Atomic clock -World's most accurate clock-How does it works?

Atomic clock is the most accurate clock:how does an atomic clock work

NIST-F1 would neither gain nor lose one second in about 100 million years

Content Highlights

• Types of clocks and basic principle behind a clock.
• What is a atomic clock?
• Principle behind an atomic clock
• How does an  Atomic Clock works?
• NIST-F1 Cesium Fountain Clock
• Definition of a second
• Applications of atomic clocks

---

Before going into the atomic clock, let us see what a clock really is. A clock's job is to measure passage of time. All clocks do this by measuring some sort of resonation of one or another material. Let us have look at some commonly used 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

the NIST-F1 in Boulder, Colorado

In an atomic clock an ‘atom’ or a ‘molecule’ is the resonator. In every clock accuracy of resonator determines accuracy of time. An atom resonates at extremely consistent frequency. This ensures the accuracy of an atomic clock. In all other clocks resonator is manufactured, so, there is limitation in accuracy that can be obtained from such resonators.

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 an Atomic clock.

There are many atomic clocks around the world. Out of which, NIST-F1 Cesium Fountain Clock is one among the most accurate clocks.It is developed by National Institute of Standards and Technology (U.S. Department of commerce) . The uncertainty of NIST-F1 is continually improving. As of January 2013, the uncertainty has been reduced to about 3 x 10-16, which means it would neither gain nor lose a second in more than 100 million years .NIST-F1 is referred to as a fountain clock because it uses a fountain-like movement of atoms to measure frequency and time interval.

Diagrammatic representation of a an atomic clock 

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-F1 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-atoms are stabilized

using six laser beams 

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.

a ball of atoms tossed up by
laser beams

when laser is turned off ,atoms fall down
due to gravity

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 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). The photons, or the tiny packets of light that they emit, are measured by a detector.

Detection of photons emitted by caesium atoms

This process is repeated many times while the microwave signal in the cavity is tuned to different frequencies. 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.

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 "

Applications of atomic clocks 

What is the need of an atomic clock?..It has got many applications in our 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.

Still atomic clock researches are going around the globe .And scientists are coming up with more and more accurate atomic clocks.

References:

1. NIST Cesium Fountains — Current Status and Future Prospects S.R. Jefferts∗ , T.P. Heavner, T.E. Parker and J.H. Shirley http://przyrbwn.icm.edu.pl/APP/PDF/112/a112z506.pdf 
2.  https://www.nist.gov