Atomic clock

Atom cesium clock.

An atomic clock is a type of clock that uses a normal atomic resonance frequency to power its counter. Early atomic clocks took their reference from a maser. The best modern atomic frequency references (or clocks) are based on more advanced physics, involving cold atoms and atomic sources. National standards agencies maintain an accuracy of 10^-9 seconds per day and an accuracy equal to the frequency of the radio transmitter pumping the maser.

Atomic clocks maintain a continuous and stable time scale, International Atomic Time (IAT). For daily use another chronological scale is disseminated: Coordinated Universal Time (UTC). UTC is derived from TAI, but is synchronized using leap seconds with Universal Time (UT1), which is based on the day-night transition based on astronomical observations.

The first was built at the Willard Frank Libby, in the USA, in 1949, based on ideas about an extremely regular phenomenon: molecular and atomic magnetic resonance, by Isidor Isaac Rabi, Nobel Prize in Physics, although the precision achieved using ammonia —a molecule used by the National Institute of Standards and Technology (NIST) prototype— was not much higher than the standards of the time, based on quartz oscillators.

Today the best atomic frequency standards are based on the physical properties of cesium emission sources. The first cesium atomic clock was built in 1955 at the National Physical Laboratory (NPL) in England. Its creators were Louis Essen and John V.L Parry.

In 1967, cesium-based atomic clocks had achieved sufficient reliability for the International Bureau of Weights and Measures to choose the atomic vibration frequency of the devices created and perfected by Essen as the new base standard for the definition of the unit of physical time. According to this pattern, one second corresponds to 9,192,631,770 cycles of the radiation associated with the hyperfine transition from the resting state of the isotope of cesium 133: (¹³³Cs).

The precision achieved with this type of atomic clock is so high that it admits only an error of one second in 30,000,000 years. The most accurate clock in the world is designed at the Paris Observatory, where current atomic clocks would take 52 million years to lose one second. The new goal of French research is to increase that period to 32 billion years. The current standard for working atomic clocks allows one second every 3.7 billion years (NIST).

History

Lord Kelvin first suggested in 1879 the idea of using atomic vibration to measure time. The practical method for doing this became MRI, developed in the 1930s by Isidor Isaac Rabi. The first atomic clock was an ammonia maser device built in 1949 at the US National Bureau of Standards. NBS, now NIST). It was less accurate than existing quartz watches, but it served to demonstrate the concept. The first exact atomic clock was a cesium standard based on a certain transition of the ¹³³Cs atom, built by Louis Essen in 1955 at the National Physical Laboratory (UK). The standard cesium atomic clock was calibrated using the ephemeris time (TE) astronomical time scale.

This led to the most recent internationally agreed definition of second, by the International System of Units (SI), based on atomic time. It has been verified that the equality of the second ET with that of the second SI (atomic clock) is of a precision of 1 part in 10¹⁰. The second SI thus inherits the effect of the decisions of the original designers of the ET time scale: ephemeris time, the determination of the duration of the second ET.

May 2009. The JILA (Joint Institute for Laboratory Astrophysics) strontium optical atomic clock is now the world's most accurate clock based on neutral atoms. A bright blue laser on ultracold strontium atoms in an optical trap testing the efficacy of a previous burst of light from a red laser has driven the atoms into an excited state. Only the atoms that remain in the lowest energy state respond to the blue laser and cause the fluorescence that is expressed here. Photography: Sebastian Blatt, JILA, University of Colorado.

Since the beginning of development in the 1950s, atomic clocks have been made on the hyperfine (microwave) basis of transitions in ¹H (hydrogen 1), ¹³³Cs and ⁸⁷Rb (rubidium 87). The first commercial atomic clock was the Atomichron manufactured by the National Company. More than 50 were sold, between 1956 and 1960. This bulky and expensive machine was later replaced by much smaller rack-mount devices such as the standard Hewlett-Packard Model 5060, of cesium frequency, released in 1964 [1].

In the late 1990s, four factors contributed to significant advances in these types of watches:

• Laser cooling and trapped in atoms.
• Fabry-Pérot high-end cavities for narrow laser lines.
• Precision laser spectroscopy.
• A convenient count of optical frequencies using optical combs.

In August 2004, NIST scientists demonstrated a chip atomic clock. According to the researchers, the size of the clock would be one hundredth of any other. It was also claimed to require only 75 milliwatts (mW), which is ideal for battery-powered applications. This technology has been commercially available since 2011 (SA.45s CSAC Chip Scale Atomic Clock. 2011. May 24, 2012).

In March 2008, physicists at NIST demonstrated a quantum logic-based clock on mercury and individual aluminum ions. These two watches are the most accurate that have been built to date. They are not slowing down, or moving forward, at a speed that exceeds more than one second in a billion years.

Recent Developments

An atomic clock implemented in an integrated circuit developed by the NIST.

Despite this, physicists continue to experiment with new variations with masers, of: a) hydrogen (Townes); b) rubidium optical pumping (Kasler); c) those recently proposed for mercury, which would allow greater precision to be achieved. The precision of the cesium clocks with lasers to cool the atoms is also constantly being improved, and that obtained in the latest NIST clock, the NIST-F1, launched in 1999, which is of the order of one second in twenty million years..

In August 2004, NIST scientists made the first demonstration of an atomic clock the size of an integrated circuit. This represents a clock one hundred times less than any other built to date, whose consumption is only 0.079 watts.

Operation

The mechanical clock relies on a pendulum to function. The atomic works through the frequency of hyperfine energy transitions (in the microwave ranges) in atoms.

At one end of the cesium clock is a furnace with a cesium plate, from which cesium ions evaporate. Ions occur in two states dependent on the spin or spin (spin) of the last cesium electron. The energy difference between these two states corresponds to a frequency of 9,192,631,770 hertz (Hz). In each state the magnetic properties of the ions are different. After evaporation, a magnet is used to separate the ions and discard those with the highest energy. The lower energy ions are relocated in a chamber.

The real clock is an electronic oscillator that generates pulses of an adjustable frequency. It is adjusted to that corresponding to the hyperfine transition of cesium by the following feedback process. A microwave radio transmitter uniformly fills the chamber cavity with radio waves of the frequency of the electronic oscillator. When the frequency of the radiated wave is matched to the frequency of the cesium hyperfine transition, the cesium ions absorb the radiation and emit light. A photoelectric cell is sensitive to the emitted light and is connected to the electronic oscillator with electronic instrumentation.

To carry out the measurement using these particles it is necessary to create an electromagnetic field that does not exist naturally in the Universe. The process is carried out inside a "magneto-optical trap": a sphere the size of a melon, into which cesium atoms are injected which, confined in a magnetic field, propagate six beams of laser light. Just as a person slows down in a gust of wind, atoms slow down when bombarded by lasers fired from all directions. Using this method, atoms can slow down to ten thousand times slower than normal. When atoms and lasers collide, a cloud of very slow or ultracold atoms is formed.

The most frequent uses of atomic clocks are:

• Phone networks.
• Global Positioning Systems (GPS).
• Time measurement.
• Equipment calibration.

Research

Most research focuses on the often conflicting goals of making watches smaller, cheaper, more accurate, and more reliable.

New technologies such as femtosecond frequency combs, optical gratings, and quantum information have enabled the next generation of atomic clocks to be prototyped. These are based on optics, rather than microwave transitions. A major obstacle to the development of an optical clock is the difficulty of directly measuring optical frequencies. This problem has been solved by the development of mode-locked self-referencing of lasers, commonly known as femtosecond frequency combs.

Prior to the demonstration of the frequency comb in 2000, terahertz techniques were necessary to bridge the gap between radio and optical frequencies. The respective systems were cumbersome and complicated. With the improvement of the frequency comb, these measurements have become much more accessible, and numerous optical clock systems are being developed around the world.

Just 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 noise phase bandwidth is also divided by that factor. Although generally such laser noise phase bandwidth is greater than stable microwave sources, after splitting it is less.

The two primary systems under study for use in optical frequency standards are ions isolated in an ion trap and neutral atoms trapped in an optical lattice. Both of these techniques allow the atoms or ions to be largely isolated from external disturbances, resulting in an extremely stable frequency reference.

• Optical clocks. They have already achieved greater stability and less systematic uncertainty than the best microwave watches. This places them in a position to replace the time standard: the cesium source clock.

• Atomic systems in consideration include Al¹⁺, Hg^{+ / 2+}, Hg, Mr.^{+ / 2+}, In^{+ / 3+}, Mg, Ca, Ca⁺Yb^{+ / 2+ / 3+} e Y.

Clock Radios

A radio clock is a clock that automatically adjusts to atomic time by means of official radio signals received by a radio receiver. Many retailers mistakenly sell clock radios as "atomic clocks." Although the radio signals they receive come from atomic clocks, these are not actual atomic clocks. They provide a means of obtaining highly accurate time from an atomic clock, over a wide area, with inexpensive equipment.

While official time broadcasts are themselves extremely accurate, many consumer clock radios sync only once a day, achieving only about a second of accuracy. To take advantage of the overall accuracy of received time signals, receiving instruments with time setting capability must be used. For every 300 kilometers (186 miles) of distance between the transmitter and the receiver there is a signal delay of approximately 1 ms (one millisecond).

The time signals generated by atomic clocks are broadcast by long-wave radio transmitters run by the governments of many countries around the world, such as DCF77 (Germany), HBG (Switzerland), JJY (Japan), MSF (United Kingdom), TDF (France) and WWVB (United States). These signals can be received from far away outside of your home country. Sometimes at night the JJY signal can be picked up even in Western Australia and Tasmania. Thus, there are very few regions of the world where precise time from atomic clocks is not available.

Applications

Atomic clocks are used to generate standard frequencies. They are installed at the sites of time signals, LORAN-C, and Alpha navigation transmitters. ^{[citation needed]} They have also been installed in some long and medium wave broadcast stations, to deliver very precise transmission frequencies, which can also function as standard frequencies. ^{[citation required]}

In addition, atomic clocks are used in long baseline interferometry in radio astronomy.

Atomic clocks are the basis of the GPS navigation system. The GPS master clock time is a weighted average of the atomic clocks located at ground stations and those located on GPS satellites. Each of them is endowed with several atomic clocks.

Aluminum atomic clock

Physicists at the National Institute of Standards and Technology (NIST) have built an improved version of an experimental atomic clock based on a single aluminum atom. As of February 2009, it is the most accurate clock, since in 3.7 billion years it does not gain or lose even a second (the cesium source atomic clock loses one second every 100 million years).

As the international definition of the second (International System of Units) is based on the cesium atom, this element remains the official time regulator. Therefore no other clock can be more accurate than the cesium clock.

Global Positioning System

The GPS system provides very accurate signals of time and frequency. A GPS receiver works by measuring the relative delay time of signals from four or more GPS satellites, each with three or four cesium or rubidium atomic clocks on board. The four relative times are mathematically transformed into three absolute distance coordinates and one absolute time coordinate.

Time precision is around 50 nanoseconds (ns). However, inexpensive GPS receivers probably don't assign a high priority to screen updating. Therefore, the displayed time may differ significantly from the internal time. References to time accuracy using GPS are marketed for use in computer networks, laboratories, and cellular communications networks. They keep the accuracy within the range of around 50 ns.

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