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An atomic clock is a type of clock that uses an atomic resonance frequency standard as its counter. Early atomic clocks were masers with attached equipment. Today's best atomic frequency standards (or clocks) are based on more advanced physics involving cold atoms and atomic fountains.
National standards agencies maintain an accuracy of 10-9 seconds per day, 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.
The first atomic clock was built in 1949 at the U.S. National Bureau of Standards. The first accurate atomic clock, based on the transition of the caesium-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.
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 atomic. It was also claimed that it requires just 75 mW, making it suitable for battery-driven applications.
Modern radio clocks are referenced to atomic clocks, and provide a way of getting high-quality atomic-derived time over a wide area using inexpensive equipment.
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 Cesium-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.
Most research focuses on way to make the clocks smaller, cheaper, more accurate, and more reliable. These goals usually conflict.
A lot of research currently focuses on various sorts of ion traps. Theoretically, a single ion suspended electromagnetically could be observed for very long periods, increasing the accuracy of the clock, while also reducing its size and power.
In practice, single-ion clocks have poor short term accuracy because the ion moves so much. Current research uses laser cooling of ions, with optical resonators to increase the short term stability of the driving optics. Much of the difficulty is related to eliminating temperature and mechanical noise effects in the resonators and lasers. The current best known practice cools sapphire resonators to the temperature of liquid helium. No laser has achieved wide use. The result is that the ion trap is very small, but the supporting equipment is still large.
Some researchers developed clocks with different geometries of ion traps, as well. Linear clouds of ions have better short term accuracy than single ions. There are trade-offs.
The best developed systems use Mercury ions. Some researchers experiment with other ions. A particular isotope of Ytterbium has a particularly precise resonant frequency in one of its hyperfine transitions. Strontium has a hyperfine transition that is not as precise, but can be driven by solid-state lasers. This might permit a very inexpensive, long-lasting compact clock.