[ExI] D-Wave's Quantum Computer

Anders Sandberg anders at aleph.se
Thu May 23 06:17:58 UTC 2013

On 2013-05-22 22:22, Kelly Anderson wrote:
> Even in the most optimistic scenarios, are quantum computers general 
> purpose computing machines? Or are they simply machines that are 
> capable of solving a certain class of problem extremely rapidly?

As far as I understand, they can be general purpose. You just use gates 
that implement universal reversible operations. I don't think they would 
be amazingly effective, but they could do it.

Incidentally, since we are talking quantum computation. I have been 
trying to track down a proper source on minimum energy dissipation for 
it. As far as I get it, by virtue of reversibility QCs can run with no 
dissipation - they do not erase bits, so the Landauer limit doesn't 
apply. But error correction will be necessary, and error correction 
schemes involve dirtying ancilla bits - these bits will have to be 
erased, at a thermodynamic cost. Does anybody know a paper placing 
bounds on how many ancillas are necessary? I know there are theorems 
stating that if the error probability per gate is under a certain 
constant in-the-large fault-tolerant computation is possible, but most 
of these constructions seem to have huge numbers of gates (literally 
millions per bit). Does anybody know a paper that instead minimizes the 
number of expected erasures?

Oh, and while still quantum:
Entanglement between particles separated in time. Unsurprising, yet 
totally rad.

Dr Anders Sandberg
Future of Humanity Institute
Oxford Martin School
Oxford University

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