Definition of “quantum computer”

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.

3 thoughts on “Definition of “quantum computer”

  1. I agree that the first definition is valid, but this is not a universal quantum computer. If machines of the first kind become common and are shown to offer some practical advantage as you describe, which they almost certainly will, then this distinction may become a problem. This also raises the issue of what is the best way to introduce the ideas of quantum computing and quantum computers to those outside of the field. A distinction such as this could easily get lost along the way, though, as each type of machine has very different experimental and technical requirements and promises very different computational benefits, it is important to ensure that they are seen as distinct goals that we are pursuing.

    When you did your scientific survey, was there any correlation between area of expertise and choice of definition?

  2. Ash: Yes there was a correlation. Experimental physicists & non-specialists tended to like the first, while theorists (comp sci & physics) tended to prefer the second.

    I don’t see any conclusive evidence to support one of your contentions though. You say that the two types of machine should provide “very different computational benefits”. I’m not convinced. If you exclude the number-theory types of algorithms (that are not practically useful) the only clear exponentially sped-up algorithms are the quantum simulations ones.

    Can you exponentially speed up some quantum chemistry calcs without having a universal QC? Maybe even just one? I don’t know. There might be a way.

    I just don’t think the current state of knowledge is sufficient to answer these types of questions.

  3. I personally like to think that quantum computers can be defined as computers that do their work in other universes.

    I guess that puts me in group one’s camp.

    I vaguely remember Deutsch’s discussions about universal virtual reality generators and universal quantum computers rendering every possible quantum situation, etc., etc.

    Once I refigured my own definition of “computer,” I could get my brain around that stuff, too.

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