An atomic clock is a clock device that uses an electron transition frequency in a microwave, optical, or ultraviolet region of the atomic electromagnetic spectrum as the frequency standard for its timing element. The atomic clock is the most accurate time and known frequency standard, and is used as the primary standard for international time distribution services, to control the frequency of broadcast television, and in global navigation satellite systems such as GPS.
The principle of atomic clock operation is based on atomic physics; it uses microwave signals that are released by electrons in the atom when they change energy levels. Early atomic clocks are based on maser at room temperature. Currently, the most accurate atomic clocks first cool the atoms to near absolute zero temperatures by slowing them down with lasers and probing them in atomic fountains in microwaved cavities. An example is the NIST-F1 atomic clock, one of the major national time and frequency standards of the United States.
Accuracy of atomic clocks depends on two factors. The first factor is the atomic temperature of the sample - the cooler atoms move much more slowly, allowing longer check times. The second factor is the intrinsic frequency and width of the electronic transition. Higher frequencies and narrower lines increase accuracy.
National standard bodies in many countries maintain aligned atomic clock networks and are continuously synchronized with an accuracy of 10 seconds (seconds) <9 seconds (about 1 part in 10 14 ). These hours collectively determine the time scale of sustainable and stable, International Atomic Time (TAI). For civil time, other time scales are disseminated, Coordinated Universal Time (UTC). UTC comes from TAI, but adds a leap second from UT1, to account for the Earth's rotation with respect to solar time.
Video Atomic clock
Histori
The idea of ​​using atomic transitions for measuring time was suggested by Lord Kelvin in 1879. Magnetic resonance, developed in 1930 by Rabbi Isidor, became a practical method for doing this. In 1945, the Rabbi first publicly declared that magnetic resonance of an atomic ray could be used as the base of the clock. The first atomic clock was an ammonia drainage device at 23870.1 MHz built in 1949 at the National Bureau of Standards (NBS, now NIST). It was less accurate than the existing quartz clock, but presented to show the concept. The first accurate atomic clock, a standard cesium based on a particular transition of cesium-133 atoms, was built by Louis Essen and Jack Parry in 1955 at the National Physical Laboratory in England. The calibrated atomic clock calibration calibration is performed by the use of astronomical time scales ephemeris time (ET). This leads to an internationally agreed definition of the newest SI seconds based on atomic time. The second ET equation with second atomic clock has been verified in 1 part in 10 10 . The second SI inherits the decision effect by the original designer of the ephemeris time scale, determining the length of the second ET.
Since the early development of the 1950s, atomic clocks have been based on hyperfine transitions in hydrogen-1, cesium-133, and rubidium-87. The first commercial atomic clock was Atomichron, produced by the National Company. More than 50 were sold between 1956 and 1960. These large and expensive instruments were later replaced by smaller rack-mountable devices, such as the standard Hewlett-Packard 5060 cesium frequency model, released in 1964.
In the late 1990s there were four factors that contributed to the great progress in hours:
- Laser cooling and atomic trap
- The so-called Fabry-PÃÆ' cavity cavity is high for the narrow laser width line
- Precise laser spectroscopy
- Convenient optical frequency calculation using optical comb.
In August 2004, NIST scientists demonstrated chip-scale atomic clocks. According to the researchers, the clock is believed to be one-hundredth the size of another size. It requires no more than 125 mW, making it suitable for battery-driven applications. This technology became commercially available in 2011. The optical clock experimental optical ion is more precise than the current cesium standard.
In April 2015, NASA announced that it plans to deploy the Deep Space Atomic Clock (DSAC), an ultra-precision atomic mercury-ion atom, into space. NASA says that DSAC will be much more stable than other navigation hours.
Maps Atomic clock
Mechanism
Since 1967, the International System of Units (SI) has defined the latter as the duration of 9 192 631 770 Ã, cycle of the radiation corresponding to the transition between two energy levels of the cesium-133 atom. In 1997, the International Committee for Weight and Size (CIPM) added that the previous definition refers to cesium atoms at rest at absolute zero temperatures.
This definition makes the cesium oscillator the main standard for measuring time and frequency, called the cesium standard. The definition of other physical units, such as volts and meters, depends on the second definition.
The actual time reference of an atomic clock consists of an electronic oscillator operating at microwave frequencies. The oscillator is set so that the frequency determinant components include elements that can be controlled by the feedback signal. The feedback signal makes the oscillator tuned in resonance with the electronic transition frequency of cesium or rubidium.
The core of the atomic clock is a melodic microwave cavity containing gas. In the hydrogen mask clock, the gas emits microwaves (gas mas ) in the hyperfine transition, the area in the cavity oscillates, and the cavity is tuned to the maximum microwave amplitude. Or, in a cesium or rubidium clock, light or gas absorbs microwaves and cavities containing electronic amplifiers to make it oscillate. For both types of atoms in the gas are prepared in one electronic state before filling them into the cavity. For the second type, the number of atoms that change the electronic state is detected and the cavity is set to the maximum detected status change.
Much of the complexity of the hours lies in this adjustment process. Adjustments try to correct undesirable side effects, such as frequencies from other electron transitions, temperature changes, and frequency spreading caused by the ensemble effect. One way to do this is to sweep the microwave oscillator frequency in the narrow range to produce a modulated signal on the detector. The detector signal can then be demodulated to apply feedback to control long-range deviations in radio frequency. In this way, the quantum mechanical properties of the atomic transition frequency of cesium can be used to set the microwave oscillator to the same frequency, except for a small number of experimental errors. When the clock is first turned on, it may take a while for the oscillator to stabilize. In practice, feedback and monitoring mechanisms are much more complex.
A number of other atomic clock schemes are being used for other purposes. Rubidium standard clock is valuable for low cost, small size (commercial standard as small as 17 cm 3 ) and short-term stability. They are used in many commercial, portable and aerospace applications. Hydrogen builders (often produced in Russia) have superior short-term stability compared to other standards, but are lower in long-term accuracy.
Often, one standard is used to fix another. For example, some commercial applications using the rubidium standard are periodically corrected by the global positioning system receiver (see GPS discipline oscillator). This achieves excellent short-term accuracy, with long-term accuracy equivalent to (and traceable to) US national time standards.
Standard life is an important practical issue. The modern rubidium standard tube lasts for more than ten years, and the price can reach US $ 50. The cesium reference tube corresponding to the current national standard lasts about seven years and costs around US $ 35,000. The long-term stability of the standard hydrogen maser decreases due to changes in cavity properties over time.
Modern clocks use magneto-optical traps to cool atoms to be more accurate.
Power consumption
The atomic clock power consumption varies with its size. Atomic clocks on a single chip scale require less than 30 milliwatts; Primary frequencies and time standards such as the US Standard Time atomic clock, NIST-F1 and NIST-F2, use a much greater amount of power.
Evaluate accuracy
The evaluated accuracy of the report of major frequency and time standards is published online by the International Bureau of Weight and Size (BIPM). Some of the frequency and timeliness groups in 2015 reported u B values ​​in 2 ÃÆ'â € "10 -16 to the range 3 ÃÆ'â € "10 -16 .
In 2011, the NEC-CsF2 cesium fountain clock operated by the National Physical Laboratory (NPL), which functions as the primary frequency and time standard of the British Empire, is enhanced in relation to two of the largest sources of uncertainty measurement - the phase of the distributed cavity and microwave shift coating. In 2011 this resulted in reduced frequency uncertainty evaluated from u B = 4.1 ÃÆ'â € "10 -16 to u B = 2,3 ÃÆ'â € "10 -16 ; - the lowest value for any major national standards at the time. At this uncertainty frequency, NPL-CsF2 is expected to not gain or lose one second in approximately 138 million ( 138 ÃÆ'â € "10 6 ) years.
The NIST-F2 fountain clock operated by the National Institute of Standards and Technology (NIST), was officially launched in April 2014, to serve as the new US civil time frequency and time standard, along with NIST-F1 standards. The planned performance level of NIST-F2 is 1 ÃÆ'â € "10 -16 . "At this planned performance level, the NIST-F2 clock will not lose a second in at least 300 million years." NIST-F2 is designed using lessons learned from NIST-F1. The main key of NIST-F2 compared to NIST-F1 is that the vertical flight tube is now cooled inside the liquid nitrogen container, at -193Ã,  ° C (-315,4Ã, ° F). Cooling this cycle dramatically lowers background radiation and thus reduces some very small measurement errors that need to be fixed in NIST-F1.
The first in-house accuracy evaluation of NIST-F2 reported u B of 1.1 ÃÆ'â € "10 -16 . However, published scientific criticisms of the NIST F-2 accuracy evaluation illustrate the problem in handling phase shifts in distributed cavities and microwave lens frequency shifts, treated significantly differently than most accurate fountain evaluation. Submission of the next NIST-F2 to BIPM in March, 2015 again reported u B of 1.5 ÃÆ'â € "10 - 16 , but does not respond to standing criticism. No further reports to BIPM from NIST-F2 nor the latest accuracy evaluations have been published.
At the request of the Italian standards organization, NIST made many duplicate components for the second version of NIST-F2, known as IT-CsF2 to be operated by Istituto Nazionale at Ricerca Metrologica (INRiM), NIST's partner in Turin, Italy. In May, October and November 2016, TI-CsF2 cesium fountain clock reported u B of 1.7 ÃÆ'â € "10
Research
Much of the research focuses on the often conflicting goals of making clocks smaller, cheaper, more portable, more energy efficient, more accurate, more stable, and more reliable. The Atomic Clock Ensemble in Space is an example of clock research.
Secondary representation of the second
The recommended frequency list for secondary representations of the latter is maintained by the International Bureau of Weights and Measures (BIPM) since 2006 and is available online. This list contains the respective standard frequency and uncertainty values ​​for rubidium microwave transitions and for some optical transitions. This secondary frequency standard is accurate at the inner level 10 -18 ; however, the uncertainties provided in the list fall within the span of 10 -14 - 10 -15 as they are limited by link to the current standard cesium standard (2015) defines the latter.
For context, femtosecond ( 1 ÃÆ' - 10 -15 Ã, s ) is the second second is about 31.71 million ( 31,71 ÃÆ' - 10 6 ) year and attosecond ( 1 ÃÆ' - 10 -18 Ã, s ) is a second second what is about 31.71 billion ( 31,71 ÃÆ' - 10 9 ) year.
The 21st century experimental atomic clock that provides non-cesium secondary representations of the latter becomes so precise that they may be used as highly sensitive detectors for things other than measuring frequency and time. For example, the atomic clock frequency is slightly altered by gravity, magnetic field, electric field, force, movement, temperature and other phenomena. The experimental clock tends to keep increasing, and leadership in performance has shifted back and forth between the various types of experimental clocks.
Quantum clock
In March 2008, physicists at NIST described the quantum logic clock based on individual ions of beryllium and aluminum. This clock is compared to the NIST mercury ion clock. These are the most accurate hours that have been built, with hours not getting or losing time at a rate that will exceed one second in over a billion years. In February 2010, NIST physicists described the enhanced second version of the quantum logic clock based on individual magnesium and aluminum ions. Considered the most precise clock in the world in 2010 with the fractional frequency imprecision of 8.6 ÃÆ'â € "10 -18 , it offers more than twice the original accuracy.
The accuracy of the experimental quantum clock has since been superseded by experimental optical lattice hours based on strontium-87 and ytterbium-171.
Optical clock
The theoretical step of the microwaves as an atomic "breakout" for light hours in the optical range (more difficult to measure but offers better performance) resulted in John L. Hall and Theodor W. HÃÆ'¤nsch Nobel Prize in Physics in 2005. One of Nobelis Physics 2012, David J. Wineland, is a pioneer in exploiting the properties of one ion stored in a trap to develop the highest stability hours.
New technologies, such as the femtosecond frequency comb, optical lattice, and quantum information, have enabled the prototype of the next generation atomic clock. This clock is based on optical transitions rather than microwaves. The main obstacle to developing optical clocks is the difficulty of directly measuring optical frequencies. This problem has been solved with the development of a laser encoded self-reference mode, commonly referred to as a femtosecond frequency comb. Prior to the frequency combo demonstration in 2000, terahertz techniques were needed to bridge the gap between radio frequency and optical frequency, and the system to do so became complicated and complicated. With the improvements of the frequency combs, these measurements are becoming much more accessible and many optical clock systems are now being developed around the world.
As in radio range, absorption spectroscopy is used to stabilize the oscillator - in this case the laser. When the optical frequency is divided into radio frequency which can be calculated by using femtosecond inserts, the bandwidth of phase noise is also divided by that factor. Although the laser phase noise bandwidth is generally larger than a stable microwave source, after division it is less.
The main systems considered for use in optical frequency standards are:
- a single ion isolated in an ion trap;
- a neutral atom trapped in an optical lattice and
- atoms packed in a three dimensional gas quantum optical lattice.
These techniques allow the atom or ion to be very isolated from external interference, resulting in a very stable reference frequency.
The atomic system being considered includes Al , Hg /2 , Hg, Sr, Sr /2 , In /3 , Mg, Ca, Ca , Yb /2/3 , Yb and Th /3 .
The rare-earth ytterbium element (Yb) is valued not so much by its mechanical properties but to complement the internal energy level. "A particular transition in Yb atoms, at a wavelength of 578 nm, currently provides one of the most accurate standard optical atomic frequencies in the world," said Marianna Safronova. Estimates of the amount of uncertainty achieved in accordance with the Yb clock uncertainty of about one second during the lifetime of the universe so far, 15 billion years, according to scientists at the Joint Quantum Institute (JQI) and University of Delaware in December 2012.
In 2013 optical lattice hours (OLCs) prove to be as good or better than cesium fountain clocks. The two optical lattice clocks containing about 10,000 atoms of strontium-87 are able to stay in sync with each other at a precision of at least 1.5 ÃÆ'â € "10 -16 , which is as accurate as measurable by experiment. These hours have been shown to offset all three hours of cesium fountain at the Paris Observatory. There are two reasons for better accuracy. First, the frequency is measured using light, which has a much higher frequency than the microwaves, and second, by using many atoms, every error is averaged. Using ytterbium-171 atoms, a new record for stability with a precision of 1,6 ÃÆ' - 10 -18 over a 7 hour period was published on August 22 2013. At this stability, two optical lattice clocks working independently of each other used by the NIST research team will differ less than one second above the age of the universe ( 13.8 ÃÆ' - 10 < soup> 9 Ã, year ); this 10 times is better than the previous experiment. The clock is dependent on 10 000 ytterbium atoms spaced cooled to 10 microkelvin and trapped in an optical lattice. Lasers at 578 nm excite atoms between two levels of their energy. After establishing the stability of the clock, the researchers are studying external influences and evaluating the remaining systematic uncertainties, in the hope that they can bring clock accuracy to their level of stability. The enhanced optical lattice clock is described in a 2014 Nature paper. In 2015 JILA evaluated the absolute frequency uncertainty of the strontium-87 optical lattice clock at 2.1 ÃÆ'â € "10 -18 , which corresponds to widening the measured gravity time for elevation changes of 2 cm (0.79 inches) on planet Earth which according to JILA/NIST Jun Ye's Fellow is "getting closer to useful for relativistic geodesy". At this uncertainty frequency, the JILA lattice optical optical clock is not expected to gain or lose a second in more than 15 billion ( 15 ÃÆ'â € "10 9 ) years.
In 2017 JILA reported a quantum optical lattice strontium optical gap clock in which the strontium-87 atom is packed into a small three-dimensional (3-D) cube at 1,000 times the current one-dimensional clock density (1-D), such as the JILA 2015 clock. synchronous clocks between the two 3D grid zones resulted in a record 5 record sync levels in 1 hour of average time. The quantum nucleus strontium optical lattice clock 3D is an unusual state of matter called degenerate Fermi gas (quantum gas for Fermi particles). The experimental data shows the quantum 3D gas clock reaching the precision of 3.5 ÃÆ'â € "10 -19 in about two hours. According to Jun Ye "This is a significant improvement over every previous demonstration." Ye further commented "The most important potential of a 3D gas quantum clock is the ability to increase the number of atoms, which will lead to a major increase in stability." and "The ability to increase the number of atoms and time coherence will make this new generation of clocks qualitatively different from previous generations." In 2018 JILA reported a 3D gas quantum clock reaching a frequency precision of 2.5 ÃÆ'â € "10 -19 for 6 hours. In the uncertainty of these frequencies, the 3D gas quantum clock is expected to not gain or lose a second in the near future ( 126.84 ÃÆ'â € "9 12 ) years or more than 0, 1 second compared to the age of the universe.
The current optical clock (2018) is still mainly a mature, research-ridden project of rubidium and microwave cesium standards, which regularly allow time for the International Bureau of Weights and Measures (BIPM) to build the International Atomic Time (TAI). When optical clock experiments move beyond their microwave counterparts in terms of accuracy and performance stability this puts them in a position to replace the current standard, the cesium fountain clock. In the future this may lead to redefining the second cesium-based microwave SI and other new dissemination techniques at the highest level of accuracy to transfer clock signals will be required which can be used both in shorter span range and longer range (frequency) between more clocks both and to explore their fundamental limitations without significantly compromising their performance.
The clock comparison technique
The development of atomic clocks has led to many advances in science and technology such as precise global and regional navigation satellite system systems, and applications on the Internet, which are heavily dependent on frequency and time standards. The atomic clock is mounted on the radio timing radio timing site. They are used in several long-wave and medium-wave broadcast stations to provide very precise carrier frequencies. Atomic clocks are used in many disciplines, such as for long-term interferometry in radioastronomy.
The Global Positioning System (GPS) operated by the US Air Force Space Command provides a very accurate time and frequency signal. A GPS receiver works by measuring the relative delay time of signals from a minimum of four, but usually more, GPS satellites, each of which has at least two cesium onboard and as much as two hours of rubidium atoms. The relative time is mathematically converted into three absolute space coordinates and an absolute time coordinate. GPS time (GPST) is a continuous time scale and is theoretically accurate to about 14 ns. However, most receivers lose accuracy in signal interpretation and are only accurate up to 100 ns. GPST is related but different from TAI (International Atomic Time) and UTC (Coordinated Universal Time). GPST remains at a constant offset with TAI (TAI - GPST = 19 seconds) and like TAI does not implement leap seconds. Periodic correction is done to the clock on the board on the satellite to keep it synchronized with the ground clock. GPS navigation messages include the difference between GPST and UTC. In July 2015, the GPST is 17 seconds ahead of UTC because the leap seconds are added to UTC on June 30, 2015. The recipient reduces this weighing from the GPS Time to calculate the UTC and a specified time zone value.
The GLObal NAvigation Satellite System (GLONASS) operated by the Russian Aerospace Defense Force provides an alternative to the Global Positioning System (GPS) system and is a second navigation system operating with global coverage and comparable precision. GLONASS Time (GLONASST) is generated by GLONASS Central Synchroniser and is usually better than 1,000 ns. Unlike GPS, the GLONASS time scale implements leap seconds, such as UTC.
Galileo's Global Navigation Satellite System is operated by the European GNSS Agency and the European Space Agency. Galileo began offering Global Initial Operating Capacity (EOC) on December 15, 2016, providing the first non-military, non-military Global Satellite Navigation System, and is expected to achieve FOC in 2019. To achieve the goal of the constellation of FOC Galileo 6 extra satellites are planned to be added. Galileo System Time (GST) is a continuous time scale generated on the field at Galileo Control Center in Fucino, Italy, by Precise Timing Facility, based on average different atomic clocks maintained by Galileo Central Segment and synchronized with TAI with nominal offset at under 50 ns. According to the European GNSS Agency, Galileo offers 30-second punctuality. The March 2018 Quarterly Performance Report by the European GNSS Service Center reported UTC Time Dissemination Service Accuracy was <= 7.6 ns, calculated by collecting samples over the previous 12 months and exceeding the target <= 30 ns. Each Galileo satellite has two passive hydrogen masers and two hours of rubidium atoms for onboard time. Galileo's navigation messages include the difference between GST, UTC and GPST (to promote interoperability).
System under construction
The BeiDou-2 satellite navigation system is in construction by 2017 but should add extra satellites planned to achieve the goal of its full-scale global coverage constellation. BeiDou Time (BDT) is a continuous time scale from 1 January 2006 at 0:00:00 UTC and synchronized with UTC within 100 ns. BeiDou started operations in China in December 2011, with 10 satellites in use, and began offering services to customers in the Asia-Pacific region in December 2012. BeiDou's global navigation system must be completed by 2020.
Time radio signal transmitter
The clock radio is a clock that automatically synchronizes itself using the radio timing signal the government receives by the radio receiver. Many retailers market the clock radio inaccurately like atomic clocks; although the radio signals they receive are from atomic clocks, they are not the atomic clock itself. Normal low-cost consumer receptors depend only on amplitude-modulated timing signals and use narrow band receivers (with 10 Hz bandwidth) with antennas and small ferrite loopin circuits with digital signal processing delay not optimal and therefore can only be expected to determine the start of a second with accuracy of practical accuracy of ± 0.1 second. This is sufficient to be controlled by hours and hours of low quality radio controlled consumers using standard quality quartz clocks for timeliness between daily sync attempts, as they will be most accurate as soon as successful synchronization and will become less accurate from that point forward until next sync. Recipient of instrument class time gives higher accuracy. The device experiences a transit delay of about 1 ms for every 300 kilometers (186 mi) distance from the radio transmitter. Many governments operate transmitters for the purpose of timers.
See also
References
External links
- National Research Council Canada, FAQ: "What is 'cesium atomic clock'?"
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S.R. Jefferts, T.P. Heavner, T.E. Parker and J.H. Shirley ( Time and Frequency of NIST Division ) (2007). "NIST Cesium Fountains - Current Status and Future Prospects" (PDF) . Acta Physica Polonica A . 112 (5): 759 ff . Bibcode: 2007AcPPA.112..759J. CS1 maint: Using the author parameters (link) - National Research Council Canada, archived content: Optical frequency standards based on one trapped ion
- United States Naval Observation Service Department Department
- PTB Braunschweig, Germany - with links in English
- National Physical Laboratory (UK) website
- NIST Internet Time Service (ITS): Set Your Computer Hours Via the Internet
- NIST press release about chip-scale atomic clocks
- The NIST website
- Web page at atomic clock by The Science Museum (London)
- Atomic Optical Clock BBC, 2005
- Optical grid clock; Journal of Japanese Physical Society
- The atomic fountain
- Canadian National Research Council, archived content: Optical Frequency Comb - Optical Frequency Measurement
Source of the article : Wikipedia
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