Which one of the following correctly describes the principle of the working of an atomic clock? 

The correct answer is Resonant frequency in cesium (or rubidium) atom.

Key PointsPrinciple of Atomic Clocks

• Atomic clocks operate on the principle of measuring the resonant frequency of atoms, typically cesium-133 or rubidium-87.
• The cesium atomic clock, for instance, uses the exact frequency of the microwave spectral line emitted by cesium atoms to keep time with extreme accuracy.
• These clocks do not rely on the vibration of a quartz crystal or any simple harmonic motion but on the quantum-mechanical properties of the atoms.
• Atomic clocks are able to keep time accurately to within a few billionths of a second per day, making them the most accurate timekeeping devices known.
• The process involves cooling cesium gas and placing it in a microwave cavity, where the atoms absorb and emit microwaves at a precise frequency. The clock's mechanism measures these vibrations to keep track of time.

Hence, statement 3 is correct.

Additional Information

• The concept of quantum mechanics plays a crucial role in the operation of atomic clocks, exploiting the precise energy levels of atoms.
• Cesium atomic clocks are so precise that they are used as the standard for International System of Units (SI) definition of the second.
• Atomic clocks are used in various applications including global positioning systems (GPS), internet data transfer synchronization, and in the study of relativity.
• The development of optical lattice clocks with strontium atoms is underway, which promises even greater accuracy, potentially reaching accuracy levels of 1 second in over 14 billion years.
• Rubidium atomic clocks, while slightly less accurate than cesium ones, are smaller, use less power, and are more cost-effective, making them suitable for many applications including telecommunications and satellite navigation.