Atomic clock

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, 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 (NBS). The first accurate atomic clock, a caesium standard 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. 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. However, radio clocks are not appropriate for high-precision scientific work. Many retailers sell radio clocks under the name "atomic clocks", but in doing so they are either mistaken or lying. Radio clocks can easily be accurate to the millisecond, but they are often off by hours or days unless the user can figure out how to get the clock to synchronize to a real atomic clock.

How they work

Frequency reference masers use glowing chambers of ionized gas, often caesium 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 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 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 caesium 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 caesium 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 stablize.

In practice, the feedback and monitoring mechanism is much more complex than described above.

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. Caesium 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.

Application

Generating of standard frequencies. Atomic clocks are installed at each site of time signal, LORAN-C,and Alpha Navigation transmitters. They are also installed at some long-wave and medium-wave broadcasting stations to deliver a very precise carrier frequency, which also has its usage as standard frequency.

Further, atomic clocks are used for long-baseline interferometry in radioastronomy.

Atomic clocks are the basis of the GPS navigation system. The GPS master clocks are Atomic clocks at the ground stations, and each of the GPS satellites has an on-board atomic clock.

Power consumption

Power consumption varies enormously, but there is a crude scaling with size. Chip scale atomic clocks can use of the order of 100 mW; NIST F1 uses orders of magnitude more power.

Research

Most research focuses on ways to make the clocks smaller, cheaper, more accurate, and more reliable. These goals often conflict.

New technologies, such as femtosecond frequency combs, optical lattices and quantum information, have enabled prototypes of next generation atomic clocks. These clocks are based on optical rather than microwave transitions. A major obstacle to developing an optical clock is the difficulty of directly measuring optical frequencies. This problem has been solved with the development of self-referenced mode-locked lasers, commonly referred to as femtosecond frequency combs. Before the demonstration of the frequency comb in 2000, terahertz techniques were needed to bridge the gap between radio and optical frequencies, and the systems for doing so were cumbersome and complicated. With the refinement of the frequency comb these measurements have become much more accessible and numerous optical clock systems are now being developed around the world.

Like in the radio range absorption spectroscopy is used to stabilize an oscillator — in this case a laser. When the optical frequency is divided down into a countable radio frequency using a femtosecond comb, the bandwidth of the phase noise is also divided by that factor. Although the bandwidth of laser phase noise is generally greater than stable microwave sources, after division it is less.

The two primary systems under consideration for use in optical frequency standards are single ions isolated in an ion trap and neutral atoms trapped in an optical lattice. These two techniques allow the atoms or ions to be highly isolated from external perturbations, thus producing an extremely stable frequency reference.

Optical clocks have already achieved better stability and lower systematic uncertainty than the best microwave clocks. This puts them in a position to replace the current standard for time, the cesium fountain clock.

Atomic systems under consideration include but are not limited to Al+, Hg+, Hg, Sr, Sr+, In+, Ca+, Ca, Yb+ and Yb.

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How they work

Application

Power consumption

Research

See also

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