanism of cavity, oscillator and feedback loop that maintains the frequency standard on which the clock is based.

A number of other atomic clock schemes are in use for other purposes. Rubidium 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 $50 US. Caesium reference tubes suitable for national standards currently last about seven years, and cost about $35,000 US. Hydrogen standards have an unlimited lifetime.