Quantum: Quantum is the minimal unit of any kind of physical entity. In simple words, it is the smallest quantity that can be thought of. For example: Photon is the smallest element of light, thus photon is the light quantum.

One another example that I can think of is the digital technology that we use intensely. We have now microprocessor, transistors and so on. But the basis of all these things starts with the concept of bits and bytes. Similarly, the physical world that we pass around is also composed of bits. These infinitesimal objects are the quanta. Everything is made up of small bits called quantum.
(Note: one quantum, two quanta)


Applications of Quantum Mechanics:-
Planck's constant is very important to quantum theory and quantum mechanics in general. It states that the energy of each quantum is equal to the frequency of the radiation multiplied by the universal constant: E=f*h, where h is 6.63 * 10E-34 Js.
Quantum mechanics can be referred to as the mathematical description of physical reality of the matter, like almost any other scientific model. According to quantum mechanics the most complete description we can make of a system is its wavefunction, which is mathematically speaking just a number varying between time and place.
Quantum theory can provide accurate descriptions for phenomenons such as black body radiation and the stability of electron orbitals.
The famous quantum theory of the atom was primarily developed as an explanation for the electron's staying in its orbital, which couldn't at that time be explained by Newton's laws of motion.
Quantum particles have one very unique property, namely they can be in different states at the same time. This is referred to as the "superposition" of two conditions. Based on this for instance the 'spin' of an electron can be pointing in two different directions at once.
Many scientists believe that super-fast quantum computer are only matter of time.
Quantum computers have also unlimited potential and they are likely able to fix problems that would normally take millions of years to solve, much faster than ordinary computers. Currently the biggest problem with development of quantum computers is that scientists can only hold a limited number of atoms in place for a very short period of time.
Quantum mechanics allows a quantum computer built on these principles (still theoretical, to perform tasks that are currently thought impossible to do efficiently on a normal computer, for instance breaking with ease all currently possible encryption standards.
Quantum mechanics plays key role for understanding how individual atoms combine covalently to form chemicals or molecules, and is so well used in chemistry that is even known under the name of quantum chemistry.
Quantum mechanics has had significant impact on today's modern technology such as lasers, transistors and diodes.

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