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To understand quantum computers, we must first be able to understand what do we mean by quantum? and from where does this idea came ?
The idea of quantum computing comes from the quantum mechanics which was based on the property of subatomic particles called ‘wave-particle duality’ where classical physics fails as these subatomic particles neither behaves like particles only nor waves only but behaves both as wave and particle. These subatomic particles in classical physics which was considered as only particles, when it was passed through the double slit experiment, the interference pattern confirmed its wave nature, where even a single electron interferes with itself to form the same pattern which is the crux of wave particle duality.
\newline With the advent of technology, the size of computers are now approaching the size of atoms. A computer is made up of computer chips which are made from logic gates which in turn are made up of transistors. A transistor is the smallest form of a data processor which basically is just a switch to block or allow information to pass through and these information are made from the classical bits – 0 and 1.Today the size of a transistor is just 14 nm . As these are shrinking to size of only few atoms, The electrons when encountering a block or a wall, instead of waiting for switch to allow its passage, it can simply pass through the block or wall transferring themselves to the other side via a phenomenon called ‘Quantum Tunneling which is seemed to allow faster than light travel, a supposed physical impossibility .

\newline In quantum realm, we describe everything as waves, the electron waves are not actual waves like a water waves but is an abstract mathematical description, operating on them known as wave function will gives the real world properties of it in terms of probabilities which is a phenomenon stated by Heisenberg’s uncertainty principle and because of this quantum uncertainty, we cannot measure everything with accuracy and precision in the universe which is experimentally demonstrated by Schrondinger’s Cat which leads us to another quantum phenomenon known as ‘Superposition’ where the quantum bits called qubits, which are the building blocks of QCs , are in superposition of probabilities of 0 and 1, and we cannot predict which state it will be unless we test or measure them, it collapses into one of these two states which is known as ‘Quantum Decoherence’ .
To demonstrate how powerful these qubits are as compared to classical bits, let us take an example of $4$ classical bits which forms $2^4 = 16$ combinations, out of which we can use only $1$, but for $4$ qubits in superposition, it can be in all the $16$ combinations at once and this number grows exponentially with each extra qubit where $20$ qubits can store a million value all in parallel. A point to note that QC is not same as parallel computing.
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