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Before diving into the concept of an atomic clock, let’s first understand what a clock is. Essentially, a clock measures the passage of time by tracking the resonation of a material. Here are some commonly used types of clocks:

Pendulum clock: In a pendulum clock, the resonator is a pendulum and the gears in the clock keep track of time by counting the resonations (swinging) of the pendulum. The pendulum resonates at a frequency of one swing per second.

Digital clock: It either measures oscillations on power line or oscillation provided by an electronics circuit. In US power supply oscillates at 60 cycles per second and in Europe and Asia power supply oscillates at 50 cycles per second.

Quartz clock: In this type of clocks ,on supply of electric current quartz crystal oscillates at a particular frequency. This resonating frequency is converted into measurable form with the help of gears.

What is an Atomic Clock?

An atomic clock uses the natural vibration of atoms or molecules as its resonator. In every clock, the precision of timekeeping depends on how consistently the resonator operates. Atoms vibrate at exceptionally stable and predictable frequencies, which allows atomic clocks to measure time with extraordinary accuracy. In contrast, traditional clocks use manufactured resonators—like pendulums, electrical circuits, or quartz crystals—which, while reliable, cannot match the remarkable precision of atomic vibrations. This is why atomic clocks are renowned for being the most accurate timekeeping devices in existence.

Basic principle behind an atomic clock

When exposed to certain frequencies of radiation, such as radio waves or microwaves the electrons that orbit an atom’s nucleus will “jump” back and forth between energy states. The electrons absorb energy to move to a higher energy level (away from the nucleus), and release energy to move down an energy level (towards the nucleus). This “jumping” happens at extremely consistent frequency .Clocks based on this jumping within atoms can therefore provide an extremely precise way to count seconds.

Commonly used atoms in an atomic clock: Currently caesium (³³Cs) is the widely used one,but rubidium (⁸⁷Rb) and thallium (²⁰⁵Tl) were used earlier.

Working of NIST-F4 Atomic clock

There are many atomic clocks around the world. Out of which, NIST-F4 Cesium Fountain Clock is one among the most accurate clocks. It is an upgrade to the previous version NIST -F1. It is developed by National Institute of Standards and Technology (U.S. Department of commerce) . The uncertainty of NIST atomic clock  is continually improving. The NIST-F4 atomic clock has a reported fractional frequency uncertainty of 2.2 parts in 10¹⁶, or 2.2 x 10⁻¹⁶, according to its accuracy evaluation published in April 2025. This level of precision means the clock would only deviate by about one second in 140 million years.NIST-F4 is referred to as a fountain clock because it uses a fountain-like movement of atoms to measure frequency and time interval.

Laser cooling

We know that a moving atom possess higher energy than the stationary one. To restrict the movement of cesium atoms a technique called LASER cooling is used in NIST-F4 clock. First, a gas of cesium atoms is introduced into the clock’s vacuum chamber. Six infrared laser beams then are directed at right angles to each other at the center of the chamber. The lasers gently push the cesium atoms together into a ball. In the process of creating this ball, the lasers slow down the movement of the atoms . Laser cooling drops the temperature of the atoms to a few millionths of a degree above absolute zero, and reduces their thermal velocity to a few centimeters per second

Laser cooling is called “cooling” because it literally reduces the kinetic energy of atoms, which is the physical definition of temperature. While we usually think of lasers as powerful beams that heat things up, laser cooling uses a very specific principle called the Doppler effect to slow down the motion of atoms.

Here’s a simple breakdown of the principle:

• The Doppler Effect: Just as an ambulance siren sounds higher in pitch as it approaches and lower as it moves away, the frequency of light also changes depending on the relative motion of the source and the observer. An atom moving toward a laser beam “sees” the light’s frequency shifted to a higher, or “blue-shifted,” frequency. An atom moving away “sees” the frequency shifted lower, or “red-shifted.”
• Targeted Slowing: In laser cooling, a laser is tuned to a frequency that is just below the atom’s natural resonant frequency. When an atom moves toward the laser, the Doppler effect shifts the light’s frequency up, making it just right for the atom to absorb a photon. The absorbed photon gives the atom a “kick” of momentum in the opposite direction of its motion, slowing it down.
• The “Kick” and Re-emission: After absorbing the photon, the atom is in an excited state. It quickly re-emits a new photon, but in a random direction. Over many absorption and re-emission cycles, the random kicks from the re-emitted photons cancel each other out. However, the slowing kicks from the absorbed photons are always in the same direction, opposite to the atom’s motion. The net effect is a gradual reduction in the atom’s velocity.

This is crucial for atomic clocks because slower atoms can be observed for a longer time, which allows for more precise measurements of their “ticks,” leading to significantly more accurate clocks.

Two vertical lasers are used to gently toss the ball upward (the “fountain” action), and then all of the lasers are turned off. This little push is just enough to loft the ball about a meter high through a microwave-filled cavity. Under the influence of gravity, the ball then falls back down through the microwave cavity. In this area the atoms are exposed to microwaves from a crystal oscillator.

Tuning to the natural frequency of caesium atoms

The round trip up and down through the microwave cavity lasts for about 1 second. During the trip, the atomic states of the atoms might or might not be altered as they interact with the microwave signal.

When caesium atom is exposed to microwave of desired frequency, it absorbs energy and its electron jumps to the next higher energy level .When their trip is finished, another laser is pointed at the atoms. Those atoms whose atomic state were altered by the microwave signal emit light (a state known as fluorescence) .Here electron jumps from higher to lower energy level by emitting photons. The photons, or the tiny packets of light that they emit, are measured by a detector.

This process is repeated many times while the microwave signal in the cavity is tuned to different frequencies. A feedback system adjusts the frequency of the crystal oscillator until it hits this maximum, ensuring it is precisely “on-tune” with the cesium atom’s vibration. Eventually, a microwave frequency is found that alters the states of most of the cesium atoms and maximizes their fluorescence. This frequency is the natural resonance frequency of the cesium atom (9,192,631,770 Hz), or the frequency used to define the second.

How a second is measured?

By counting the number of microwave cycles, the clock can precisely measure the passage of time. One second is counted when the microwave oscillator completes exactly 9,192,631,770 cycles.

Definition of a second: According to The International System of Units (SI) “The second is the duration of 9192631770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium 133 atom. This definition refers to a caesium atom at rest at a temperature of 0 K “

Why Cesium atom is the ideal choice for atomic clocks?

Stable resonant frequency:The cesium-133 atom has a unique and extremely stable resonant frequency when exposed to specific microwave radiation.

Hyperfine States:The cesium atom has two closely spaced energy levels, called hyperfine states, in its ground state. These states allow for the atom to absorb microwave energy at its specific resonant frequency, causing a “quantum jump” between the energy levels. 

Convenient Microwave Frequency:The frequency of this quantum jump (9,192,631,770 Hz) is at a convenient microwave range, making it easy to generate and count electrically. 

Simplicity of Isotope:Cesium has only one stable isotope, Cesium-133. This means a pure gas of Cesium-133 can be readily obtained without needing extensive isotopic separation. 

Convenient Properties: At room temperature, cesium is a soft, metallic element that vaporizes easily, making it simple to create a beam of atoms in a vacuum. The atoms are also relatively heavy, so they move slowly, allowing for a longer interaction time with the microwaves that “tick” the clock.

Rubidium vs. Cesium in Atomic Clocks

While cesium clocks are the international standard for defining the second, rubidium clocks are also a key technology for timekeeping. The primary difference is their resonant frequency and application.

• Cesium: The cesium-133 atom’s resonant frequency is about 9.19 GHz. This higher frequency allows for more precise measurements, making cesium clocks the most accurate for long-term stability. They are considered primary frequency standards and are used to maintain national and international time.
• Rubidium: The rubidium-87 atom’s resonant frequency is about 6.83 GHz(6,834,682,610.9043126 Hz). While slightly less accurate than cesium clocks, rubidium clocks are significantly more compact, affordable, and power-efficient. This makes them the “workhorses” of the industry, widely used in telecommunications, GPS satellites, and various military applications where portability and cost are crucial.

In a rubidium clock One second is counted when the microwave oscillator completes exactly 6,834,682,610.9043126 cycles.

Applications of atomic clocks 

What is the need of an atomic clock? It has got many applications in our modern  life. Some of them are listed below.

• 1.GPS and similar navigation systems:In GPS our position is calculated based on the time taken by the signal to travel between GPS satellites and receiver. To determine position with high precision even billionths of a second is significant. 
• 2. Telecommunications systems require synchronization better than 100 billionths of a second .
• 3. Electrical power companies use synchronized systems to accurately determine the location of faults (for example, lightning damage) when they occur and to control the stability of their distribution systems.
• 4. In space exploration, radio observations of distant objects in the universe, require exceedingly good atomic reference clocks. And navigation of probes within our solar system depends critically on well-synchronized control stations on earth.
• 5. The time-related quantity called frequency, basically the rate at which a clock runs, is needed by the radio and television broadcast industry to maintain proper control of transmissions and thus avoid interference between stations.
• 6.Using the internet every computer can synchronize its time setting with a centralized time server – the atomic clock time server – so that all the computer times all around the world can use standardized settings, even though they are scattered through different time zones. Most operating systems (i.e. Windows, Mac, Linux) have an option to automatically synchronize the system clock periodically using an NTP (network time protocol) server: NIST is offering a network time service to deliver UT1(Universal Time) time.

In essence, atomic clocks serve as the invisible backbone of our interconnected, technology-driven society. From the synchrony of telecommunications and the pinpoint navigation of GPS to the exploration of distant galaxies, their unparalleled accuracy underpins countless innovations and conveniences of modern life. As science and technology progress, atomic timekeeping will remain an essential foundation—quietly ensuring that our world runs not only on time, but in harmony.