ATOMIC TIME - A (VERY BRIEF) INTRODUCTION TO ATOMIC CLOCKS
- Miguel Lino
- Sep 20
- 13 min read
Updated: Sep 21
Atomic clocks are the most precise timekeeping instruments ever created by humankind. They rely on the quantum physics of atoms to define the second with a precision capable of supporting GPS, synchronizing digital networks, ensuring the security of global systems, and paving the way for new scientific discoveries. From cesium-133 to the optical clocks under development, atomic clocks represent the union between theory and technology, between the stability of matter and the universal need to measure time with absolute accuracy.
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Summary of contents
The following text is a summary of the CIRCLE dedicated to Atomic Time , held on September 18th at the Medeiros e Almeida Museum in Lisbon. During this session, Joana Ferreira presented the acknowledgments on behalf of the Medeiros e Almeida Museum, Nuno Margalha on behalf of the IPR - Portuguese Institute of Watchmaking, and Miguel Lino presented, in approximately 1.5 hours, a possible and accessible explanation of the fascinating universe of Atomic Time, the time of our days.
Part 1 — What is an Atomic Clock? What is an Atom?
1) The concept of time
Before defining the atomic clock, it's worth addressing the concept of time itself. Time can be understood as the period during which an event, process, or condition exists or evolves; as the continuous, seemingly irreversible progression of events; as what's read on a clock; or even as a social construct, an illusion. These perspectives demonstrate that time is simultaneously a physical reality and a human convention. Measuring instruments allow us to unite these two dimensions, and the atomic clock is the most sophisticated among them.
2) What is an atomic clock?
An atomic clock is an instrument that uses atoms to measure time. Its operating principle is not based on a pendulum, a balance wheel, or a quartz crystal, but on an internal transition within the atom, capable of providing a reference frequency. Examples of current primary standards include the Microchip 5071A, a cesium clock that illustrates this category of instruments.
3) The second in the International System
For centuries, the second was defined as a fraction of the mean solar day, equivalent to 1/86,400 of one rotation of the Earth. Later, in 1960, it was defined as 1/31,556,925.9747 of the tropical year 1900. These definitions were astronomical and suffered from limitations, since the Earth's rotation is not completely regular.
In 1967, the definition changed radically: the second became the duration of 9,192,631,770 periods of radiation corresponding to the hyperfine transition of the ground state of the cesium-133 atom. In 2018, the note was added that this transition should be measured in the atom's unperturbed ground state. This leap removed the unit of time from the sphere of astronomy and placed it in the domain of atomic physics, ensuring greater stability and universality.
The historical timeline shows this evolution: in 1657, the pendulum clock made it possible to measure seconds consistently; around 1874, systems already allowed for more accurate readings; in 1927, quartz clocks emerged as a stable alternative; and finally, in 1967, the international standard for the second was anchored to cesium-133. It is also important to remember that the concept of the second, as a subdivision of time, originates in the sexagesimal system of antiquity, a purely mathematical division used to track celestial bodies.
4) What defines a standard
For a definition to be accepted as a standard, it must meet four essential requirements: it must be universal, that is, independent of time and place; it must be reproducible, replicable in any laboratory with the appropriate equipment; it must be invariant, not changing over time; and it must be precise, providing the highest possible accuracy. Furthermore, a standard must result from international agreement in order to be widely accepted. The atom, because it exhibits well-defined and stable transitions, meets these conditions.
5) The place of the second in SI
The second is one of the seven fundamental units of the International System of Units. The others are: candela (luminous intensity), kelvin (temperature), ampere (electric current), meter (length), kilogram (mass), and mole (quantity of substance). Each of these units has a definition associated with a fundamental constant or invariable physical phenomenon. The second, defined by the frequency of the hyperfine transition of cesium-133, is on the same level of precision as these other quantities.
6) What is an atom?
The atom is the elementary unit that maintains the properties of a chemical element. It consists of a nucleus formed by protons and neutrons, around which electrons orbit. Historically, the 1920 Bohr model introduced the idea of discrete energy levels, in which electrons could jump from one orbit to another, releasing or absorbing energy proportional to the frequency of a photon. In 1925, Schrödinger's model described electrons not as particles in fixed orbits, but as probability distributions, known as orbitals.
Electronic transitions are quantified phenomena: whenever an electron changes levels, there is emission or absorption of energy proportional to a defined frequency, according to the Planck-Einstein relation. This means that each atom possesses a kind of unique signature, a frequency "note" that can serve as a reference.
The hyperfine transition
Among the possible transitions, the hyperfine transition is the most stable. It results from the interaction between the magnetic moment of the nucleus and that of the electrons. This transition is much less sensitive to external disturbances such as magnetic fields, electric fields, pressure, or temperature. It is precisely the hyperfine transition of cesium-133 that defines the latter, due to its stability and regularity.
7) Criteria for an atom to be used as a reference
For an atom or isotope to be adopted as a temporal reference, it must meet certain conditions: it must have stable and defined energy levels; it must exhibit a specific and measurable transition; it must be insensitive to external influences such as temperature and magnetic fields; it must have a high-frequency transition, which increases the possible precision; and it must be stable over the long term.
Cesium-133 meets these requirements. Its hyperfine transition is high-frequency, extremely stable, and easily measurable. The exact value of 9,192,631,770 Hz corresponds to the duration of one second.
8) Why cesium-133
The frequency of electronic transitions varies from atom to atom, as it depends on the number of protons in the nucleus and the interactions between electrons. Among all these, cesium-133 stands out for meeting the ideal conditions to serve as a time standard: its hyperfine transition has a high, stable, and easily measurable frequency , allowing for consistent and universally reproducible levels of precision. Furthermore, it is a stable, non-radioactive isotope available in adequate quantities , characteristics that make its use viable on an international scale.
From a chemical perspective, cesium is an alkali metal that is highly reactive in contact with air , requiring careful handling and storage. This reactivity, however, does not affect its function as a metrological reference.
It's important to emphasize that cesium-133 isn't the only atomic species used in reference clocks. Hydrogen masers also play an essential role, particularly as short-term stability standards, complementing cesium in establishing international time scales.
Other elements find application in different architectures. In microwave clocks , rubidium is often used as a practical alternative in portable and commercial systems. Optical clocks , currently under development and refinement, use atoms such as strontium , ytterbium , and aluminum , whose optical transitions, at a much higher frequency than those of cesium, offer the potential to significantly surpass current precision.
Despite these advances and the growing interest in optical clocks, caesium-133 remains the official international standard for defining the second , recognized by the International System of Units and supported by decades of use in time and frequency metrology.
9) Summary of Part 1
An atomic clock is an instrument that uses atoms to measure time, using the hyperfine electron transition as a reference. This reference translates into a specific and extremely stable frequency. Cesium-133 atomic clocks are those that define the fundamental unit of time: the second.
The atom, in turn, is the elementary unit of matter that retains the properties of an element. Electrons orbit the nucleus in discrete levels, and their transitions between levels release or absorb energy proportional to precise frequencies. It is this property that allows atoms to be used as universal standards for measuring time.
Part 2 — Accuracy and Uses of Atomic Clocks
1) The search for precision
The history of time measurement can be read as a succession of efforts to reduce error. Humans have always sought instruments that would allow them to organize social life, agriculture, navigation, and science with greater precision. From solar observation to lasers applied to optical clocks, chronology reveals a progression toward accuracy.
The Egyptian sundial, circa 1176 BC, allowed for the regulation of daily and religious activities, but it presented errors of hours. The medieval mechanical clock, which emerged around 1300 in Europe, reduced this margin to about 15 minutes per day, useful primarily for religious and scientific purposes. In 1657, with Christiaan Huygens' pendulum clock, accuracy reached about one minute per day, enabling a significant leap forward.
The H4 chronometer, created by John Harrison in 1759, achieved accuracy of tenths of a second per day and revolutionized maritime navigation. In the 19th and early 20th centuries, watchmakers refined mechanisms, but in 1927 a new solution emerged: the quartz watch, developed by Marrison and Horton, which reduced the error to microseconds per day, enabling industrial, transportation, and telecommunications applications.
The major turning point came in 1948, when Isidor Rabi and Harold Lyons pioneered the atomic clock. With this, accuracy rose to a few tenths of a second per year, quickly refined to values such as 1 second in millions, and today even billions, of years. In 1955, Louis Essen and Jack Parry built the first cesium beam atomic clock, definitively laying the foundation for the redefinition of the second.
2) Types of atomic clocks
Atomic clocks are not a homogeneous group: there are different technologies that operate in different frequency and energy ranges.
The microwave clock category includes:
Beam (cesium atomic beam) , the classic technology.
Fountain (atomic fountain) , in which cooled atoms are launched vertically and pass through the microwave cavity in free fall.
Hydrogen maser , which uses microwaves generated in resonance by hydrogen atoms.
Vapor cell , more compact and applicable to smaller-scale devices.
In the category of optical watches are:
Optical lattices (lattice clocks) , in which atoms are trapped in a lattice of laser light.
Trapped ions , where one or a few ions are confined in electromagnetic traps.
Nuclear or solid-state clocks , still experimental, that explore energy transitions at even higher scales.
Microwave clocks use frequencies on the order of gigahertz, while optical clocks operate at much higher frequencies, reaching terahertz or petahertz. Therefore, optical clocks achieve even greater precision, reaching error rates of just one second in hundreds of billions of years.
3) What limits accuracy
It's intuitive to think that the higher the reference frequency, the greater the clock's accuracy. But this reasoning isn't always straightforward. Other factors affect its operation:
In mechanical watches , disturbances include temperature, gravity, friction, impacts, and build quality.
In quartz watches , temperature, aging, capacitance and available energy have an influence.
In atomic clocks , relativistic effects, magnetic fields and aging of electronic components occur.
There are systematic errors, which can be predicted and corrected, and random errors, which require probabilistic models. In any case, atomic clocks far outperform the alternatives, but they are still not perfect.
4) What are atomic clocks for?
The usefulness of atomic clocks is vast and critical to the functioning of contemporary society.
Standardization
Atomic clocks define the unit of time in the International System. The second is established by the hyperfine transition of Cs-133, and this standardization ensures global consistency. Without a single, universal standard, each country or system could drift over time, creating unacceptable discrepancies in science, industry, and everyday life.
Navigation
Satellite navigation systems, such as GPS and other GNSS systems, rely entirely on atomic clocks installed in satellites. The time difference between the signal emitted by the satellite and that received by the device (a cell phone, for example) allows the user's position to be calculated. At least four satellites are required to accurately determine latitude, longitude, and altitude. Accurate time measurement directly translates to accurate location accuracy.
Global synchronization
The digital society demands that information circulate synchronously. Protocols like NTP (Network Time Protocol) allow computers, servers, and networks to share a common reference time, based on UTC. This is essential for financial transactions, digital security, media, telecommunications, online meetings, and content streaming. Even scientific research relies on this synchronization to process data coherently.
Security and defense
Atomic clocks are also strategic. They allow countries to maintain internal standards independent of international networks, ensuring resilience in the event of attacks or failures. They are indispensable in coordinating military operations, electronic warfare, and encrypting communications. They contribute to database security, preventing catastrophic failures, and enable the synchronization of electrical networks, preventing blackouts. Their use in phase measurement units ensures the stability of modern power grids.
Industry and science
In industry, they ensure high-precision manufacturing and automation processes. In science, they enable studies of extremely rapid or subtle phenomena. They enable research in fundamental physics, experiments with relativity, radio astronomy observations, and measurements associated with the detection of gravitational waves. In short, they are indispensable tools for better understanding the universe and exploring new technologies.
5) UTC and the globalization of time
To ensure the world maintains a coordinated time, an international framework was created. International Atomic Time (TAI) is the weighted average of approximately 450 atomic clocks, distributed across 85 laboratories in 80 countries. Each month, the data is gathered and processed to provide a universal reference.
Coordinated Universal Time (UTC) derives from TAI, but is adjusted for Earth's rotation using the UT1 scale. When necessary, leap seconds are added or subtracted to ensure UTC does not deviate too far from solar time. UTC was officially recognized in 1975 by the General Conference on Weights and Measures and is now the global standard for telecommunications, transportation, digital networks, and nearly all areas that depend on synchronization.
Currently, the difference between UT1 and TAI is only a few tens of seconds, which explains the leap second mechanism. UTC is thus the most concrete expression of international collaboration on time.
6) Summary of Part 2
Atomic clocks are the most accurate timekeeping instruments available today. They fall into two main groups: microwave and optical, with various technologies adapted to different purposes. Their accuracy ranges from 1 second in thousands of years to 1 second in hundreds of billions of years.
Their role goes far beyond theory: they guarantee the standardization of units, enable global navigation, ensure the synchronization of digital and telecommunications networks, strengthen national and international security, and make scientific and industrial advances possible.
They are also the result of exemplary international collaboration, involving 80 countries and hundreds of clocks that, together, support International Atomic Time and Coordinated Universal Time. Without atomic clocks, modern life as we know it would be impossible.
Part 3 — How an Atomic Clock Works
1) The fundamental principle
An atomic clock doesn't "generate" time from the atom: it uses the atom as a reference tuning fork to tune an electronic oscillator, usually quartz. The oscillator provides an electrical signal (e.g., 10 MHz), which can then be converted to 1 Hz, corresponding to one second. The atom's role is to keep the oscillator constantly corrected so that it never drifts.
The classic architecture that best illustrates this process is the cesium beam clock . The model may seem complex, but each step has a precise purpose.
2) The main elements
The atom
The base is the stable, non-radioactive isotope cesium-133 , obtained from the mineral pollucite. This atom contains the hyperfine transition used as the standard.
The atomic furnace
The first step is to transform solid cesium into a beam of atoms. This is done in an atomic furnace , which heats the material until it vaporizes. The beam is collimated, that is, organized into parallel paths, through microchannels. The furnace operates in a molecular flux regime , allowing the atoms to escape without excessive shocks.
The vacuum
The beam enters an ultra-high vacuum (UHV) chamber , below 10⁻⁹ mbar. This environment prevents collisions between cesium atoms and air molecules, preserving the beam's coherence.
Magnetic filter A
A magnet, similar to the Stern–Gerlach device, selects only atoms in the correct ground state . It's a quantum state filtering process: only the desired atoms move forward.
Radiofrequency cavity
The heart of the system is the Ramsey radiofrequency cavity . Here, radiofrequency waves interact with atoms. The RF signal comes from a voltage-controlled quartz oscillator ( VCXO ) and passes through a synthesizer and a frequency mixer. The cavity applies energy to the atoms, inducing the hyperfine transition if the frequency is correctly adjusted. The goal is to align the oscillator signal with the natural frequency of the transition: 9.192 631 770 GHz.
Frequency synthesizer and mixer
The synthesizer converts the base frequency (e.g., 10 MHz of the oscillator) to much higher values, in the gigahertz range. The mixer allows the signal to be finely tuned until it exactly matches the frequency of the cesium hyperfine transition.
Magnetic filter B
After the cavity, a second magnetic filter selects the atoms that have actually passed to the upper level of the transition. As with the initial filter, it is a state selection mechanism.
Detection
Next comes detection , performed by ionization. Atoms are ionized by colliding with a heated wire ( hot wire ), and the generated ions are collected by an ion collector (Faraday cup). The number of atoms detected is translated into an electrical signal, indicating the efficiency of the transition.
Amplification and comparison
The output signal is amplified and compared with the frequency mixer signal. The difference between the two is expressed as a voltage difference. If there is no difference (ΔV = 0), this means the quartz oscillator is perfectly tuned to the cesium transition frequency. If there is a deviation, the voltage automatically corrects the VCXO.
Frequency output
The corrected oscillator provides a stable 10 MHz output , which can be converted by a frequency divider to 1 Hz , or one pulse per second. This is the usable signal that powers electronic systems, telecommunications networks, satellites, and all other infrastructure that relies on precise timing.
3) Historical examples
The first cesium atomic clock was built in 1955 by Louis Essen and Jack Parry . A few years later, in 1959, the NBS-1 appeared in the United States. In 1972, Hewlett-Packard released the 5061A model , a commercial standard that became iconic and widely used in laboratories and telecommunications.
Over the decades, beam clocks have been replaced by atomic sources, trapped ions, and, more recently, optical clocks. However, the cesium beam remains the basis for the international standard for the second.
4) Modern variations and miniaturization
Traditional beam clocks consume between 20 and 40 W and achieve an accuracy of about 1 second in 3.17 million years. But miniaturization has opened up new possibilities: so-called Chip-Scale Atomic Clocks (CSAC) , based on techniques such as Coherent Population Trapping (CPT) , are smaller in size and consume only 100 mW. Although less accurate (on the order of 1 second in 3.17 million years), they are sufficiently stable for mobile, portable, and military applications.
CPT is based on a curious phenomenon: when atoms enter a specific energized state, they stop absorbing energy and become transparent to laser light. This allows frequency measurement in a more compact and efficient way.
5) Summary of Part 3
An atomic clock operates on an architecture that combines quantum physics and electronic engineering. The cesium atom provides the stable and unchanging reference; the quartz oscillator, continuously tuned to this reference, generates the usable signal.
The process involves atom preparation (furnace, vacuum, magnetic filters), radiofrequency excitation, state detection, signal comparison, and automatic oscillator correction. The result is a precise output frequency, converted to seconds.
The atom is the tuning fork and the oscillator is the instrument : together they produce the exact cadence on which the modern measurement of time is based.
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