These are the impossible machines of the twenty first century; they are the ultramodern equivalent of the perpetual motion machine. You can read all about them at (Give some Web addresses). It is claimed that Quantum Computers can store a spectacularly immense amount of information more than ordinary ones, but they are based on the quantum-mechanical fallacy (more simply, and more accurately, the mechanical fallacy) that there is no noise at zero degrees Kelvin. Actually, we know from Stochastic Electrodynamics that this is false. The would-be whizzkids of the twenty first century are rather like King Canute, who tried to suppress the "noise" of the tides!
Recently quantum computers have been linked with another old science-fiction fantasy, namely Time Travel. See also quantum cryptography