Over the course of the last few weeks I have been discussing with a variety of folks what is meant by the words “quantum computer”.
In the course of this highly unscientific survey I have found that people have two very different definitions that appear with about the same frequency.
The first is this:
A quantum computer is any computing device that makes direct use of quantum mechanical phenomena, such as superposition and entanglement, to solve computational problems.
The second is this:
A quantum computer is any computing device that comprises a set of globally phase coherent qubits, which can simulate any other possible quantum computer.
Obviously the first definition is weaker.
An interesting point is that no machine consistent with the second definition has ever been built, while several of the first type have been.
What do you guys think? Is there an accepted meaning for the term? If not which of these makes more sense?
From my point of view the property of QCs that makes them QCs is the computational scaling advantage, and not anything having to do with global phase coherence (or any other specific physical quantity, such as entanglement etc.) unless these are clearly required to get the aforementioned scaling advantage. My vote goes to the first definition.