Scott Fulton III put together a simple primer on quantum computing.
A quantum computer is — or, perhaps more accurately phrased, would be — a wholly different order of mechanism than anything the human species has ever constructed. Today, there are working machines that perform some small part of what a full quantum computer may eventually do.
Uh, OK. Maybe a definition would help.
The atom-level device that actually performs the quantum calculations will, as a result of its work and as is its nature, self-destruct when it’s done. A quantum computing mechanism would actually be a machine that automatically builds the computing device out of atoms (calcium atoms are good candidates), sustains the operating conditions of that device for the duration of its program, applies the program, allows it to execute, looks the other way (because quantum logic gates are shy and will explode if anyone sees them), interprets the final state of its registers as the final probability table of results, then resets itself to rebuild another mechanism all over again.
Maybe we can figure it out when Apple puts one into a Mac.