Atomic clock

Atomic clock

Overview
An atomic clock is a clock
Clock
A clock is an instrument used to indicate, keep, and co-ordinate time. The word clock is derived ultimately from the Celtic words clagan and clocca meaning "bell". A silent instrument missing such a mechanism has traditionally been known as a timepiece...

 that uses an electronic transition frequency
Frequency
Frequency is the number of occurrences of a repeating event per unit time. It is also referred to as temporal frequency.The period is the duration of one cycle in a repeating event, so the period is the reciprocal of the frequency...

 in the microwave
Microwave
Microwaves, a subset of radio waves, have wavelengths ranging from as long as one meter to as short as one millimeter, or equivalently, with frequencies between 300 MHz and 300 GHz. This broad definition includes both UHF and EHF , and various sources use different boundaries...

, optical
Light
Light or visible light is electromagnetic radiation that is visible to the human eye, and is responsible for the sense of sight. Visible light has wavelength in a range from about 380 nanometres to about 740 nm, with a frequency range of about 405 THz to 790 THz...

, or ultraviolet
Ultraviolet
Ultraviolet light is electromagnetic radiation with a wavelength shorter than that of visible light, but longer than X-rays, in the range 10 nm to 400 nm, and energies from 3 eV to 124 eV...

 region of the electromagnetic spectrum
Electromagnetic spectrum
The electromagnetic spectrum is the range of all possible frequencies of electromagnetic radiation. The "electromagnetic spectrum" of an object is the characteristic distribution of electromagnetic radiation emitted or absorbed by that particular object....

 of atoms as a frequency standard
Frequency standard
A frequency standard is a stable oscillator used for frequency calibration or reference. A frequency standard generates a fundamental frequency with a high degree of accuracy and precision. Harmonics of this fundamental frequency are used to provide reference points.Since time is the reciprocal of...

 for its timekeeping element. Atomic clocks are the most accurate time
Time standard
A time standard is a specification for measuring time: either the rate at which time passes; or points in time; or both. In modern times, several time specifications have been officially recognized as standards, where formerly they were matters of custom and practice. An example of a kind of time...

 and frequency standard
Frequency standard
A frequency standard is a stable oscillator used for frequency calibration or reference. A frequency standard generates a fundamental frequency with a high degree of accuracy and precision. Harmonics of this fundamental frequency are used to provide reference points.Since time is the reciprocal of...

s known, and are used as primary standard
Primary standard
A primary standard in metrology is a standard that is accurate enough that it is not calibrated by or subordinate to other standards. Primary standards are defined via other quantities like length, mass and time. Primary standards are used to calibrate other standards referred to as working...

s for international time distribution services, to control the wave frequency of television broadcasts, and in global navigation satellite system
Global Navigation Satellite System
A satellite navigation or SAT NAV system is a system of satellites that provide autonomous geo-spatial positioning with global coverage. It allows small electronic receivers to determine their location to within a few metres using time signals transmitted along a line-of-sight by radio from...

s such as GPS.

The principle of operation of an atomic clock is not based on nuclear physics
Nuclear physics
Nuclear physics is the field of physics that studies the building blocks and interactions of atomic nuclei. The most commonly known applications of nuclear physics are nuclear power generation and nuclear weapons technology, but the research has provided application in many fields, including those...

, but rather on atomic physics
Atomic physics
Atomic physics is the field of physics that studies atoms as an isolated system of electrons and an atomic nucleus. It is primarily concerned with the arrangement of electrons around the nucleus and...

 and using the microwave
Microwave
Microwaves, a subset of radio waves, have wavelengths ranging from as long as one meter to as short as one millimeter, or equivalently, with frequencies between 300 MHz and 300 GHz. This broad definition includes both UHF and EHF , and various sources use different boundaries...

 signal that electron
Electron
The electron is a subatomic particle with a negative elementary electric charge. It has no known components or substructure; in other words, it is generally thought to be an elementary particle. An electron has a mass that is approximately 1/1836 that of the proton...

s in atoms emit when they change energy level
Energy level
A quantum mechanical system or particle that is bound -- that is, confined spatially—can only take on certain discrete values of energy. This contrasts with classical particles, which can have any energy. These discrete values are called energy levels...

s.
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Encyclopedia
An atomic clock is a clock
Clock
A clock is an instrument used to indicate, keep, and co-ordinate time. The word clock is derived ultimately from the Celtic words clagan and clocca meaning "bell". A silent instrument missing such a mechanism has traditionally been known as a timepiece...

 that uses an electronic transition frequency
Frequency
Frequency is the number of occurrences of a repeating event per unit time. It is also referred to as temporal frequency.The period is the duration of one cycle in a repeating event, so the period is the reciprocal of the frequency...

 in the microwave
Microwave
Microwaves, a subset of radio waves, have wavelengths ranging from as long as one meter to as short as one millimeter, or equivalently, with frequencies between 300 MHz and 300 GHz. This broad definition includes both UHF and EHF , and various sources use different boundaries...

, optical
Light
Light or visible light is electromagnetic radiation that is visible to the human eye, and is responsible for the sense of sight. Visible light has wavelength in a range from about 380 nanometres to about 740 nm, with a frequency range of about 405 THz to 790 THz...

, or ultraviolet
Ultraviolet
Ultraviolet light is electromagnetic radiation with a wavelength shorter than that of visible light, but longer than X-rays, in the range 10 nm to 400 nm, and energies from 3 eV to 124 eV...

 region of the electromagnetic spectrum
Electromagnetic spectrum
The electromagnetic spectrum is the range of all possible frequencies of electromagnetic radiation. The "electromagnetic spectrum" of an object is the characteristic distribution of electromagnetic radiation emitted or absorbed by that particular object....

 of atoms as a frequency standard
Frequency standard
A frequency standard is a stable oscillator used for frequency calibration or reference. A frequency standard generates a fundamental frequency with a high degree of accuracy and precision. Harmonics of this fundamental frequency are used to provide reference points.Since time is the reciprocal of...

 for its timekeeping element. Atomic clocks are the most accurate time
Time standard
A time standard is a specification for measuring time: either the rate at which time passes; or points in time; or both. In modern times, several time specifications have been officially recognized as standards, where formerly they were matters of custom and practice. An example of a kind of time...

 and frequency standard
Frequency standard
A frequency standard is a stable oscillator used for frequency calibration or reference. A frequency standard generates a fundamental frequency with a high degree of accuracy and precision. Harmonics of this fundamental frequency are used to provide reference points.Since time is the reciprocal of...

s known, and are used as primary standard
Primary standard
A primary standard in metrology is a standard that is accurate enough that it is not calibrated by or subordinate to other standards. Primary standards are defined via other quantities like length, mass and time. Primary standards are used to calibrate other standards referred to as working...

s for international time distribution services, to control the wave frequency of television broadcasts, and in global navigation satellite system
Global Navigation Satellite System
A satellite navigation or SAT NAV system is a system of satellites that provide autonomous geo-spatial positioning with global coverage. It allows small electronic receivers to determine their location to within a few metres using time signals transmitted along a line-of-sight by radio from...

s such as GPS.

The principle of operation of an atomic clock is not based on nuclear physics
Nuclear physics
Nuclear physics is the field of physics that studies the building blocks and interactions of atomic nuclei. The most commonly known applications of nuclear physics are nuclear power generation and nuclear weapons technology, but the research has provided application in many fields, including those...

, but rather on atomic physics
Atomic physics
Atomic physics is the field of physics that studies atoms as an isolated system of electrons and an atomic nucleus. It is primarily concerned with the arrangement of electrons around the nucleus and...

 and using the microwave
Microwave
Microwaves, a subset of radio waves, have wavelengths ranging from as long as one meter to as short as one millimeter, or equivalently, with frequencies between 300 MHz and 300 GHz. This broad definition includes both UHF and EHF , and various sources use different boundaries...

 signal that electron
Electron
The electron is a subatomic particle with a negative elementary electric charge. It has no known components or substructure; in other words, it is generally thought to be an elementary particle. An electron has a mass that is approximately 1/1836 that of the proton...

s in atoms emit when they change energy level
Energy level
A quantum mechanical system or particle that is bound -- that is, confined spatially—can only take on certain discrete values of energy. This contrasts with classical particles, which can have any energy. These discrete values are called energy levels...

s. Early atomic clocks were based on maser
Maser
A maser is a device that produces coherent electromagnetic waves through amplification by stimulated emission. Historically, “maser” derives from the original, upper-case acronym MASER, which stands for "Microwave Amplification by Stimulated Emission of Radiation"...

s at room temperature. Currently, the most accurate atomic clocks first cool the atoms to near absolute zero
Absolute zero
Absolute zero is the theoretical temperature at which entropy reaches its minimum value. The laws of thermodynamics state that absolute zero cannot be reached using only thermodynamic means....

 temperature by slowing them with lasers and probing them in atomic fountain
Atomic fountain
An atomic fountain is a cloud of atoms that are tossed upwards by lasers in the Earth's gravitational field. If visible, it would resemble the water in a fountain. While weightless in the toss, the atoms are measured to set the frequency of an atomic clock....

s in a microwave-filled cavity. An example of this is the NIST-F1
NIST-F1
NIST-F1 is a cesium fountain clock or atomic clock in the National Institute of Standards and Technology in Boulder, Colorado, and serves as the United States' primary time and frequency standard...

 atomic clock, the U.S. national primary time and frequency standard.

The accuracy of an atomic clock depends on the temperature of the sample atoms—colder atoms move much more slowly, allowing longer probe times, as well as having reduced collision rates—and on the frequency and intrinsic width of the electronic transition. Higher frequencies and narrow lines increase the precision.

National standards agencies maintain an accuracy of 10−9 seconds per day (approximately 1 part in 1014), and a precision set by the radio transmitter pumping the maser
Laser pumping
Laser pumping is the act of energy transfer from an external source into the gain medium of a laser. The energy is absorbed in the medium, producing excited states in its atoms. When the number of particles in one excited state exceeds the number of particles in the ground state or a less-excited...

. These clocks collectively define a continuous and stable time scale, International Atomic Time
International Atomic Time
International Atomic Time is a high-precision atomic coordinate time standard based on the notional passage of proper time on Earth's geoid...

 (TAI). For civil time, another time scale is disseminated, Coordinated Universal Time
Coordinated Universal Time
Coordinated Universal Time is the primary time standard by which the world regulates clocks and time. It is one of several closely related successors to Greenwich Mean Time. Computer servers, online services and other entities that rely on having a universally accepted time use UTC for that purpose...

 (UTC). UTC is derived from TAI, but approximately synchronized, by using leap seconds, to UT1
Universal Time
Universal Time is a time scale based on the rotation of the Earth. It is a modern continuation of Greenwich Mean Time , i.e., the mean solar time on the Prime Meridian at Greenwich, and GMT is sometimes used loosely as a synonym for UTC...

, which is based on actual rotations of the earth with respect to the solar time
Solar time
Solar time is a reckoning of the passage of time based on the Sun's position in the sky. The fundamental unit of solar time is the day. Two types of solar time are apparent solar time and mean solar time .-Introduction:...

.

History


The idea of using atomic transitions to measure time was first suggested by Lord Kelvin in 1879. Magnetic resonance, developed in the 1930s by Isidor Rabi, became the practical method for doing this. In 1945, Rabi first publicly suggested that atomic beam magnetic resonance might be used as the basis of a clock. The first atomic clock was an ammonia
Ammonia
Ammonia is a compound of nitrogen and hydrogen with the formula . It is a colourless gas with a characteristic pungent odour. Ammonia contributes significantly to the nutritional needs of terrestrial organisms by serving as a precursor to food and fertilizers. Ammonia, either directly or...

 maser
Maser
A maser is a device that produces coherent electromagnetic waves through amplification by stimulated emission. Historically, “maser” derives from the original, upper-case acronym MASER, which stands for "Microwave Amplification by Stimulated Emission of Radiation"...

 device built in 1949 at the U.S. National Bureau of Standards (NBS, now NIST
National Institute of Standards and Technology
The National Institute of Standards and Technology , known between 1901 and 1988 as the National Bureau of Standards , is a measurement standards laboratory, otherwise known as a National Metrological Institute , which is a non-regulatory agency of the United States Department of Commerce...

). It was less accurate than existing quartz clock
Quartz clock
A quartz clock is a clock that uses an electronic oscillator that is regulated by a quartz crystal to keep time. This crystal oscillator creates a signal with very precise frequency, so that quartz clocks are at least an order of magnitude more accurate than good mechanical clocks...

s, but served to demonstrate the concept. The first accurate atomic clock, a caesium standard
Caesium standard
A caesium standard or caesium atomic clock is a primary frequency standard in which electronic transitions between the two hyperfine ground states of caesium-133 atoms are used to control the output frequency. They are one of the most accurate types of atomic clock...

 based on a certain transition of the caesium-133 atom, was built by Louis Essen
Louis Essen
Louis Essen FRS O.B.E. was an English physicist whose most notable achievements were in the precise measurement of time and the determination of the speed of light...

 in 1955 at the National Physical Laboratory
National Physical Laboratory, UK
The National Physical Laboratory is the national measurement standards laboratory for the United Kingdom, based at Bushy Park in Teddington, London, England. It is the largest applied physics organisation in the UK.-Description:...

 in the UK. Calibration of the caesium standard atomic clock was carried out by the use of the astronomical time scale ephemeris time
Ephemeris time
The term ephemeris time can in principle refer to time in connection with any astronomical ephemeris. In practice it has been used more specifically to refer to:...

(ET). This led to the internationally agreed definition of the latest SI second being based on atomic time. Equality of the ET second with the (atomic clock) SI second has been verified to within 1 part in 1010. The SI second thus inherits the effect of decisions by the original designers of the ephemeris time
Ephemeris time
The term ephemeris time can in principle refer to time in connection with any astronomical ephemeris. In practice it has been used more specifically to refer to:...

 scale, determining the length of the ET second.


Since the beginning of development in the 1950s, atomic clocks have been based on the hyperfine (microwave) transitions in hydrogen-1, caesium-133, and rubidium-87.
The first commercial atomic clock was the Atomichron
Atomichron
The Atomichron was the world's first commercial atomic clock, built by the National Company, Inc of Malden, Massachusetts. It was also the first self-contained portable atomic clock and was a caesium standard clock. More than 50 clocks with the trademarked Atomichron name were produced.- External...

, manufactured by the National Company
National Radio Company
The National Radio Company, headquartered in Malden, Massachusetts, USA was an American manufacturer of radio equipment from 1914 to 1991.-History:...

. More than 50 were sold between 1956 and 1960. This bulky and expensive instrument was subsequently replaced by much smaller rack-mountable devices, such as the Hewlett-Packard
Hewlett-Packard
Hewlett-Packard Company or HP is an American multinational information technology corporation headquartered in Palo Alto, California, USA that provides products, technologies, softwares, solutions and services to consumers, small- and medium-sized businesses and large enterprises, including...

 model 5060 caesium frequency standard, released in 1964.

In the late 1990s four factors contributed to major advances in clocks:
  • Laser cooling
    Laser cooling
    Laser cooling refers to the number of techniques in which atomic and molecular samples are cooled through the interaction with one or more laser light fields...

     and trapping of atoms
  • So-called high-finesse Fabry–Pérot cavities for narrow laser line widths
  • Precision laser spectroscopy
  • Convenient counting of optical frequencies using optical combs
    Frequency comb
    A frequency comb is the graphic representation of the spectrum of a mode locked laser. An octave spanning comb can be used for mapping radio frequencies into the optical frequency range or it can be used to steer a piezoelectric mirror within a carrier envelope phase correcting feedback loop...

    .


In August 2004, NIST
National Institute of Standards and Technology
The National Institute of Standards and Technology , known between 1901 and 1988 as the National Bureau of Standards , is a measurement standards laboratory, otherwise known as a National Metrological Institute , which is a non-regulatory agency of the United States Department of Commerce...

 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. This device could conceivably become a consumer product.

Mechanism


Since 1967, the International System of Units
International System of Units
The International System of Units is the modern form of the metric system and is generally a system of units of measurement devised around seven base units and the convenience of the number ten. The older metric system included several groups of units...

 (SI) has defined the second
Second
The second is a unit of measurement of time, and is the International System of Units base unit of time. It may be measured using a clock....

 as the duration of of radiation corresponding to the transition between two energy levels of the caesium-133 atom.

This definition makes the caesium oscillator the primary standard for time and frequency measurements, called the caesium standard
Caesium standard
A caesium standard or caesium atomic clock is a primary frequency standard in which electronic transitions between the two hyperfine ground states of caesium-133 atoms are used to control the output frequency. They are one of the most accurate types of atomic clock...

. Other physical quantities, e.g., the volt
Volt
The volt is the SI derived unit for electric potential, electric potential difference, and electromotive force. The volt is named in honor of the Italian physicist Alessandro Volta , who invented the voltaic pile, possibly the first chemical battery.- Definition :A single volt is defined as the...

 and the metre
Metre
The metre , symbol m, is the base unit of length in the International System of Units . Originally intended to be one ten-millionth of the distance from the Earth's equator to the North Pole , its definition has been periodically refined to reflect growing knowledge of metrology...

, rely on the definition of the second in their own definitions.

The actual time-reference of an atomic clock consists of an electronic oscillator operating at microwave frequency. The oscillator is arranged so that its frequency-determining components include an element that can be controlled by a feedback signal. The feedback signal keeps the oscillator tuned in resonance
Resonance
In physics, resonance is the tendency of a system to oscillate at a greater amplitude at some frequencies than at others. These are known as the system's resonant frequencies...

 with the frequency of the electronic transition of caesium or rubidium.

The core of the atomic clock is a tunable microwave cavity
Microwave cavity
A microwave cavity is a closed metal structure that confines electromagnetic fields in the microwave region of the spectrum. Such cavities act as resonant circuits with extremely low loss at their frequency of operation...

 containing the gas. In a hydrogen maser
Hydrogen maser
A Hydrogen maser, also known as hydrogen frequency standard, is a specific type of maser that uses the intrinsic properties of the hydrogen atom to serve as a precision frequency reference....

 clock the gas emits microwaves (the gas mases
Maser
A maser is a device that produces coherent electromagnetic waves through amplification by stimulated emission. Historically, “maser” derives from the original, upper-case acronym MASER, which stands for "Microwave Amplification by Stimulated Emission of Radiation"...

) 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 beam or 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 number of atoms which change electronic state is detected and the cavity is tuned for a maximum of detected state changes.

Most of the complexity of the clock lies in this adjustment process. 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
Lock-in amplifier
A lock-in amplifier is a type of amplifier that can extract a signal with a known carrier wave from an extremely noisy environment . It is essentially a homodyne with an extremely low pass filter...

 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 stabilize. 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
Rubidium standard
A rubidium standard or rubidium atomic clock is a frequency standard in which a specified hyperfine transition of electrons in rubidium-87 atoms is used to control the output frequency...

 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 compared 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 periodically corrected by a global positioning system
Global Positioning System
The Global Positioning System is a space-based global navigation satellite system that provides location and time information in all weather, anywhere on or near the Earth, where there is an unobstructed line of sight to four or more GPS satellites...

 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 trap
Magneto-optical trap
A magneto-optical trap is a device that uses both laser cooling with magneto-optical trapping in order to produce samples of cold, trapped, neutral atoms at temperatures as low as several microkelvins, two or three times the recoil limit.By combining the small momentum of a single photon with a...

s to cool the atoms for improved precision.

Physics package realizations


There exists a number of methods of utilizing the hyperfine splitting. These methods have their benefits and draw-backs and have influenced the development of commercial devices and laboratory standards. By tradition the hardware which is used to probe the atoms is called the physics package.

Atomic beam standard


The atomic beam standard is a direct extension of the Stern-Gerlach atomic splitting experiment. The atoms of choice are heated in an oven to create gas, which is collimated into a beam. This beam passes through a state-selector magnet A, where atoms of the wrong state are separated out from the beam. The beam is exposed to an RF field at or near the transition. The beam then passes through a space before it is again exposed to the RF field. The RF field and a static homogeneous magnetic field from the C-field coil will change the state of the atoms. After the second RF field exposure the atomic beam passes through a second state selector magnet B, where the atom state being selected out of the beam at the A magnet is being selected. This way, the detected amount of atoms will relate to the ability to match the atomic transition. After the second state-selector a mass-spectrometer using an ionizer will detect the rate of atoms being received.

Modern variants of this beam mechanism use optical pumping to transition all atoms to the same state rather than dumping half the atoms. Optical detection using scintillation can also be used.

The most common isotope for beam devices is caesium
Caesium
Caesium or cesium is the chemical element with the symbol Cs and atomic number 55. It is a soft, silvery-gold alkali metal with a melting point of 28 °C , which makes it one of only five elemental metals that are liquid at room temperature...

 (133Cs), but rubidium
Rubidium
Rubidium is a chemical element with the symbol Rb and atomic number 37. Rubidium is a soft, silvery-white metallic element of the alkali metal group. Its atomic mass is 85.4678. Elemental rubidium is highly reactive, with properties similar to those of other elements in group 1, such as very rapid...

 (87Rb) and thallium
Thallium
Thallium is a chemical element with the symbol Tl and atomic number 81. This soft gray poor metal resembles tin but discolors when exposed to air. The two chemists William Crookes and Claude-Auguste Lamy discovered thallium independently in 1861 by the newly developed method of flame spectroscopy...

 (205Tl) are examples of others used in early research.

The frequency errors can be made very small for a beam device, or predicted (such as the magnetic field pull of the C-coil) in such a way that a high degree of repeatability and stability can be achieved. This is why an atomic beam can be used as a primary standard.

Atomic gas cell standard


The atomic gas cell standard builds on a confined reference isotope (often an alkali metal such as Rubidium (87Rb)) inside an RF cavity. The atoms are excited to a common state using optical pumping; when the applied RF field is swept over the hyperfine spectrum, the gas will absorb the pumping light, and a photodetector provides the response. The absorption peak steers the fly-wheel oscillator.

A typical rubidium gas-cell uses a rubidium (87Rb) lamp heated to 108-110 degrees Celsius, and an RF field to excite it to produce light, where the D1 and D2 lines are the significant wavelengths. An 85Rb cell filters out the D1 line so that only the D2 line pumps the 87Rb gas cell in the RF cavity.

Among the significant frequency pulling mechanisms inherent to the gas cell are wall-shift, buffer-gas shift, cavity-shift and light-shift. The wall-shift occurs as the gas bumps into the wall of the glass container. Wall-shift can be reduced by wall coating and compensation by buffer gas. The buffer gas shift comes from the reference atoms which bounce into buffer gas atoms such as neon and argon; these shifts can be both positive and negative. The cavity shift comes from the RF cavity, which can deform the resonance amplitude response; this depends upon cavity center frequency and resonator Q-value. Light-shift is an effect where frequency is pulled differently depending on the light intensity, which often is modulated by the temperature shift of the rubidium lamp and filter cell.

There are thus many factors in which temperature and aging can shift frequency over time, and this is why a gas cell standard is unfit for a primary standard, but can become a very inexpensive, low-power and small-size solution for a secondary standard or where better stability compared to crystal oscillators is needed, but not the full performance of a caesium beam standard. The rubidium gas standards have seen use in telecommunications systems and portable instruments.

Active maser standard


The active maser standard is a development from the atomic beam standard in which the observation time was incremented by using a bounce-box. By controlling the beam intensity spontaneous emission will provide sufficient energy to provide a continuous oscillation, which is being tapped and used as a reference for a fly-wheel oscillator.

The active maser is sensitive to wall-shift and cavity pulling. The wall-shift is mitigated by using PTFE coating (or other suitable coating) to reduce the effect. The cavity pulling effect can be reduced by automatic cavity tuning. In addition the magnetic field pulls the frequency.

While not being long-term stable as caesium beams, it remains one of the most stable sources available. The inherent pulling effects makes repeatability troublesome and does prohibits its use as being primary standard, but it makes an excellent secondary standard. It is used as low-noise fly-wheel standard for caesium beam standards.

Fountain standard


The fountain standard is a development from the beam standard where the beam has been folded back to itself such that the first and second RF field becomes the same RF cavity. A ball of atoms is laser cooled, which reduces black body temperature shifts. Phase errors between RF cavities are essentially removed. The length of the beam is longer than many beams, but the speed is also much slower such that the observation time becomes significantly longer and hence a higher Q value is achieved in the Ramsey fringes.

Caesium fountains has been implemented in many laboratories, but rubidium has even greater ability to provide stability in the fountain configuration.

Ion trap standard


The ion trap standard is a set of different approaches, but their common property is that atoms used in their ion form is confined in a electrostatic field and cooled down. The hyperfine region of the available electron is then being tracked similar to that of a gas cell standard.

Ion traps has been tried for numerous ions, where mercury 199Hg+ was an early candidate.

Power consumption



The power consumption of atomic clocks varies with their size. One chip scale atomic clocks require power less than 75 mW
Watt
The watt is a derived unit of power in the International System of Units , named after the Scottish engineer James Watt . The unit, defined as one joule per second, measures the rate of energy conversion.-Definition:...

; NIST-F1 uses power orders of magnitude greater.

Research



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

New technologies, such as femtosecond frequency combs, optical lattices and quantum information
Quantum information
In quantum mechanics, quantum information is physical information that is held in the "state" of a quantum system. The most popular unit of quantum information is the qubit, a two-level quantum system...

, 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 comb
Frequency comb
A frequency comb is the graphic representation of the spectrum of a mode locked laser. An octave spanning comb can be used for mapping radio frequencies into the optical frequency range or it can be used to steer a piezoelectric mirror within a carrier envelope phase correcting feedback loop...

s. 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
Phase noise
Phase noise is the frequency domain representation of rapid, short-term, random fluctuations in the phase of a waveform, caused by time domain instabilities...

 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 caesium fountain clock.

Atomic systems under consideration include Al
Aluminium
Aluminium or aluminum is a silvery white member of the boron group of chemical elements. It has the symbol Al, and its atomic number is 13. It is not soluble in water under normal circumstances....

+, Hg
Mercury (element)
Mercury is a chemical element with the symbol Hg and atomic number 80. It is also known as quicksilver or hydrargyrum...

+/2+, Hg
Mercury (element)
Mercury is a chemical element with the symbol Hg and atomic number 80. It is also known as quicksilver or hydrargyrum...

, Sr
Strontium
Strontium is a chemical element with the symbol Sr and the atomic number 38. An alkaline earth metal, strontium is a soft silver-white or yellowish metallic element that is highly reactive chemically. The metal turns yellow when exposed to air. It occurs naturally in the minerals celestine and...

, Sr
Strontium
Strontium is a chemical element with the symbol Sr and the atomic number 38. An alkaline earth metal, strontium is a soft silver-white or yellowish metallic element that is highly reactive chemically. The metal turns yellow when exposed to air. It occurs naturally in the minerals celestine and...

+/2+, In
Indium
Indium is a chemical element with the symbol In and atomic number 49. This rare, very soft, malleable and easily fusible post-transition metal is chemically similar to gallium and thallium, and shows the intermediate properties between these two...

+/3+, Mg
Magnesium
Magnesium is a chemical element with the symbol Mg, atomic number 12, and common oxidation number +2. It is an alkaline earth metal and the eighth most abundant element in the Earth's crust and ninth in the known universe as a whole...

, Ca
Calcium
Calcium is the chemical element with the symbol Ca and atomic number 20. It has an atomic mass of 40.078 amu. Calcium is a soft gray alkaline earth metal, and is the fifth-most-abundant element by mass in the Earth's crust...

, Ca
Calcium
Calcium is the chemical element with the symbol Ca and atomic number 20. It has an atomic mass of 40.078 amu. Calcium is a soft gray alkaline earth metal, and is the fifth-most-abundant element by mass in the Earth's crust...

+, Yb
Ytterbium
Ytterbium is a chemical element with the symbol Yb and atomic number 70. A soft silvery metallic element, ytterbium is a rare earth element of the lanthanide series and is found in the minerals gadolinite, monazite, and xenotime. The element is sometimes associated with yttrium or other related...

+/2+/3+ and Yb
Ytterbium
Ytterbium is a chemical element with the symbol Yb and atomic number 70. A soft silvery metallic element, ytterbium is a rare earth element of the lanthanide series and is found in the minerals gadolinite, monazite, and xenotime. The element is sometimes associated with yttrium or other related...

.

Quantum clocks


In March 2008, physicists at NIST
National Institute of Standards and Technology
The National Institute of Standards and Technology , known between 1901 and 1988 as the National Bureau of Standards , is a measurement standards laboratory, otherwise known as a National Metrological Institute , which is a non-regulatory agency of the United States Department of Commerce...

 described a quantum logic clock
Quantum Clock
A quantum clock is a type of clock that confines aluminum and beryllium ions together in an electromagnetic trap and cools them by lasers to near absolute zero temperatures. Developed by National Institute of Standards and Technology physicists, the clock is 37 times more precise than the existing...

 based on individual ion
Ion
An ion is an atom or molecule in which the total number of electrons is not equal to the total number of protons, giving it a net positive or negative electrical charge. The name was given by physicist Michael Faraday for the substances that allow a current to pass between electrodes in a...

s of beryllium and aluminium
Aluminium
Aluminium or aluminum is a silvery white member of the boron group of chemical elements. It has the symbol Al, and its atomic number is 13. It is not soluble in water under normal circumstances....

. This clock was compared to NIST's mercury
Mercury (element)
Mercury is a chemical element with the symbol Hg and atomic number 80. It is also known as quicksilver or hydrargyrum...

 ion clock. These were the most accurate clocks that had been constructed, with neither clock gaining nor losing time at a rate that would exceed a second in over a billion years. In February 2010, NIST physicists described a second, enhanced version of the quantum logic clock based on individual ion
Ion
An ion is an atom or molecule in which the total number of electrons is not equal to the total number of protons, giving it a net positive or negative electrical charge. The name was given by physicist Michael Faraday for the substances that allow a current to pass between electrodes in a...

s of magnesium
Magnesium
Magnesium is a chemical element with the symbol Mg, atomic number 12, and common oxidation number +2. It is an alkaline earth metal and the eighth most abundant element in the Earth's crust and ninth in the known universe as a whole...

 and aluminium
Aluminium
Aluminium or aluminum is a silvery white member of the boron group of chemical elements. It has the symbol Al, and its atomic number is 13. It is not soluble in water under normal circumstances....

. Considered the world's most precise clock, it offers more than twice the precision of the original.

Evaluated accuracy


In 2011, the NPL-CsF2 cesium fountain clock operated by the National Physical Laboratory (NPL), which serves as the United Kingdom primary time
Time
Time is a part of the measuring system used to sequence events, to compare the durations of events and the intervals between them, and to quantify rates of change such as the motions of objects....

 and frequency standard, was improved regarding the two largest sources of measurement uncertainties — distributed cavity phase and microwave lensing frequency shifts. As of 2011 this resulted in an evaluated frequency uncertainty reduction from 4.1 x 10-16 to 2.3 x 10-16 — the lowest value for any primary national standard so far. At this frequency uncertainty the NPL-CsF2 is expected to neither gain nor lose a second in more than 138 million years.

Applications


The development of atomic clocks has led to many scientific and technological advances such as a worldwide system of precise position measurement (Global Positioning System), and applications in the Internet
Internet
The Internet is a global system of interconnected computer networks that use the standard Internet protocol suite to serve billions of users worldwide...

, which depend critically on frequency and time standards.
Atomic clocks are installed at sites of time signal
Time signal
A time signal is a visible, audible, mechanical, or electronic signal used as a reference to determine the time of day.-Audible and visible time signals:...

 radio transmitters. They are used at some long wave and medium wave broadcasting stations to deliver a very precise carrier frequency. Atomic clocks are used in many scientific disciplines, such as for long-baseline interferometry
Interferometry
Interferometry refers to a family of techniques in which electromagnetic waves are superimposed in order to extract information about the waves. An instrument used to interfere waves is called an interferometer. Interferometry is an important investigative technique in the fields of astronomy,...

 in radioastronomy.

Global Positioning System


The Global Positioning System
Global Positioning System
The Global Positioning System is a space-based global navigation satellite system that provides location and time information in all weather, anywhere on or near the Earth, where there is an unobstructed line of sight to four or more GPS satellites...

 (GPS) provides very accurate timing and frequency signals. A GPS receiver works by measuring the relative time delay of signals from a minimum of four, but usually more GPS satellites, each of which has four onboard caesium or rubidium atomic clocks. The relative times are mathematically transformed into three absolute spatial coordinates and one absolute time coordinate. The time is accurate to within about 50 nanoseconds. However, inexpensive GPS receivers may not assign a high priority to updating the display, so the displayed time may differ perceptibly from the internal time. Precision time references that use GPS are marketed for use in computer networks, laboratories, and cellular communications networks, and do maintain accuracy to within about .

Time signal radio transmitters


A radio clock
Radio clock
A radio clock or radio-controlled clock is a clock that is synchronized by a time code bit stream transmitted by a radio transmitter connected to a time standard such as an atomic clock...

 is a clock that automatically synchronizes itself by means of government radio time signal
Time signal
A time signal is a visible, audible, mechanical, or electronic signal used as a reference to determine the time of day.-Audible and visible time signals:...

s received by a radio receiver. Many retailers market radio clocks inaccurately as atomic clocks; although the radio signals they receive originate from atomic clocks, they are not atomic clocks themselves. They are inexpensive time-keeping devices with an accuracy of about a second. Instrument grade time receivers provide higher accuracy. Such devices incur a transit delay of approximately 1 ms
Millisecond
A millisecond is a thousandth of a second.10 milliseconds are called a centisecond....

 for every 300 kilometres (186 mi) of distance from the radio transmitter. Many governments operate transmitters for time-keeping purposes.

See also


  • Atomichron
    Atomichron
    The Atomichron was the world's first commercial atomic clock, built by the National Company, Inc of Malden, Massachusetts. It was also the first self-contained portable atomic clock and was a caesium standard clock. More than 50 clocks with the trademarked Atomichron name were produced.- External...

  • Atomic fountain
    Atomic fountain
    An atomic fountain is a cloud of atoms that are tossed upwards by lasers in the Earth's gravitational field. If visible, it would resemble the water in a fountain. While weightless in the toss, the atoms are measured to set the frequency of an atomic clock....

  • Optical Clockwork
  • International Atomic Time
    International Atomic Time
    International Atomic Time is a high-precision atomic coordinate time standard based on the notional passage of proper time on Earth's geoid...

  • Magneto-optical trap
    Magneto-optical trap
    A magneto-optical trap is a device that uses both laser cooling with magneto-optical trapping in order to produce samples of cold, trapped, neutral atoms at temperatures as low as several microkelvins, two or three times the recoil limit.By combining the small momentum of a single photon with a...

  • Network Time Protocol
    Network Time Protocol
    The Network Time Protocol is a protocol and software implementation for synchronizing the clocks of computer systems over packet-switched, variable-latency data networks. Originally designed by David L...

  • NIST-F1
    NIST-F1
    NIST-F1 is a cesium fountain clock or atomic clock in the National Institute of Standards and Technology in Boulder, Colorado, and serves as the United States' primary time and frequency standard...

  • Primary Atomic Reference Clock in Space
    Primary Atomic Reference Clock in Space
    The Primary Atomic Reference Clock in Space or PARCS is an atomic-clock mission once scheduled to fly on the International Space Station in 2008, but cancelled to make way for the Vision for Space Exploration. The mission, to have been funded by NASA, involved a laser-cooled caesium atomic clock,...

  • Sense of time
  • Télé Distribution Française
    Télé Distribution Francaise
    Télé Diffusion de France, or TDF, is a time signal service, broadcast on longwave radio by the French "Laboratoire primaire du temps et des frequences" . This time signal is generated by an extremely accurate caesium fountain atomic clock...

  • Quantum Clock
    Quantum Clock
    A quantum clock is a type of clock that confines aluminum and beryllium ions together in an electromagnetic trap and cools them by lasers to near absolute zero temperatures. Developed by National Institute of Standards and Technology physicists, the clock is 37 times more precise than the existing...


External links