A quantum computer is a special type of computer which uses entangled photons for processing. The general computer we use uses electrons but a quantum computer uses photons.That is why it is more powerful than general computer.Just like our computer uses bits the quantum computer uses qbits.
As of 2017, the development of actual quantum computers is still in its infancy, but experiments have been carried out in which quantum computational operations were executed on a very small number of quantum bits.[7] Both practical and theoretical research continues, and many national governments and military agencies are funding quantum computing research in an effort to develop quantum computers for civilian, business, trade, environmental and national security purposes, such as cryptanalysis.
A quantum computer with a given number of qubits is fundamentally different from a classical computer composed of the same number of classical bits. For example, representing the state of an n-qubit system on a classical computer requires the storage of 2n complex coefficients, while to characterize the state of a classical n-bit system it is sufficient to provide the values of the n bits, that is, only n numbers. Although this fact may seem to indicate that qubits can hold exponentially more information than their classical counterparts, care must be taken not to overlook the fact that the qubits are only in a probabilistic superposition of all of their states. This means that when the final state of the qubits is measured, they will only be found in one of the possible configurations they were in before the measurement. It is in general incorrect to think of a system of qubits as being in one particular state before the measurement, since the fact that they were in a superposition of states before the measurement was made directly affects the possible outcomes of the computation.
Quantum computers are extremely good at completing very hard calculations faster that classical computers. For example the factorization of very big prime numbers, a task which is closely related to cryptography and security of passwords.
It has to be said that quantum computers can not be used in the same way in which we use normal computers such as playing video games.
Moreover they require (at least for now) a lot of space and very low temperatures in order to work.
