Re: Quantum Computation

Nick Maclaren wrote:
Ralph Hartley <hartley@xxxxxxxxxxxxxxxx> wrote: 
Simulating quantum systems.

Given a description of a quantum system, the problem is to answer questions about its behavior (Simple questions like "Is the probability to go from state A to state B in time t more than 0.5").

You might not consider that a "practical" problem, but I would. I view anything that could be used to design a better transistor, or understand high temperature superconductivity etc. as practical. Quantum simulation wouldn't guarantee a solution to either problem, but it sure would help. 

Now, I will dispute that.  That is not a practical approach, on its
own.  Before you can call it practical, you have to ensure that you
can both set up the required initial state and extract the information
you need from the final one.  I know of several experts who believe
that those processes may be as expensive as the classical approaches.

That you can set up the initial, and read the final, states is implicit in the definition of a Quantum Computer. If you can't do that, you don't have one.

Of course quantum algorithms are not practical without a Quantum Computer.

There is also the question of whether each problem will need a new
computer built from scratch, or whether there will be a way of
programming such state manipulations.

All the proposed implementations I have seen are programmable. Typically they allow a complete set of operations to be performed in an arbitrary order (the sequence of operations is classically determined, but that's all you need). They are complete for quantum computation.

All *actual* implementations to date have so few bits, and can perform so few operations before failing, that the phrase "arbitrary sequence" is a bit of an exaggeration.

Ralph Hartley

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