Quantum computers won't be delivering your e-mail any time soon, but in theory computers based on the principles of quantum mechanics could be much more powerful than today's models. But now researchers have discovered a rule, called the no-deleting rule, that could set limits on the power of quantum computers.
What makes the principle of quantum computers quite different from classical computers is how they store their basic unit of information. Regular bits of information can be only 0 or 1. A quantum bit has the additional talent of being able to be both zero and one at the same time--a property called superposition--but only until you measure it. In the 1980s, scientists realized that, because of this property, you can't duplicate an unknown quantum bit. Copying bits of information in a standard computer is as simple as sticking a floppy into a disk drive. But to make a copy in a quantum computer, you'd have to measure the unknown quantum bit, destroying the superposition and ruining the original. This dilemma is referred to as the "no-cloning rule." You can make multiple copies of quantum bits by using the same production processes over and over, but because you're creating the quantum bits yourself, the states aren't unknown.
Now physicists Arun Pati and Samuel Braunstein of the University of Wales have come up with a theorem similar to the no-cloning ordinance: the "no-deleting rule." According to the new rule, if you have several copies of an unknown quantum bit, "you cannot delete one or more copies keeping the others intact," Pati says. "If you try to delete one, the other copies will be disturbed." Thus, unlike a regular computer, a quantum computer might have trouble clearing out its memory when you want to get rid of a superfluous copy of a piece of data.
Yet there may be a silver lining. Pati speculates that the no-deleting rule might give users of quantum computers the ability to keep track of all copies of their files in a way that would be impossible for classical computers: A saboteur who can't define a file can't delete it. "I believe that the laws of quantum mechanics might provide security," he says. While Los Alamos physicist Wojciech Zurek is not so sure about this, he finds the result quite important, nonetheless. "It will clarify in what sense quantum information is quantum."