On Thu, Jun 20, 2019 at 10:04 PM Lawrence Crowell <
[email protected]> wrote:
*> There is a bit of a major hurdle to leap over. That is decoherence. If
> you write a simple program on the QE it runs in less than 10^{-6} seconds
> and that is about all the time you have.*
>
Yes, decoherence has always been the big problem, that's why Microsoft
wants to build a Topological Quantum Computer that uses braided non-Abelian
Majorana sudo-particles called "anyons "; they are much less susceptible to
decoherence than anything else for the same reason a random bump is
unlikely to untie a knot. In the August 23 2017 issue of the journal Nature
the Microsoft people report they have managed to braid anyons in a hashtag
# arrangement of wires. These braids can encode a Qbit of quantum
information and they can be manipulated.
http://www.nature.com/nature/journal/v548/n7668/full/nature23468.html
There is also a short video that gives a outline of the above:
Nano-hashtags could provide definite proof of Majorana particles
<https://www.youtube.com/watch?v=aakSpSXLSYY>
> *> The highly entangled quantum states become mixed state entangled with
> the environment states and the execution of the algorithm falls
> apart.Quantum error correction has its limits, which is related to the
> limitations of using Hamming distance.*
>
The biggest difficulty comes from the No Cloning Theorem, it says you can't
make an identical copy of an unknown quantum state, in other words you
can't copy a Qbit. So quantum error correction algorithms are longer and
more complex than for conventional error correction, and thus you have to
worry more about errors produced when running the quantum error algorithm
itself.
Nevertheless It has been proven you could theoretically make a arbitrary
large quantum computer even if the underlining quantum gate had a error
rate as high as 3%, although the amount of overhead needed for error
correction would be so large it probably wouldn't be practical. But the
overhead drops fast with increased gate reliability, if you can get the
error rate down to 1% then it looks like a Quantum Computer of arbitrary
size is practical as well as possible.
Quantum Computing with Very Noisy Devices
<https://arxiv.org/pdf/quant-ph/0410199.pdf>
> *> for now quantum computers are in a bit of a niche market of sorts, and
> their practical use is largely with encryption.*
Somebody will certainly use Shor's factoring algorithm to break RSA
encryption once they get their hands on a Quantum Computer, but I think the
killer application will be in efficient quantum simulation; imagine
dreaming up a complex shape you want a protein to fold up into and the
computer telling you what sequence of amino acids will fold up into exactly
that shape. It would revolutionize medicine and chemistry not to mention
give a big shot in the arm to Drexler style Nanotechnology.
* > I do though suspect before long ordinary computers will have some qubit
> processor that will be executed in some algorithms.*
>
Maybe but I doubt it. Nobody knows if a conventional computer could solve
all nondeterministic polynomial time problems in polynomial time, we don't
even know if a Quantum Computer can do that, but there has been a recent
development that I find suggestive. Even if, to everybody's surprise, it
turned out that P=NP and even if we had a algorithm that could solve NP
problems on a conventional computer in polynomial time there would STILL be
a class of problems a conventional computer couldn’t solve efficiently but
a quantum computer could. Yes this class of problems is very exotic and
nobody knowns if they have any fundamental interest in themselves or if
they're interesting only because a conventional computer can’t solve them,
but still...
Oracle Separation of BQP and PH
<https://eccc.weizmann.ac.il/report/2018/107/>
John K Clark
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