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An atomic clock is a
clock A clock or a timepiece is a device used to Measurement, measure and indicate time. The clock is one of the oldest Invention, human inventions, meeting the need to measure intervals of time shorter than the natural units: the day, the lunar ...

clock
whose timekeeping mechanism is based on the interaction of
electromagnetic radiation In physics Physics is the natural science that studies matter, its Elementary particle, fundamental constituents, its Motion (physics), motion and behavior through Spacetime, space and time, and the related entities of energy and force. ...

electromagnetic radiation
with the excited states of certain
atoms An atom is the smallest unit of ordinary that forms a . Every , , , and is composed of neutral or atoms. Atoms are extremely small, typically around 100 s across. They are so small that accurately predicting their behavior using —as ...

atoms
. Specifically, either a hyperfine transition in the
microwave Microwave is a form of electromagnetic radiation In physics Physics (from grc, φυσική (ἐπιστήμη), physikḗ (epistḗmē), knowledge of nature, from ''phýsis'' 'nature'), , is the natural science that studies mat ...

microwave
, or electron transition in the
optical Optics is the branch of physics Physics is the natural science that studies matter, its Elementary particle, fundamental constituents, its Motion (physics), motion and behavior through Spacetime, space and time, and the related entities ...

optical
or
ultraviolet Ultraviolet (UV) is a form of electromagnetic radiation In physics Physics (from grc, φυσική (ἐπιστήμη), physikḗ (epistḗmē), knowledge of nature, from ''phýsis'' 'nature'), , is the natural science that stud ...

ultraviolet
region of the
emission spectrum The emission spectrum of a chemical element Image:Simple Periodic Table Chart-blocks.svg, 400px, Periodic table, The periodic table of the chemical elements In chemistry, an element is a pure substance consisting only of atoms that all have ...
of an atom is used as a
frequency standard These precision 100 kHz oven controlled crystal oscillators at the US Bureau of Standards (now National Institute of Standards and Technology, NIST) served as the frequency standard for the United States in 1929. A frequency standard is a stable ...
for the timekeeping element. Atomic clocks are the most accurate
time Time is the continued of and that occurs in an apparently succession from the , through the , into the . It is a component quantity of various s used to events, to compare the duration of events or the intervals between them, and to of ...
and frequency standards known, and are used as
primary standardA primary standard in metrology 290px, alt=Man in white standing in front of a large machine, A scientist stands in front of the Microarcsecond Metrology (MAM) testbed. Metrology is the scientific study of measurement ' Measurement is the numb ...
s for international time distribution services, to control the wave frequency of television broadcasts, and in
global navigation satellite system A satellite navigation or satnav system is a system that uses satellites to provide autonomous geo-spatial positioning. It allows small electronics, electronic receivers to determine their location (longitude, latitude, and altitude/elevation) t ...
s such as
GPS The Global Positioning System (GPS), originally Navstar GPS, is a satellite-based radionavigation system owned by the United States government The federal government of the United States (U.S. federal government) is the national ...

GPS
. The principle of operation of an atomic clock is based on atomic physics; it measures the electromagnetic signal that
electron The electron is a subatomic particle In physical sciences, subatomic particles are smaller than atom An atom is the smallest unit of ordinary matter In classical physics and general chemistry, matter is any substance that has ma ...

electron
s in atoms emit when they change
energy level Energy levels for an electron in an atom: ground state and excited state">atom.html" ;"title="electron in an atom">electron in an atom: ground state and excited states. After absorbing energy, an electron may "jump" from the ground state to a high ...
s. Early atomic clocks were based on
maser (see description below) A maser (, an acronym for microwave amplification by stimulated emission of radiation) is a device that produces coherence (physics), coherent electromagnetic waves through amplification by stimulated emission. The firs ...

maser
s at room temperature. Since 2004, more accurate atomic clocks first cool the atoms to near
absolute zero Absolute zero is the lowest limit of the thermodynamic temperature Thermodynamic temperature is the measure of ''absolute temperature'' and is one of the principal parameters of thermodynamics. A thermodynamic temperature reading of zero deno ...
temperature by slowing them with lasers and probing them in
atomic fountain An atomic fountain is a cloud of atoms that is tossed upwards in the Earth's gravitational field by lasers A laser is a device that emits light Light or visible light is electromagnetic radiation within the portion of the electromagnet ...

atomic fountain
s in a
microwave Microwave is a form of electromagnetic radiation In physics Physics (from grc, φυσική (ἐπιστήμη), physikḗ (epistḗmē), knowledge of nature, from ''phýsis'' 'nature'), , is the natural science that studies mat ...

microwave
-filled cavity. An example of this is the
NIST-F1 250px, NIST-F1, source of the official time of the United States NIST-F1 is a cesium fountain clock, a type of atomic clock, in the National Institute of Standards and Technology The National Institute of Standards and Technology (NIST) is a ...
atomic clock, one of the national primary time and frequency standards of the United States. The accuracy of an atomic clock depends on two factors: the first is temperature of the sample atoms—colder atoms move much more slowly, allowing longer probe times, the second is the frequency and intrinsic linewidth of the electronic or hyperfine transition. Higher frequencies and narrow lines increase the precision. National standards agencies in many countries maintain a network of atomic clocks which are intercompared and kept synchronized to an accuracy of 10−9 seconds per day (approximately 1 part in 1014). These clocks collectively define a continuous and stable time scale, the
International Atomic Time International Atomic Time (TAI, from the French name ) is a high-precision atomic coordinate In geometry Geometry (from the grc, γεωμετρία; ''wikt:γῆ, geo-'' "earth", ''wikt:μέτρον, -metron'' "measurement") is, with a ...
(TAI). For civil time, another time scale is disseminated,
Coordinated Universal Time Coordinated Universal Time or UTC is the primary 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 o ...
(UTC). UTC is derived from TAI but has added
leap seconds A leap second is a one-second adjustment that is occasionally applied to Coordinated Universal Time (UTC), to accommodate the difference between precise time (as measured by atomic clocks) and imprecise solar time#Mean solar time, observed solar ...
from
UT1 Universal Time (UT) is a time standard based on Earth's rotation. There are several versions of Universal Time, which differ by up to a few seconds. The most commonly used are Coordinated Universal Time Coordinated Universal Time or UTC is t ...

UT1
, to account for variations in the
rotation of the Earth A rotation is a circular movement of an object around a center (or point) of rotation. The geometric plane along which the rotation occurs is called the '' rotation plane'', and the imaginary line extending from the center and perpendicular ...
with respect to the
solar time Solar time is a calculation of the passage of time Time is the continued of and that occurs in an apparently succession from the , through the , into the . It is a component quantity of various s used to events, to compare the dura ...
.


History

The idea of using atomic transitions to measure time was suggested by
Lord Kelvin William Thomson, 1st Baron Kelvin, (26 June 182417 December 1907) was a British mathematician A mathematician is someone who uses an extensive knowledge of mathematics Mathematics (from Ancient Greek, Greek: ) includes the study of ...

Lord Kelvin
in 1879. Magnetic resonance, developed in the 1930s by
Isidor Rabi Isidor Isaac Rabi (; born Israel Isaac Rabi, July 29, 1898 – January 11, 1988) was an American physicist A physicist is a scientist A scientist is a person who conducts Scientific method, scientific research to advance knowledge in ...
, 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 is a compound Compound may refer to: Architecture and built environments * Compound (enclosure), a cluster of buildings having a shared purpose, usually inside a fence or wall ** Compound (fortification), a version of the above fort ...

ammonia
absorption line device at 23870.1 MHz built in 1949 at the U.S.
National Bureau of Standards The National Institute of Standards and Technology (NIST) is a physical sciences Physical science is a branch of natural science that studies abiotic component, non-living systems, in contrast to life science. It in turn has many branches, e ...
(NBS, now
NIST The National Institute of Standards and Technology (NIST) is a physical sciences Physical science is a branch of natural science that studies abiotic component, non-living systems, in contrast to life science. It in turn has many branches, e ...
). It was less accurate than existing
quartz clock Quartz clocks and quartz watches are timepieces that use an electronic oscillator regulated by a quartz crystal to keep time. This crystal oscillator creates a signal with very precise frequency, so that quartz clocks and watches are at least an ...

quartz clock
s, but served to demonstrate the concept. The first accurate atomic clock, a
caesium standard The caesium standard is a primary frequency standard in which the photon absorption by transitions between the two hyperfine ground states of caesium-133 Caesium Caesium (IUPAC spelling) (American and British English spelling differences, a ...
based on a certain transition of the
caesium-133 Caesium Caesium (IUPAC spelling) (American and British English spelling differences, also spelled cesium in American English) is a chemical element with the Symbol (chemistry), symbol Cs and atomic number 55. It is a soft, silvery-golde ...

caesium-133
atom, was built by Louis Essen and Jack Parry in 1955 at the National Physical Laboratory in the UK. Calibration of the caesium standard atomic clock was carried out by the use of the astronomical time scale ''
ephemeris time The term ephemeris time (often abbreviated ET) can in principle refer to time in association with any ephemeris (itinerary of the trajectory of an astronomical object). In practice it has been used more specifically to refer to: # a former stand ...
'' (ET). In 1967, this led the scientific community to redefine the
second The second (symbol: s, also abbreviated: sec) is the of in the (SI) (french: Système International d’unités), commonly understood and historically defined as of a – this factor derived from the division of the day first into 24 s, th ...
in terms of a specific atomic frequency. 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 The term ephemeris time (often abbreviated ET) can in principle refer to time in association with any ephemeris (itinerary of the trajectory of an astronomical object). In practice it has been used more specifically to refer to: # a former stand ...
scale, determining the length of the ET second. Since the beginning of development in the 1950s, atomic clocks have been based on the hyperfine transitions in
hydrogen-1 Hydrogen Hydrogen is the chemical element Image:Simple Periodic Table Chart-blocks.svg, 400px, Periodic table, The periodic table of the chemical elements In chemistry, an element is a pure substance consisting only of atoms that all ...

hydrogen-1
,
caesium-133 Caesium Caesium (IUPAC spelling) (American and British English spelling differences, also spelled cesium in American English) is a chemical element with the Symbol (chemistry), symbol Cs and atomic number 55. It is a soft, silvery-golde ...

caesium-133
, and
rubidium-87 Rubidium Rubidium is the chemical element Image:Simple Periodic Table Chart-blocks.svg, 400px, Periodic table, The periodic table of the chemical elements In chemistry, an element is a pure substance consisting only of atoms that all have ...
. 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 instrument was subsequently replaced by much smaller rack-mountable devices, such as the
Hewlett-Packard The Hewlett-Packard Company, commonly shortened to Hewlett-Packard ( ) or HP, was an American multinational company headquartered in . HP developed and provided a wide variety of hardware components, as well as software and related servic ...

Hewlett-Packard
model 5060 caesium frequency standard, released in 1964. In the late 1990s, four factors contributed to major advances in clocks: *
Laser cooling Laser cooling and laser trapping include a number of techniques in which atomic and molecular samples are cooled down to near absolute zero Absolute zero is the lowest limit of the thermodynamic temperature Thermodynamic temperature is the m ...
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. In August 2004,
NIST The National Institute of Standards and Technology (NIST) is a physical sciences Physical science is a branch of natural science that studies abiotic component, non-living systems, in contrast to life science. It in turn has many branches, e ...
scientists demonstrated a
chip-scale atomic clock A chip scale atomic clock (CSAC) is a compact, low-power atomic clock fabricated using techniques of microelectromechanical systems Microelectromechanical systems (MEMS), also written as micro-electro-mechanical systems (or microelectronic and m ...
. Available on-line at:
NIST.gov
/ref> According to the researchers, the clock was believed to be one-hundredth the size of any other. It requires no more than 125  mW, making it suitable for battery-driven applications. This technology became available commercially in 2011. Ion trap experimental optical clocks are more precise than the current caesium standard. In April 2015, NASA announced that it planned to deploy a (DSAC), a miniaturized, ultra-precise mercury-ion atomic clock, into outer space. NASA said that the DSAC would be much more stable than other navigational clocks.


Mechanism

Since 1968, the
International System of Units The International System of Units, known by the international abbreviation SI in all languages and sometimes Pleonasm#Acronyms_and_initialisms, pleonastically as the SI system, is the modern form of the metric system and the world's most wi ...
(SI) has defined the
second The second (symbol: s, also abbreviated: sec) is the of in the (SI) (french: Système International d’unités), commonly understood and historically defined as of a – this factor derived from the division of the day first into 24 s, th ...
as the duration of of radiation corresponding to the transition between two energy levels of the ground state of the
caesium-133 Caesium Caesium (IUPAC spelling) (American and British English spelling differences, also spelled cesium in American English) is a chemical element with the Symbol (chemistry), symbol Cs and atomic number 55. It is a soft, silvery-golde ...

caesium-133
atom. In 1997, the
International Committee for Weights and Measures The General Conference on Weights and Measures (GCWM; french: Conférence Générale des Poids et Mesures, CGPM) is the supreme authority of the International Bureau of Weights and Measures (BIPM), the intergovernmental organization established in ...
(CIPM) added that the preceding definition refers to a caesium atom at rest at a temperature of
absolute zero Absolute zero is the lowest limit of the thermodynamic temperature Thermodynamic temperature is the measure of ''absolute temperature'' and is one of the principal parameters of thermodynamics. A thermodynamic temperature reading of zero deno ...
. This definition makes the caesium oscillator the primary standard for time and frequency measurements, called the
caesium standard The caesium standard is a primary frequency standard in which the photon absorption by transitions between the two hyperfine ground states of caesium-133 Caesium Caesium (IUPAC spelling) (American and British English spelling differences, a ...
. The definitions of other physical units, e.g., the
volt The volt is the derived unit for electric potential The electric potential (also called the ''electric field potential'', potential drop, the electrostatic potential) is defined as the amount of work (physics), work energy needed to move a ...

volt
and the
metre The metre ( Commonwealth spelling) or meter (American spelling Despite the various English dialects spoken from country to country and within different regions of the same country, there are only slight regional variations in English o ...
, rely on the definition of the second. In this particular design, the 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 describes the phenomenon of increased amplitude The amplitude of a Periodic function, periodic Variable (mathematics), variable is a measure of its change in a single Period (mathematics), period (such as frequency, time or Wavelen ...

resonance
with the frequency of the hyperfine transition of caesium or rubidium. The core of the
radio frequency Radio frequency (RF) is the oscillation Oscillation is the repetitive variation, typically in time Time is the indefinite continued sequence, progress of existence and event (philosophy), events that occur in an apparently irreversible p ...
atomic clock is a tunable
microwave cavity A microwave cavity or ''radio frequency Radio frequency (RF) is the oscillation rate of an Alternating_current, alternating electric current or voltage or of a Magnetic_field, magnetic, electric or electromagnetic field or mechanical system in ...
containing a gas. In a
hydrogen maser Diagram of a hydrogen maser A hydrogen maser, also known as hydrogen frequency standard These precision 100 kHz oven controlled crystal oscillators at the US Bureau of Standards (now NIST) served as the frequency standard for the United States ...

hydrogen maser
clock the gas emits
microwave Microwave is a form of electromagnetic radiation In physics Physics (from grc, φυσική (ἐπιστήμη), physikḗ (epistḗmē), knowledge of nature, from ''phýsis'' 'nature'), , is the natural science that studies mat ...

microwave
s (the gas ''
masesMases ( grc, Μάσης, ἡ Μάσητος) was a city in the district Hermionis Ermioni (Greek language, Greek , Ancient Greek Hermione ) is a small port town and a former municipality in Argolis, Peloponnese (region), Peloponnese, Greece on the ...

mases
'') 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 hyperfine state prior to filling them into the cavity. For the second type, the number of atoms that change hyperfine 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 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. A number of other atomic clock schemes used for other purposes. Rubidium standard clocks are prized for their low cost, small size (commercial standards are as small as 17 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 The Global Positioning System (GPS), originally Navstar GPS, is a satellite-based radionavigation system owned by the United States government The federal government of the United States (U.S. federal government) is the national ...
receiver (see GPS disciplined oscillator). This achieves excellent short-term accuracy, with long-term accuracy equal to (and traceable to) the US 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 A magneto-optical trap (MOT) is an apparatus which uses laser cooling and a spatially-varying magnetic field to create a trap which can produce samples of cold, trapped, neutral atoms. Temperatures achieved in a MOT can be as low as several microke ...
s to cool the atoms for improved precision.


Power consumption

The power consumption of atomic clocks varies with their size. Atomic clocks on the scale of one chip require less than 30
milliwatt The watt (symbol: W) is a unit of power Power typically refers to: * Power (physics) In physics, power is the amount of energy transferred or converted per unit time. In the International System of Units, the unit of power is the watt, equa ...

milliwatt
; Primary frequency and time standards like the United States Time Standard atomic clocks, NIST-F1 and NIST-F2, use far higher power.


Evaluated accuracy

The evaluated accuracy ''u''''B'' reports of various primary frequency and time standards ar
published online
by the International Bureau of Weights and Measures (BIPM). Several frequency and time standards groups as of 2015 reported ''u''''B'' values in the to range. In 2011, the NPL-CsF2 caesium fountain clock operated by the National Physical Laboratory (NPL), which serves as the United Kingdom primary frequency and time standard, was improved regarding the two largest sources of measurement uncertainties—distributed cavity phase and microwave lensing frequency shifts. In 2011 this resulted in an evaluated frequency uncertainty reduction from ''u''''B'' = to ''u''''B'' = —the lowest value for any primary national standard at the time. At this frequency uncertainty, the NPL-CsF2 is expected to neither gain nor lose a second in about 138 million () years. The
NIST-F2 NIST-F2 is a caesium fountain, cesium fountain atomic clock that, along with NIST-F1, serves as the United States' primary time and frequency standard. NIST-F2 was brought online on 3 April 2014. Accuracy NIST-F1, a cesium fountain atomic clock us ...
caesium fountain clock operated by the National Institute of Standards and Technology (NIST), was officially launched in April 2014, to serve as a new U.S. civilian frequency and time standard, along with the
NIST-F1 250px, NIST-F1, source of the official time of the United States NIST-F1 is a cesium fountain clock, a type of atomic clock, in the National Institute of Standards and Technology The National Institute of Standards and Technology (NIST) is a ...
standard. The planned ''u''''B'' performance level of NIST-F2 is . "At this planned performance level the NIST-F2 clock will not lose a second in at least 300 million years." NIST-F2 was designed using lessons learned from NIST-F1. The NIST-F2 key advance compared to the NIST-F1 is that the vertical flight tube is now chilled inside a container of liquid nitrogen, at . This cycled cooling dramatically lowers the background radiation and thus reduces some of the very small measurement errors that must be corrected in NIST-F1. The first in-house accuracy evaluation of NIST-F2 reported a ''u''''B'' of . However, a published scientific criticism of that NIST F-2 accuracy evaluation described problems in its treatment of distributed cavity phase shifts and the microwave lensing frequency shift, which is treated significantly differently than in the majority of accurate fountain clock evaluations. The next NIST-F2 submission to the BIPM in March, 2015 again reported a ''u''''B'' of , but did not address the standing criticism. There have been neither subsequent reports to the BIPM from NIST-F2 nor has an updated accuracy evaluation been published. At the request of the Italian standards organization, NIST fabricated many duplicate components for a second version of NIST-F2, known as IT-CsF2 to be operated by the Istituto Nazionale di Ricerca Metrologica (INRiM), NIST's counterpart in Turin, Italy. As of February 2016 the IT-CsF2 caesium fountain clock started reporting a ''u''''B'' of in the BIPM reports of evaluation of primary frequency standards.


Research

Most research focuses on the often conflicting goals of making the clocks smaller, cheaper, more portable, more energy efficient, more
accurate In a set of measurements, accuracy is closeness of the measurements to a specific value, while precision is the closeness of the measurements to each other. ''Accuracy'' has two definitions: # More commonly, it is a description of ''systematic err ...

accurate
, more stable and more reliable. The
Atomic Clock Ensemble in Space ''Atomic Clock Ensemble in Space'' (ACES) is a project led by the European Space Agency which will place ultra-stable atomic clocks on the International Space Station. Operation in the Micro-g environment, microgravity environment of the ISS will pr ...
is an example of clock research.


Secondary representations of the second

A list of frequencies recommended for secondary representations of the second is maintained by the International Bureau of Weights and Measures (BIPM) since 2006 and i
available online
The list contains the frequency values and the respective standard uncertainties for the rubidium microwave transition and for several optical transitions. These secondary frequency standards are accurate at the level of parts in ; however, the uncertainties provided in the list are in the range of parts in – since they are limited by the linking to the caesium primary standard that currently (2015) defines the second. For context, a
femtosecond A femtosecond is the SI unit of time equal to 10 or of a second The second (symbol: s, abbreviation: sec) is the SI base unit, base unit of time in the International System of Units (SI) (French: Système International d’unités), commonly ...
() is to a second what a second is to about 31.71 million () years and an
attosecond An attosecond is 1×10−18 of a second The second (symbol: s, abbreviation: sec) is the SI base unit, base unit of time in the International System of Units (SI) (French: Système International d’unités), commonly understood and historical ...
() is to a second what a second is to about 31.71 billion () years. Twenty-first-century experimental atomic clocks that provide non-caesium-based secondary representations of the second are becoming so precise that they are likely to be used as extremely sensitive detectors for other things besides measuring frequency and time. For example, the frequency of atomic clocks is altered slightly by gravity, magnetic fields, electrical fields, force, motion, temperature and other phenomena. The experimental clocks tend to continue improving, and leadership in performance has been shifted back and forth between various types of experimental clocks.


Quantum clocks

In March 2008, physicists at
NIST The National Institute of Standards and Technology (NIST) is a physical sciences Physical science is a branch of natural science that studies abiotic component, non-living systems, in contrast to life science. It in turn has many branches, e ...
described a quantum logic clock based on individual
ion An ion () is an atom An atom is the smallest unit of ordinary matter In classical physics and general chemistry, matter is any substance that has mass and takes up space by having volume. All everyday objects that can be touched are ...
s of
beryllium Beryllium is a chemical element upright=1.0, 500px, The chemical elements ordered by link=Periodic table In chemistry Chemistry is the science, scientific study of the properties and behavior of matter. It is a natural science t ...
and
aluminium Aluminium (aluminum in and ) is a with the  Al and  13. Aluminium has a density lower than those of other common , at approximately one third that of . It has a great affinity towards , and of on the surface when exposed to air ...

aluminium
. This clock was compared to NIST's
mercury Mercury usually refers to: * Mercury (planet) Mercury is the smallest planet in the Solar System and the closest to the Sun. Its orbit around the Sun takes 87.97 Earth days, the shortest of all the Sun's planets. It is named after the Roman g ...

mercury
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 An ion () is an atom An atom is the smallest unit of ordinary matter In classical physics and general chemistry, matter is any substance that has mass and takes up space by having volume. All everyday objects that can be touched are ...
s of
magnesium Magnesium is a chemical element upright=1.0, 500px, The chemical elements ordered by link=Periodic table In chemistry Chemistry is the science, scientific study of the properties and behavior of matter. It is a natural science ...

magnesium
and
aluminium Aluminium (aluminum in and ) is a with the  Al and  13. Aluminium has a density lower than those of other common , at approximately one third that of . It has a great affinity towards , and of on the surface when exposed to air ...

aluminium
. Considered the world's most precise clock in 2010 with a fractional frequency inaccuracy of , it offers more than twice the precision of the original. In July 2019, NIST scientists demonstrated such an Al+ Quantum-Logic clock with total uncertainty of , which is the first demonstration of such a clock with uncertainty below . The accuracy of experimental quantum clocks has since been superseded by experimental optical lattice clocks based on
strontium-87 The alkaline earth metal The alkaline earth metals are six chemical element Image:Simple Periodic Table Chart-blocks.svg, 400px, Periodic table, The periodic table of the chemical elements In chemistry, an element is a pure substance consis ...
and
ytterbium-171 Naturally occurring ytterbium (70Yb) is composed of 7 stable isotopes, 168Yb–176Yb, with 174Yb being the most abundant (31.83% natural abundance). Twenty-seven radioisotopes have been characterized, with the most stable being 169Yb with a half ...
.


Optical clocks

The theoretical move from microwaves as the atomic "escapement" for clocks to light in the optical range (harder to measure but offering better performance) earned and the
Nobel Prize in Physics The Nobel Prize in Physics is a yearly award given by the Royal Swedish Academy of Sciences for those who have made the most outstanding contributions for mankind in the field of physics. It is one of the five Nobel Prizes established by the will ...
in 2005. One of 2012's Physics Nobelists,
David J. Wineland David Jeffrey Wineland (born February 24, 1944) is an American Nobel-laureate physicist at the National Institute of Standards and Technology The National Institute of Standards and Technology (NIST) is a physical sciences laboratory and ...
, is a pioneer in exploiting the properties of a single ion held in a trap to develop clocks of the highest stability. New technologies, such as femtosecond frequency combs, optical lattices, and
quantum information Quantum information is the information of the quantum state, state of a quantum system. It is the basic entity of study in quantum information theory, and can be manipulated using quantum information processing techniques. Quantum information ref ...
, 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 combIn optics Optics is the branch of physics Physics (from grc, φυσική (ἐπιστήμη), physikḗ (epistḗmē), knowledge of nature, from ''phýsis'' 'nature'), , is the natural science that studies matter, its Motion (physics), ...
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 combIn optics Optics is the branch of physics Physics (from grc, φυσική (ἐπιστήμη), physikḗ (epistḗmē), knowledge of nature, from ''phýsis'' 'nature'), , is the natural science that studies matter, its Motion (physics), ...
, these measurements have become much more accessible and numerous optical clock systems are now being developed around the world. As 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 Bandwidth commonly refers to: * Bandwidth (signal processing) or ''analog bandwidth'', ''frequency bandwidth'', or ''radio bandwidth'', a measure of the width of a frequency range * Bandwidth (computing), the rate of data transfer, bit rate or thr ...
of the
phase noise In signal processing Signal processing is an electrical engineering Electrical engineering is an engineering discipline concerned with the study, design, and application of equipment, devices, and systems which use electricity, electroni ...
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 primary systems under consideration for use in optical frequency standards are: * single ions isolated in an ion trap; * neutral atoms trapped in an optical lattice and * atoms packed in a three-dimensional quantum gas optical lattice. These techniques allow the atoms or ions to be highly isolated from external perturbations, thus producing an extremely stable frequency reference. Atomic systems under consideration include +, +/2+, , , +/2+, +/3+, , , +, +/2+/3+, and +/3+. The rare-earth element ytterbium (Yb) is valued not so much for its mechanical properties but for its complement of internal energy levels. "A particular transition in Yb atoms, at a wavelength of 578 nm, currently provides one of the world's most accurate optical atomic frequency standards," said Marianna Safronova. The estimated amount of uncertainty achieved corresponds to a Yb clock uncertainty of about one second over the lifetime of the universe so far, 15 billion years, according to scientists at the Joint Quantum Institute (JQI) and the
University of Delaware The University of Delaware (colloquially UD, UDel or Delaware) is a Public university, public Land-grant university, land-grant research university located in Newark, Delaware. UD is the largest university in Delaware. It offers three associate' ...
in December 2012. In 2013 optical lattice clocks (OLCs) were shown to be as good as or better than caesium fountain clocks. Two optical lattice clocks containing about of
strontium-87 The alkaline earth metal The alkaline earth metals are six chemical element Image:Simple Periodic Table Chart-blocks.svg, 400px, Periodic table, The periodic table of the chemical elements In chemistry, an element is a pure substance consis ...
were able to stay in synchrony with each other at a precision of at least , which is as accurate as the experiment could measure. These clocks have been shown to keep pace with all three of the caesium fountain clocks at the
Paris Observatory #REDIRECT Paris Observatory #REDIRECT Paris Observatory The Paris Observatory (french: Observatoire de Paris ), a research institution of the Paris Sciences et Lettres University, is the foremost astronomy, astronomical observatory of France, a ...

Paris Observatory
. There are two reasons for the possibly better precision. Firstly, the frequency is measured using light, which has a much higher frequency than microwaves, and secondly, by using many atoms, any errors are averaged. Using
ytterbium-171 Naturally occurring ytterbium (70Yb) is composed of 7 stable isotopes, 168Yb–176Yb, with 174Yb being the most abundant (31.83% natural abundance). Twenty-seven radioisotopes have been characterized, with the most stable being 169Yb with a half ...
atoms, a new record for stability with a precision of over a 7-hour period was published on 22 August 2013. At this stability, the two optical lattice clocks working independently from each other used by the
NIST The National Institute of Standards and Technology (NIST) is a physical sciences Physical science is a branch of natural science that studies abiotic component, non-living systems, in contrast to life science. It in turn has many branches, e ...
research team would differ less than a second over the
age of the universe In physical cosmology Physical cosmology is a branch of cosmology Cosmology (from Ancient Greek, Greek κόσμος, ''kosmos'' "world" and -λογία, ''-logia'' "study of") is a branch of astronomy concerned with the study of the chro ...
(); this was better than previous experiments. The clocks rely on atoms cooled to and trapped in an optical lattice. A laser at excites the atoms between two of their energy levels. Having established the stability of the clocks, the researchers are studying external influences and evaluating the remaining systematic uncertainties, in the hope that they can bring the clock's accuracy down to the level of its stability. An improved optical lattice clock was described in a 2014 Nature paper. In 2015
JILA JILA, formerly known as the Joint Institute for Laboratory Astrophysics, is a physical science Physical science is a branch of natural science that studies abiotic component, non-living systems, in contrast to life science. It in turn has many br ...

JILA
evaluated the absolute frequency uncertainty of a
strontium-87 The alkaline earth metal The alkaline earth metals are six chemical element Image:Simple Periodic Table Chart-blocks.svg, 400px, Periodic table, The periodic table of the chemical elements In chemistry, an element is a pure substance consis ...
optical lattice clock at , which corresponds to a measurable gravitational time dilation for an elevation change of on planet Earth that according to JILA/NIST Fellow
Jun Ye Jun Ye (; born 1967) is a Chinese-American physicist at JILA, National Institute of Standards and Technology, and the University of Colorado Boulder, working primarily in the field of Atomic, molecular, and optical physics, atomic, molecular and ...
is "getting really close to being useful for relativistic geodesy". At this frequency uncertainty, this JILA optical lattice clock is expected to neither gain nor lose a second in more than 15 billion () years. In 2017 JILA reported an experimental 3D quantum gas strontium optical lattice clock in which strontium-87 atoms are packed into a tiny three-dimensional (3-D) cube at 1,000 times the density of previous one-dimensional (1-D) clocks, like the 2015 JILA clock. A synchronous clock comparison between two regions of the 3D lattice yielded a record level of synchronization of in 1 hour of averaging time. The 3D quantum gas strontium optical lattice clock's centerpiece is an unusual state of matter called a degenerate matter, degenerate Fermi gas (a quantum gas for Fermi particles). The experimental data shows the 3D quantum gas clock achieved a precision of in about two hours. According to Jun Ye "This represents a significant improvement over any previous demonstrations." Ye further commented "The most important potential of the 3D quantum gas clock is the ability to scale up the atom numbers, which will lead to a huge gain in stability." and "The ability to scale up both the atom number and coherence time will make this new-generation clock qualitatively different from the previous generation." In 2018 JILA reported the 3D quantum gas clock reached a frequency precision of over 6 hours. At this frequency uncertainty, this 3D quantum gas clock would lose or gain about 0.1 seconds over the age of the universe. Recently it is proved that the quantum entanglement can help to further enhance the clock stability. Optical clocks are currently (2021) still primarily research projects, less mature than rubidium and caesium microwave standards, which regularly deliver time to the International Bureau of Weights and Measures (BIPM) for establishing International Atomic Time, International Atomic Time (TAI). As the optical experimental clocks move beyond their microwave counterparts in terms of accuracy and stability performance this puts them in a position to replace the current standard for time, the caesium fountain clock. In the future this might lead to redefine the caesium microwave based SI second and other new dissemination techniques at the highest level of accuracy to transfer clock signals will be required that can be used in both shorter-range and longer-range (frequency) comparisons between better clocks and to explore their fundamental limitations without significantly compromising their performance.


Nuclear (optical) clock concept

One theoretical possibility for improving the performance of atomic clocks is to use a ''nuclear'' energy transition (between different nuclear isomers) rather than the atomic electron transitions which current atomic clocks measure. Most nuclear transitions operate at far too high a frequency to be measured, but in 2003, Ekkehard Peik and Christian Tamm noted that the exceptionally low excitation energy of is within reach of current frequency-measurement techniques, making a clock possible. In 2012, it was shown that a nuclear clock based on a single ion could provide a total fractional frequency inaccuracy of , which is better than existing 2019 atomic clock technology. Although it remains an unrealized theoretical possibility, significant progress toward the development of an experimental nuclear clock has been made. A nuclear energy transition offers the following potential advantages: # Higher frequency. All other things being equal, a higher-frequency transition offers greater stability for simple statistical reasons (fluctuations are averaged over more cycles per second). # Immunity to environmental effects. Due to its small size and the shielding effects of the surrounding electrons, an atomic nucleus is much less sensitive to ambient electromagnetic fields than an electron. # Greater numbers of atoms. Because of the aforementioned immunity to ambient fields, it is not necessary to have the clock atoms well-separated in a dilute gas. In fact, it would be possible to take advantage of the Mössbauer effect and place the atoms in a solid, which would allow billions of atoms to be interrogated.


Clock comparison techniques

In June 2015, the European National Physical Laboratory (United Kingdom), National Physical Laboratory (NPL) in Teddington, UK; the French Paris Observatory, department of Time-Space Reference Systems at the Paris Observatory (LNE-SYRTE); the German Physikalisch-Technische Bundesanstalt, German National Metrology Institute (PTB) in Braunschweig; and Italy's :it:Istituto nazionale di ricerca metrologica, Istituto Nazionale di Ricerca Metrologica (INRiM) in Turin labs have started tests to improve the accuracy of current state-of-the-art satellite comparisons by a factor 10, but it will still be limited to one part in . These 4 European labs are developing and host a variety of experimental optical clocks that harness different elements in different experimental set-ups and want to compare their optical clocks against each other and check whether they agree. In a next phase, these labs strive to transmit comparison signals in the visible spectrum through fibre-optic cables. This will allow their experimental optical clocks to be compared with an accuracy similar to the expected accuracies of the optical clocks themselves. Some of these labs have already established fibre-optic links, and tests have begun on sections between Paris and Teddington, and Paris and Braunschweig. Fibre-optic links between experimental optical clocks also exist between the American NIST lab and its partner lab
JILA JILA, formerly known as the Joint Institute for Laboratory Astrophysics, is a physical science Physical science is a branch of natural science that studies abiotic component, non-living systems, in contrast to life science. It in turn has many br ...

JILA
, both in Boulder, Colorado but these span much shorter distances than the European network and are between just two labs. According to Fritz Riehle, a physicist at PTB, "Europe is in a unique position as it has a high density of the best clocks in the world". In August 2016 the French LNE-SYRTE in Paris and German PTB in Braunschweig reported the comparison and agreement of two fully independent experimental strontium lattice optical clocks in Paris and Braunschweig at an uncertainty of via a newly established phase-coherent frequency link connecting Paris and Braunschweig, using of telecom fibre-optic cable. The fractional uncertainty of the whole link was assessed to be , making comparisons of even more accurate clocks possible.


Applications

The development of atomic clocks has led to many scientific and technological advances such as a system of precise global and regional satellite navigation, navigation satellite systems, and applications in the Internet, which depend critically on frequency and time standards. Atomic clocks are installed at sites of time signal 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 in radioastronomy.


Global Navigation Satellite Systems

The Global Positioning System (GPS) operated by the US Air Force Space Command 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 at least two onboard caesium and as many as two rubidium atomic clocks. The relative times are mathematically transformed into three absolute spatial coordinates and one absolute time coordinate. GPS Time (GPST) is a continuous time scale and theoretically accurate to about 14 nanosecond, ns. However, most receivers lose accuracy in the interpretation of the signals and are only accurate to 100 ns. The GPST is related to but differs 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 corrections are performed to the on-board clocks in the satellites to keep them synchronized with ground clocks. The GPS navigation message includes the difference between GPST and UTC. As of July 2015, GPST is 17 seconds ahead of UTC because of the leap second added to UTC on 30 June 2015. Receivers subtract this offset from GPS Time to calculate UTC and specific time zone values. The GLONASS, GLObal NAvigation Satellite System (GLONASS) operated by the Russian Aerospace Defence Forces provides an alternative to the Global Positioning System (GPS) system and is the second navigational system in operation with global coverage and of comparable precision. GLONASS Time (GLONASST) is generated by the GLONASS Central Synchroniser and is typically better than 1,000 ns. Unlike GPS, the GLONASS time scale implements leap seconds, like UTC. The Galileo (satellite navigation), Galileo Satellite navigation#Global satellite navigation systems, Global Navigation Satellite System is operated by the European GNSS Agency and European Space Agency and is near to achieving full operating global coverage. Galileo started offering global Early Operational Capability (EOC) on 15 December 2016, providing the third and first non-military operated Global Navigation Satellite System, and is expected to reach Full Operational Capability (FOC) in 2019. To achieve Galileo's FOC coverage constellation goal 6 planned extra satellites need to be added. Galileo System Time (GST) is a continuous time scale which is generated on the ground at the Galileo Control Centre in Fucino, Italy, by the Precise Timing Facility, based on averages of different atomic clocks and maintained by the Galileo Central Segment and synchronised with TAI with a nominal offset below 50 ns. According to the European GNSS Agency Galileo offers 30 ns timing accuracy. The March 2018 Quarterly Performance Report by the European GNSS Service Centre reported the UTC Time Dissemination Service Accuracy was ≤ 7.6 ns, computed by accumulating samples over the previous 12 months and exceeding the ≤ 30 ns target. Each Galileo satellite has two passive
hydrogen maser Diagram of a hydrogen maser A hydrogen maser, also known as hydrogen frequency standard These precision 100 kHz oven controlled crystal oscillators at the US Bureau of Standards (now NIST) served as the frequency standard for the United States ...

hydrogen maser
and two Rubidium standard, rubidium atomic clocks for onboard timing. The Galileo navigation message includes the differences between GST, UTC and GPST (to promote interoperability). The BeiDou Navigation Satellite System#Global system (BeiDou Navigation Satellite System or BeiDou-2), BeiDou-2/BeiDou-3 satellite navigation system is operated by the China National Space Administration. BeiDou Time (BDT) is a continuous time scale starting at 1 January 2006 at 0:00:00 UTC and is synchronised with UTC within 100 ns. BeiDou became operational in China in December 2011, with 10 satellites in use, and began offering services to customers in the Asia-Pacific region in December 2012. On 27 December 2018 the BeiDou Navigation Satellite System started to provide global services with a reported timing accuracy of 20 ns. The 35th and the final BeiDou-3 satellite for global coverage was launched into orbit on 23 June 2020.


Time signal radio transmitters

A radio clock is a clock that automatically synchronizes itself by means of government radio time signals received by a radio receiver. Many retailers market radio clocks inaccurately as atomic clocks;Michael A. Lombardi
"How Accurate is a Radio Controlled Clock?"
National Institute of Standards and Technology, 2010.
although the radio signals they receive originate from atomic clocks, they are not atomic clocks themselves. Normal low-cost consumer-grade receivers solely rely on the amplitude-modulated time signals and use narrow-band receivers (with 10 Hz bandwidth) with small ferrite Loop antenna#AM broadcast receiver loop antennas, loopstick antennas and circuits with non-optimal digital signal processing delay and can therefore only be expected to determine the beginning of a second with a practical accuracy uncertainty of ± 0.1 second. This is sufficient for radio-controlled low-cost consumer-grade clocks and watches using standard-quality
quartz clock Quartz clocks and quartz watches are timepieces that use an electronic oscillator regulated by a quartz crystal to keep time. This crystal oscillator creates a signal with very precise frequency, so that quartz clocks and watches are at least an ...

quartz clock
s for timekeeping between daily synchronization attempts, as they will be most accurate immediately after a successful synchronization and will become less accurate from that point forward until the next synchronization. Instrument grade time receivers provide higher accuracy. Such devices incur a transit delay of approximately 1 millisecond, ms for every 300 kilometres (186 mi) of distance from the radio transmitter. Many governments operate transmitters for time-keeping purposes. The typical consumer-grade receivers use a ferrite rod and temperature compensation to ensure stability over time, typically a capacitor is chosen with equal and opposite (ie NTC) dielectric and thermally coupled to the ferrite rod so temperature changes don't affect the resonant frequency. The front end is normally a variant on the MK484 or IC7642 with a digital circuit that periodically turns it on to capture the time signal so as to ensure long battery life. In some cases where high interference is present, aiming the centre of the coil at the compass bearing for Anthorn or MSF will give better results as will putting the clock away from smart meters and metallic objects.


See also

* Clock drift * List of atomic clocks * Network Time Protocol * Primary Atomic Reference Clock in Space * Pulsar clock * Speaking clock * Time metrology * Time transfer


References


External links

* National Research Council Canada, FAQ
"What is a 'cesium atomic clock'?"
* National Research Council Canada, archived content


United States Naval Observatory Time Service Department

PTB Braunschweig, Germany – with link in English language

National Physical Laboratory (UK) time website

NIST Internet Time Service (ITS): Set Your Computer Clock Via the Internet

NIST press release about chip-scaled atomic clock

NIST website

Web pages on atomic clocks
by Science Museum (London), The Science Museum (London)
Optical Atomic Clock
BBC, 2005
Optical lattice clock
Journal of the Physical Society of Japan
The atomic fountain
* National Research Council of Canada, archived content

{{DEFAULTSORT:Atomic Clock Atomic clocks, Electronic test equipment