Computers that we have nowadays are based on electronics. Therefore if you want to have better computers, you have to make your electronical circuits smaller and smaller. However there is a limit for that. The smallest size is a few nanometer size. (1 million nanometer is equal to 1 millimeter). If you go down further in size, one can not use electronics since you enter the quantum mechanical world. Everything in there is governed by unusual quantum physics effects. In fact there are bad and good news. The bad news is that it is very very difficult to control your very tiny circuits. The good news is that once you control them, you will have very powerful computers that have never been realized. Everything in our life will be directly or indirectly affected by quantum computers. Nowadays, many scientist are studying on quantum computers. Other people are developing computer programs that work for quantum computers.
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From @denizyuce