An atomic clock is a clock whose time cycle is derived from the characteristic frequency of the radiation transitions of the 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 are also called primary clocks .
The measured values of over 400 atomic clocks at over 60 time institutes distributed around the world are compared using GPS time comparisons, now increasingly using two-way time and frequency comparisons (TWSTFT). The results are sent to the International Bureau of Weights and Measures (BIPM), which uses them to form a weighted average which is the basis of International Atomic Time (TAI) published by BIPM.
The foundations of the atomic clock were developed by the American physicist Isidor Isaac Rabi at Columbia University , who received the Nobel Prize in Physics for this in 1944 . Another Nobel Prize in connection with atomic clocks was awarded in 1989 to the US physicist Norman Ramsey for the improvement of measurement technology in atomic energy transitions.
The more constant the oscillation of their clock, the more precisely clocks can indicate the time. In the case of wheel clocks, this is the pendulum or the balance wheel ; in the case of a quartz clock , it is an oscillating quartz that keeps the frequency of a quartz oscillator constant. Atomic clocks make use of the property of atoms to emit or absorb electromagnetic waves of a certain frequency when transitioning between two energy states.
A temperature-compensated quartz oscillator generates an alternating electromagnetic field to which the atoms are exposed. At a specific frequency , the atoms absorb a lot of energy and radiate it in other directions. This resonance is used to keep the frequency of the crystal oscillator extremely stable by means of a control loop : If the frequency deviates from the resonance, this is recognized. The frequency of the quartz oscillator is then adjusted accordingly in order to meet the resonance frequency of the atoms again. The stability of the resonance itself now determines the frequency stability of the output signal. Finally, the time signal from the quartz watch is read out.
History and developments
Before the development of atomic clocks, the Riefler precision pendulum clock was the most precise clock with an accuracy of ± 4 × 10 −4 s / day. The Munich University Observatory received the first of these watches on July 27, 1891. It was used in over 150 observatories around the world. A total of 635 copies were made by 1965. To this day, it has remained the most precise mechanical watch.
Building on his investigations into magnetic resonance processes carried out in the 1930s, the American physicist Isidor Isaac Rabi suggested the construction of an atomic clock in 1945 . A first atomic clock was constructed by Harold Lyons in 1949 at the National Bureau of Standards (NBS) in the United States using ammonia molecules as the vibration source . However, since it did not yet provide the expected gain in accuracy, the watch was revised three years later and converted to use cesium atoms . It was named NBS-1 .
Due to the excellent rate results of these clocks, atomic time was defined as the international standard for the second. Since October 1967 the duration of one second in the international system of units is by definition […] 9,192,631,770 times the period of the radiation corresponding to the transition between the two hyperfine levels of the ground state of atoms of the nuclide 133 Cs .
Over the years the accuracy of atomic clocks has been improved again and again. By the end of the 1990s, a relative standard deviation to the ideal SI second of around 5 · 10 −15 was achieved, at the beginning of 2015 it was already 5 · 10 −18 .
A further increase in accuracy is expected from a clock that uses the excited level of an atomic nucleus instead of the atomic shell . For this, the excitation energy may only be a few electron volts , an extremely small value for nuclei. In September 2019, a first candidate for a usable level in the isotope thorium-229 was measured so precisely that the construction of such a more precise nuclear clock could move into the realm of possibility.
High-precision atomic clocks
Cesium , rubidium , hydrogen and, more recently, strontium are the most common atoms with which atomic clocks are operated. The table compares their properties. For comparison, the values for a heated quartz oscillator, the so-called quartz furnace (OCXO), and ammonia are included.
|Relative standard deviation of
|Quartz furnace (OCXO)||5 to 10||10 -8|
|NH 3||23 786||10 −11|
|133 Cs||note 19 192.631 77||10 −13|
|87 Rb||6 834.682 610 904 324||10 -15|
|1 H.||1,420,405,751 77||10 -15|
|Optical atomic clock ( 87 strontium)||429 228 004,229 874||10 −17|
In addition to cesium, rubidium and hydrogen, other atoms or molecules are also used for atomic clocks.
In newer atomic clocks one works with thermally decelerated atoms in order to increase the accuracy. In the "cesium fountain" (Engl .: cesium fountain ) cesium atoms are greatly cooled to it that only about one centimeter per second are fast. The slow atoms are then accelerated upwards with a laser and run through a ballistic trajectory (hence the term cesium fountain ). This allows the effective interaction time of the atoms with the radiated microwaves to be extended, which allows a more precise frequency determination. The relative standard deviation of the cesium fountain NIST-F1 was only about 10 −15 in 1999 , which corresponds to a deviation of one second in 20 million years.
The frequency of an atomic resonance is measured in an atomic clock. The higher the frequency of the resonance, the more precise it is. Visible light has a frequency about 50,000 times higher than the microwave radiation used in cesium. For this reason, an atomic clock that works with optical resonance can be significantly more accurate. For some years, work has therefore been carried out on the implementation of an optical atomic clock that is more accurate than the cesium clocks currently in use.
For this purpose, experiments are made with elements that have suitable transitions at optical wavelengths. This enables frequencies of hundreds of terahertz to be achieved instead of the conventional 9 GHz. In these experiments, individual atoms are stored in an ion cage . A laser is stabilized on a narrow-band transition. The stability of the frequency of this laser light is then transferred to a periodic electrical signal without any loss of accuracy. This is achieved with a frequency comb . The usual frequency for the electrical signal is 10 MHz.
Atomic clocks based on optical lattices were introduced in 2001 by Hidetoshi Katori (Optical lattice clock), who demonstrated them in 2003 and developed them down to a relative inaccuracy in time measurement of 10 −18 .
In February 2008, physicists from JILA in Boulder (Colorado) presented an optical atomic clock based on spin-polarized 87 strontium atoms, which are trapped in a grid of laser light . With the help of its portable frequency comb, the PTB succeeded in verifying a frequency of 429.228.004.229.874 ± 1 Hz. The record at the beginning of 2008 was 10 −17 , measured on an ultracooled aluminum atom.
In August 2013, in collaboration with NIST at the same institute, the precision (not to be confused with accuracy ) of an optical atomic clock could be improved to 10 −18 . This was achieved by comparing two identical clocks, which are based on spin-polarized atoms as above, but here on approx. 1,000 ytterbium atoms each . The larger number of atoms allows a comparatively quick determination of the precision of the clocks by averaging over the measurement data.
At the level of precision achieved, a multitude of effects become visible that influence the observed frequency. These include B. the Zeeman effect , collision interaction between the atoms, the AC-Stark effect or the gravitational redshift .
In July 2012, China presented for the first time an optical clock based on calcium ions developed at the Academy of Sciences in Wuhan . After the USA, Germany, Great Britain, France, Canada, Austria and Japan, China became the eighth country that can develop optical clocks.
Small format atomic clocks for practical use
Another line of development besides high-precision clocks is the construction of inexpensive, small, lighter and energy-saving clocks, e.g. B. for use in satellites of satellite navigation systems such as GPS , GLONASS or Galileo , in order to increase the positioning accuracy. In 2003 it was possible to build a rubidium atomic clock , which only takes up a volume of 40 cm³ and consumes an electrical power of one watt. It achieves a relative standard deviation of approx. 3 · 10 −12 . This corresponds to a deviation of one second in 10,000 years. This means that the clock is significantly less precise than the large stationary atomic clocks, but considerably more accurate than a quartz clock. (Accurate, non-temperature-compensated quartz watches have a deviation of around one second in a month. Compared to these, this small atomic clock is 120,000 times more accurate.)
Hydrogen maser clocks for stimulating the oscillation are also highly accurate, but more difficult to operate. The first hydrogen maser in earth orbit was transported into orbit on the Galileo navigation satellite Giove-B on April 27, 2008 as a time base for location determination.
Atomic clocks in integrated circuits
In 2011, a portable Chip Scale Atomic Clock (CSAC) with a volume of 17 cm³ came onto the civilian market at a price of $ 1500.
Use in Germany, Austria and Switzerland
In Germany, four atomic clocks are in operation at the Physikalisch-Technische Bundesanstalt (PTB) in Braunschweig , including two "cesium fountains" in regular operation. Since 1991 the cesium clock CS2 has been providing the time standard for the seconds of legal time. Radio clocks can receive this time via the time signal transmitter DCF77 ; it is also available on the internet via NTP .
In Austria , the Federal Office for Metrology and Surveying (Laboratory for Frequency, Time) operates several atomic clocks. The master clock supplies UTC (BEV). Computers can receive this time from the Stratum 1 servers via the NTP.
In Switzerland , the Laboratory for Time and Frequency of the Federal Office of Metrology (METAS) operates several atomic clocks with which the Swiss atomic time TAI (CH) is kept and the Swiss world time UTC (CH) is calculated. This can be queried over the Internet using the NTP protocol . Until 2011, radio clocks could also receive this time signal via the HBG time signal transmitter.
Atomic clocks are used, on the one hand, to measure the exact time of processes, and, on the other, to determine the exact time and coordinate different time systems and scales. For example, by comparing the internationally determined atomic time (TAI) with astronomical time ( UT1 ), the coordinated universal time (UTC) is created. In Central Europe, radio clocks receive the UTC-based time signal via the DCF77 transmitter stationed in Germany . The British counterpart is the channel MSF .
- The cesium clock model 5071A, originally developed by Hewlett-Packard and later sold by Agilent, then Symmetricom and finally Microsemi, is used in many standards institutes around the world. B. in the atomic clock laboratory of the US Naval Observatory .
- In the Atomic Clock Ensemble in Space (ACES), part of the Columbus space laboratory , two cesium atomic clocks are to be tested for use at Galileo.
- Rubidium clocks can be manufactured in compact dimensions and inexpensively. They are used in telecommunications, energy supply and for calibration in industry. A very sophisticated model works in the latest generation of satellites in the GPS navigation system.
- A rubidium oscillator stabilized the carrier frequency of the long wave radio station Donebach .
- The time impulses of numerous atomic clocks are made freely available to everyone on the Internet using the Network Time Protocol (NTP).
- Rubidium clocks are used in high quality word clock generators to synchronize groups of digital audio devices with one another.
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