
Quantum computer emulated by a classical system - jonbaer
http://phys.org/news/2015-05-quantum-emulated-classical.html
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Strilanc
The paper's conclusion [1] does a much better job of summarizing what they did
than the posted article. For example, they explicitly note limitations:

> _Because the QMT representation uses frequencies to encode the state, the
> number of qubits would be limited by bandwidth. Using an electronic voltage
> as a physical representation, we find that frequencies in the practical
> range of 0.1 Hz to 100 GHz would allow for about n=40 entangled qubits to be
> realized. [...] Since the quantum state is explicitly encoded in the signal
> spectrum, security applications such as quantum key distribution would not
> seem to be amenable to this approach, although they may be emulated._

And they even note that their approach has not been demonstrated to be
superior:

> _It remains to be shown that this approach can be realized in a fault-
> tolerant manner, using existing quantum error correction techniques, and
> that it ultimately has utility and merit over current approaches to quantum
> computing—or standard digital computers, for that matter._

My takeaway is that the paper's intended goal is to explain a neat, perhaps
practical, way to emulate some qubits. It doesn't do anything "magical", it
encodes all of the amplitudes into the signal (in a real quantum computer,
you'd have no direct access to the amplitudes) and explains how to emulate
operations being applied.

1:
[http://iopscience.iop.org/1367-2630/17/5/053017/article#njp5...](http://iopscience.iop.org/1367-2630/17/5/053017/article#njp512897s9)

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questerzen
This is something that has been noticed before mathematically, but hard to do
in practice. The article is maybe a little confused. The limitation on size
comes from the limit on frequency/amplitude resolution - for a given scale and
resolution, the need to distinguish states grows combinatorially. It can be
overcome by making the system smaller. Eventually you can rely on quantum
effects to help, when you are no longer emulating a quantum system, you've
basically built one.

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winestock
The article twice mentions that the emulator would use analog circuits. So
there must be some reason why digital circuits would not work.

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moron4hire
Probably has something to do with the determinism of digital systems.

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db48x
Analog systems are equally deterministic. In fact, a digital circuit is just
an analog circuit where the interpretation of the voltages is quantized.

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moron4hire
An analog radio tuned to read nothing but static is not deterministic.

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db48x
Just because you can't determine what it will do doesn't mean that it's
nondeterministic. You just don't have enough information about the hundreds of
billions of radio sources scattered all throughout the galaxy. You probably
don't have any better information about what the radio station a few kilohertz
away will broadcast next, either.

