Brief overview of quantum computers

Computers use a device known as transistor, it is a simple device which can enable or disable the flow of electric current through it, transistor is like a switch it can be controlled by applying certain voltage to it. By combining transistors with different circuit configuration, it can compute possibly anything which is computable. The language of a transistor is binary and so is computer, due to decrease in size of computers the transistors are also getting smaller, the size of transistors are approaching to the size of atoms.  

Regular transistor (these are not used in computers now a days, the working principle is same) 

How does transistor control the flow of electric current?

Transistor are basically semiconductors, they are generally doped with N-type semiconductor or P-type semiconductor. When the N-type and P-type semiconductors are mixed, a depletion region is formed in the between, that kind of mixture is called P-N junction (hence a diode is created). But to create a transistor another kind of doping is required either a PNP or NPN type, here 2 depletion regions are created. This depletion region is responsible for controlling the flow of electric current. 

The red colored region is the depletion region

Limitation of transistors in smaller scale?

Due to the decrease in size of transistors quantum mechanical challenges are occurring, electricity is flow of electrons but a phenomenon known as quantum tunneling, which allows electrons to transfer from one place to another even through a barrier between them. Hence very small transistors cannot control the electrons due to quantum tunneling.

Quantum computers

Regular computers use bits either 0 or 1, but a quantum computer uses Qbit. Qbit is state of a quantum system, which is in the superposition of probability for both 0 and 1 at same time until it is observed. Example of such quantum system are horizontally or vertically polarized photon, up spin or down spin particle etc. 

When the Qbits are observed, it collapses to either 0 or 1 (which is unpredictable). Classical bits can have 2^4 different configurations at a time, which can be used just once by classical computers. Qbits have all the 2^4 combination at a time, and all can be used by quantum computers at once.  

With the help of quantum entanglement we can build logic gates which can compute way faster then classical computers. 

Application of quantum computers

Quantum computers can be used in databases, cyber security, password hashing algorithms, encryption systems, complex scientific simulations and stock manipulations. These kind of tasks need very complex and fast computation, which a classical computer lacks. Quantum computers is not for home users.