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I read some posts on this forum and some articles which repeatedly state that it is not impossible to build q-comps but to make it successful, physics needs a great breakthrough. I tried finding but most of the material talks about how it works and what are current limitations, but how physics is going to help solving it?

Can somebody please explain precisely 'what is that breakthrough?'.

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3 Answers 3

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It can be argued that there are two primary challenges associated with quantum computing. One is a coding challenge, and the other is a purely mechanical (or in some minds physical) challenge.

As Ron indicated, viable Quantum Error Correction Code (QECC) is probably the largest breakthrough that makes quantum computers possible. Peter Shor was the first to demonstrate a viable Quantum Error Correction Code that can also be characterized as a Decoherence Free Subspace.

At this point, it can be argued that most of the problems associated with Fault Tolerant Quantum Computing are resolvable in terms of theory. However, their is a question of scaling as it relates to the number of qubits that are required to make the system viable, and the physical size of the system that is required to support those qubits. Most arguments that say there is a needed breakthrough in physics can be traced to this question of scalability.

In many cases, the difficulties are related to decoherence times and the speed at which the computers can perform calculations, which would allow the QECC time to operate. At this point those are now considered largely engineering challanges.

As far as physical breakthroughs, it is likely that those conversations are in regards to Topological Quantum Computers, which rely upon the construction of Anyons in order to operate. Currently these are largely viewed as pure mathematical constructions, however, the construction and use of stable anyons, even as quasiparticles, as part of topological quantum computer would be a major physical breakthrough.

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upvote specially for the last paragraph !! –  Rorschach Sep 25 '12 at 13:39

Overcome decoherence and losses. Since q-bits are superposition of states, any thermal interaction with a bath (i.e. the environment around the experimental system, including vacuum) will collapse them to some eigen-state. The bigger the number of q-bits involved the more sensitive the system is to losses and decoherence.

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it's impossible to overcome it...as far as I know ! –  Rorschach Aug 26 '12 at 8:19
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@PankajSejwal: Look up Shor error correction--- this is the true breakthrough in quantum computing. It made it feasable to do quantum computation without impossible environmental isolation. –  Ron Maimon Aug 26 '12 at 11:03
    
you don't need to overcome it completely, just enough until the computation is over :) the trend is to work at an experimental realization of qubits that will inherently be more protected to decoherence (such as topological qubits). –  natan Aug 26 '12 at 16:05
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This answer and the one by Soham only state what makes it hard to construct a q.computer, but do not answer the question, which asks for an explanation of what breakthrough in physics is needed. –  MBN Sep 25 '12 at 10:52

Basically what we need is a completely (or nearly completely) isolated environment for the Q-comp. And that is quite hard.

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