Atomic Clock

Any ordinary time clock that we use in our everyday life, is usually connected to a machine that moves to and fro periodically. This constant periodic motion is used to control the movement of the hands of the clock or to change the digits on the face of the clock. The name, atomic clock, may suggest that it is a clock that uses radioactivity, but this is not so. Actually, the atomic clock is an instrument for time keeping that uses the oscillations associated with atoms to measure time. In other words, it uses the electromagnetic waves emitted by the atoms.

History of Atomic Clock

The first atomic clock came into being in the year 1949 at the US National Bureau Standards, now known as the National Institute of Standards and Technology (NIST). It was a device that used coherent electromagnetic waves emitted from ammonia molecule. That concept was correct but the accuracy level was lower than the current quartz clocks. A more precise form of atomic clocks came up in 1955 at the National Physical Laboratory in UK. It was built by Louis Essen. These clocks used the microwaves emitted during the transition of electrons of cesium-133 atoms, from one energy level to another. Later, some more atomic clocks were built where electromagnetic waves emitted by atoms like Hydrogen-1 and rubidium-87 were used.

Working Principle of Atomic Clock

The atomic clock consists of a microwave cavity. There is an electronic amplifier in the cavity. The cavity is filled up with the core elements (usually cesium-133 or rubidium-87 or Hydrogen-1). It is then heated to release atoms. The released atoms are not neutral, they carry variable electrical charges. These released atoms are made to go through a vacuum tube and then through a magnetic field. It should be noted that only a selected few atoms which are in a low energy state can pass through this magnetic field.

After passing through a magnetic field, these atoms need to cross a microwave field, which is under the control of a crystal oscillator. The oscillator is made to vibrate the microwave field with a certain frequency. The microwave field often tends to have a variable vibration. But the variation is very small and the microwave field attains the required frequency in a cyclic order. A low-energy atom can change to a high energy state, if it can pass through the microwave field, when it is oscillating at the needed frequency. The oscillation frequency is subsequently converted to pulses of one second by a special device.

At the end of the vacuum tube, there is a device to monitor those atoms which have changed their energy state. If it is found that the number of atoms in the specified energy state are below the intended level, the crystal oscillator is adjusted in such a way that it can vibrate at a proper frequency. This adjustment process is very crucial for the working of an atomic clock. It controls certain side effects like changes in temperature and incoherence of electromagnetic waves.

Uses of Atomic Clock

The time standard of atomic clocks is very precise. Due to their high accuracy level, they are used at the sites of time signals and various radio navigation transmitters like Long Range Aid to Navigation (LORAN). International time distribution services use the atomic clock as their primary standard.

The GPS navigation system signals are controlled by atomic clocks. A GPS tracking system consists of a total of 27 satellites and each of them have 3 atomic clocks. The atomic clocks help in measuring the exact position of a vehicle in a very precise manner. At a ground station, the GPS unit receives signals from four different satellites and the positioning is calculated using the time differences of signals from those four satellites.

There are some long-wave and medium-wave broadcasting stations that use atomic clocks to deliver an accurate carrier frequency. Other communication companies, like mobile services and internet services also make use of atomic clocks for the same reason.

Some other branches of science also utilize atomic clocks. In geology, they are used to measure shifts in the Earth's crust. In radio astronomy, they are used for studying different properties of the waves emitted by different celestial objects.

Constant research is on, to make the atomic clock more reliable and accurate. Efforts are also being made to reduce its size and make it affordable to the masses. The possibility of some other applications, for consumer or scientific purpose, are also being explored. We hope to make better use of the consistency and performance of atomic clocks and derive greater benefits from them.