Computers we use for everyday's work use transistors, and its computational power depends upon the no of transistors it contains. Gordon Moore noticed that the number of transistors per square inch on integrated circuits had doubled every year since their invention. Moore's law predicts that this trend will continue occur in near future. But Moore's law is going to fail in near future as there is a limit the number of transistors we can fit in the given chip.
As the technology is growing, the thrust of increased processing power is snowballing and since Moore's law is going to fail in near future, we need alternate way to fulfill the hunger. If this trend continues to be followed then by 2040 we will not have the capability to power all of the machines around the globe. Thus the industry is focused on finding ways to make computing more energy efficient, but classical computers are limited by the minimum amount of energy it takes them to perform one operation. So to fulfill our requirement we need to reach to subatomic particles to find a way where operations can be done much more quickly using less energy than classical computers.
A bit is single piece of information that can only exist in two states: 1 or 0. Quantum computer uses quantum bits also known as 'qubits'. These quantum systems exist in two states. These can store much more information than just 1 or 0, as they values can exist in any superposition. If the qubit is in a superposition of the 1 state and the 0 state, and it performed a calculation with another qubit in the same superposition, then one calculation actually obtains 4 results: a 1/1 result, a 1/0 result, a 0/1 result, and a 0/0 result.
This is due to the result of mathematics applied on quantum system while in a state of de-coherence, which lasts when it is in superposition of states until it collapses down into one state. The exact physical mechanism at work within the quantum computer is somewhat theoretically complex and intuitively disturbing.
Thus Quantum Computers are next generation computers and are amazingly fast. Last year team of Google and NASA scientists found a D-wave quantum computer which is 100 million times faster than a conventional computer. But moving quantum computing to an industrial scale is difficult.