Atomic clock

An atomic clock is a clock whose time clock is derived from the characteristic frequency of radiative transitions of electrons of free atoms. The time display of a reference clock is continuously compared with the clock and adjusted. Atomic clocks are currently the most accurate clocks and watches are called primary.

From the measured values ​​of about 260 atomic clocks at over 60 institutions worldwide distributed by the Bureau International des Poids et Mesures ( BIPM) in Paris, the International Atomic Time (TAI ) establishes as a reference time.

Developed the fundamentals of atomic clock from the U.S. physicist Isidor Isaac Rabi at Columbia University, who in 1944 received the Nobel Prize for Physics for this. Another Nobel Prize in connection with atomic clocks in 1989 awarded to the U.S. physicist Norman Ramsey for the improvement of metrology in atomic energy transitions.

• 4.1 Application Examples

Operation

Watches can specify the time more accurate, the constant is the vibration of their clock. In mechanical clocks, this is the pendulum or balance wheel, in the quartz watch it is an oscillator whose frequency is kept constant by means of the quartz crystal. In atomic clocks one takes the property of atoms advantage of emitting electromagnetic waves of a certain frequency in the transition between two energy states or absorb.

A quartz oscillator generates an electromagnetic alternating field to which are exposed to the atoms. At a certain frequency, the atoms absorb a lot of energy and radiate it off in other directions. This response is used to keep the frequency of the crystal oscillator is extremely stable by means of a control loop. The stability of the resonance even now determines the quality of the output signal.

As a time reference is usually a temperature-compensated quartz watch. Deviates from the reference clock generated by the frequency of the clock, the reference clock is corrected. From it can be picked up a highly accurate time signal and processed.

History and developments

Building on its carried out in the 1930s, studies on magnetic resonance methods, suggested in 1945 the U.S. American physicist Isidor Isaac Rabi building a atomic clock. The first atomic clock was introduced in 1946 by Willard Frank Libby, another was constructed in 1949 at the National Bureau of Standards (NBS ) in the United States using ammonia molecules as vibration source by Harold Lyons. But since they do not yet provided the hoped-for gain in accuracy, the clock was three years later revised and converted to the use of cesium atoms. This became known as NBS -1.

1955, followed by an even more accurate cesium clock by physicist Louis Essen and JVL Parry at the National Physical Laboratory in the UK.

Due to the excellent results of these clocks, the atomic transition time was defined as the international standard for the second. Since October 1967, the time period of one second in the International System of Units is by definition [ ... ] the 9.192.631.770fache the period of the transition between the two hyperfine levels of the ground state of atoms of the nuclide 133Cs radiation corresponding.

Over the years, the accuracy of atomic clocks has continuously been improved and by the end of the 1990s, a relative standard deviation to a SI second from about 5:10 -15 was reached.

High-precision atomic clocks

Cesium, rubidium and hydrogen are the most common atoms with which atomic clocks operate. The table compares their properties. For comparison, the values ​​for a temperature compensated quartz and ammonia are added with.

(* ) HI- line

In addition to cesium, rubidium and hydrogen also other atoms or molecules for atomic clocks are used.

Cesium fountain

In more recent atomic clocks working with thermally decelerated atoms to increase the accuracy. In the " cesium fountain " (English: Cesium fountain ) cesium atoms to be quite chilled, so they only about an inch per second are still fast. The slow atoms are then accelerated by a laser up and run through a ballistic trajectory (hence the term cesium fountain ), thus the effective interaction time of the atoms with the irradiated microwaves can be extended, allowing a more accurate frequency determination. The relative standard deviation of the NIST -F1 cesium fountain in 1999 was only about 10-15, which corresponds to a difference of one second in 20 million years.

Optical clock

In an atomic clock, the frequency of an atomic resonance is measured. This is achieved by the more accurate the higher is the frequency of resonance. Visible light has an approximately 50,000 -fold higher frequency than the microwave radiation used in the cesium. An atomic clock, which operates at a resonant optical, may be more accurate for this reason. For some years, therefore, worked on the realization of an optical atomic clock, which has a higher accuracy than the currently used cesium clocks.

For this purpose, experiments made ​​with elements having suitable transitions for optical wavelengths. This achieves frequencies of hundreds of terahertz in place of the conventional 9 GHz. In these experiments individual ions in a Paul trap, are stored, and a laser is applied to a narrow-band transition (typically a quadrupole or octupole transition) stabilized. The technical challenge here is to divide the highly stable laser frequency to electronically measurable ( countable ) frequencies down. For this purpose, the frequency comb was developed at the Max Planck Institute of Quantum Optics.

Physicists from the JILA Institute at the University of Colorado at Boulder ( Colorado) have presented an optical atomic clock in February 2008, based on spin-polarized 87Strontium atoms, which are trapped in a lattice of laser light. It succeeded the PTB to verify with the help of their transportable frequency comb frequency of 429,228,004,229,874 ± 1 Hz. The record was 10-17 at the beginning of 2008, as measured by an ultra -cooled aluminum atom.

In August 2013 was at the same institute in collaboration with the NIST precision (not to be confused with accuracy ) of an optical atomic clock on 10-18 be improved. This was achieved by comparing two identically constructed clocks, based on spin-polarized atoms as above, but here each have about 10000 ytterbium atoms. The larger number of atoms allows a relatively rapid determination of the precision of the clocks by averaging the measured data.

On the level of precision achieved a variety of effects can be seen that influence the observed frequency. These include, for example, the Zeeman effect, shock interaction between the atoms, the AC Stark effect or the gravitational redshift.

In July 2012, China presented for the first time, developed at the Academy of Sciences in Wuhan optical clock based on calcium ions. China was behind the USA, Germany, UK, Canada, Austria and Japan, the seventh country can develop the optical clocks.

Small-size atomic clocks for the practical application

Another line of development in addition to the high-precision watches tracked the construction cheaper, smaller, lighter and more energy efficient watches, for example for use in satellites and satellite navigation systems such as GPS, GLONASS or Galileo. Use atomic clocks in their satellites to increase with the help of precise time information, the positioning accuracy. In 2003 it was possible to build a rubidium atomic clock, which occupies only a volume of 40 cm3 and has a power consumption of one watt. In this case, it reaches a relative standard deviation of about 10.3 -12. This corresponds to a difference of one second in 10,000 years. Thus, the clock is indeed significantly less accurate than the large stationary atomic clocks, but considerably more accurate than a quartz watch. ( Precise quartz watches have a difference of one second in a month. Compared with these, this small atomic clock is 120,000 times more accurate. )

In 2011 came a portable Chip Scale Atomic Clock ( CSAC ) with a volume of 17 cm3 at a price of $ 1.500 on the civilian market.

Hydrogen maser clocks for the excitation of vibration are also highly accurate, but more difficult to operate. The first hydrogen maser in Earth orbit has been transported on the Galileo navigation satellite Giove- B on 27 April 2008 as the time base for the determination of position in the orbit. He is expected to reach a stability of 1 ns per day.

Use in Germany, Austria and Switzerland

In Germany four atomic clocks at the Physikalisch -Technische Bundesanstalt are (PTB ) in Braunschweig in operation, including two " cesium fountain " in control mode. Since 1991, the cesium clock CS2 provides the time standard for the seconds for the official time. This time can receive radio clocks via the DCF77; it is also available on the Internet via NTP.

In Austria, the Federal Office of Metrology and Surveying operates ( Laboratory of frequency, time) several atomic clocks. The master clock is based on UTC ( BEV). This time can receive computer via the NTP from a stratum 1 servers.

In Switzerland, the laboratory operates several atomic clocks, performed with the Swiss Atomic Time TAI (CH) and the Swiss world time UTC (CH ) is calculated for time and frequency of the Federal Office of Metrology ( METAS). These can be queried via the Internet through the NTP protocol. Until 2011, it was radio controlled watches receive this signal time via the time signal transmitter HBG.

Areas of application

Atomic clocks are used for a precise time measurement of processes, on the other hand the precise timing and coordination of various systems and time - scales. The result is approximately by comparing the internationally designated Atomic Time ( TAI ) with the astronomical time (UT1 ) Coordinated Universal Time (UTC). In Central Europe, radio clocks obtain the UTC - based time signal transmitter DCF77 stationed in Germany. The British equivalent is the transmitter MSF.

Application Examples

• In many standards Institutes around the world, the cesium clock model 5071, originally developed by Hewlett -Packard, Agilent later, then sold Symmetricom used. See, eg, atomic clock laboratory of the U.S. Naval Observatory.
• The Atomic Clock Ensemble in Space ( ACES ), a part of the Columbus space laboratory, to two cesium atomic clocks for the use of Galileo to be tested.
• Rubidiumuhren can be made in compact size and low cost. They are used in the telecommunications, energy supply and for calibrations in the industry. A very sophisticated model works in the latest generation of satellites of the GPS navigation system.
• A rubidium oscillator stabilizes the carrier frequency of the long wave radio station Donebach.
• On the Internet, the timing pulses of many atomic clocks using Network Time Protocol ( NTP) be made freely available for everyone.
• Rubidiumuhren come in high quality word clock generators are used to synchronize associations digital audio devices together.