
Physicists track a quantum system’s wanderings through quantum state space - jonbaer
http://news.wustl.edu/news/Pages/27133.aspx
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troymc
I believe this is the paper (letter) they're talking about:

[http://www.nature.com/nature/journal/v511/n7511/full/nature1...](http://www.nature.com/nature/journal/v511/n7511/full/nature13559.html)

and here's the version at arxiv.org:

[http://arxiv.org/abs/1403.4992](http://arxiv.org/abs/1403.4992)

Note the use of the Bloch sphere to represent the state space:

[https://en.wikipedia.org/wiki/Bloch_sphere](https://en.wikipedia.org/wiki/Bloch_sphere)

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akavel
fragment describing the gist of the experiment:

 _" The [entering] microwaves are so far off resonance with the
[superconducting] circuit that they cannot drive it between its ground and its
excited state. So instead of being absorbed, they leave the box bearing
information about the quantum system in the form of a phase shift (the
position of the troughs and peaks of the photons’ wavefunctions)._

 _Although there is information about the quantum system in the exiting
microwaves, it is only a small amount of information._

 _“Every time we nudge the system, something different happens,” Murch said.
“That’s because the photons we use to measure the quantum system are quantum
mechanical as well and exhibit quantum fluctuations. So it takes many of these
measurements to distinguish the system’s signal from the quantum fluctuations
of the photons probing it.” Or, as physicists put it, these are weak
measurements. "_

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Yardlink
What happened to the old idea that when you observe a quantum system, its
wavefunction collapses? Is that not true anymore? It seems like these guys
observed it "gently" enough that it only sort of mostly collapsed with some
bias?

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hliyan
According to the article, what they measured doesn't really sound like a
'real' quantum system: it's a superconducting circuit with two discrete states
with an infinite number of previously immeasurable superposition states in
between. They call this an 'artificial atom'. Unless I'm mistaken, it's an
emulation of sorts.

[http://arxiv.org/abs/1202.1923](http://arxiv.org/abs/1202.1923)

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gaze
It is absolutely a real quantum system. It's an artificial atom. It has
basically approximately the same Hamiltonian as a trapped atom but at 6 GHz,
and we can control the coupling.

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debt
Couldn't they use this method to communicate over vast distances? I mean shit
you could build a computer that uses something like this to measure the shift
in a superimposed particle; this would allow communication to anywhere on
earth without physical or wireless connection. All that's need is superimposed
particles.

~~~
darkmighty
[https://en.wikipedia.org/wiki/No-
communication_theorem](https://en.wikipedia.org/wiki/No-communication_theorem)

