Quantum measurements: Common sense is not enough

Quantum measurements: Common sense is not enough
Ion trap. Photo: C. Lackner

(PhysOrg.com) -- In comparison to classical physics, quantum physics predicts that the properties of a quantum mechanical system depend on the measurement context, i.e. whether or not other system measurements are carried out. A team of physicists from Innsbruck, Austria, led by Christian Roos and Rainer Blatt, have for the first time proven in a comprehensive experiment that it is not possible to explain quantum phenomena in non-contextual terms. The scientists report on their findings in the current issue of Nature.

Quantum mechanics describes the physical state of light and matter and formulates concepts that totally contradict the classical conception we have of nature. Thus, physicists have tried to explain non-causal phenomena in by classical models of hidden variables, thereby excluding randomness, which is omnipresent in .

In 1967, however, the physicists Simon Kochen and Ernst Specker proved that measurements have to be contextual when explaining quantum phenomena by hidden variables. This means that the result of one measurement depends on which other measurements are performed simultaneously. Interestingly, the simultaneous measurements here are compatible and do not disturb each other.

The physicists led by Christian Roos and Rainer Blatt from the Institute of and (IQOQI) of the Austrian Academy of Sciences and the University of Innsbruck have now been able to prove this proposition and rule out non-contextual explanations of quantum theory experimentally. In a series of measurements on a quantum system consisting of two ions they have shown that the measurement of a certain property is dependent on other measurements of the system.

Technological headstart

The experiment was carried out by the PhD students Gerhard Kirchmair and Florian Zähringer as well as Rene Gerritsma, a Dutch postdoc at the IQOQI. The scientists trapped a pair of laser-cooled calcium ions in an electromagnetic trap and carried out a series of measurements.

"For this experiment we used techniques we had previously designed for building a quantum computer. We had to concatenate up to six quantum gates for this experiment," explains Christian Roos. "We were able to do this because, it is only recently that we can perform a quantum gate with high fidelity."

Only last year, a team of scientists led by Rainer Blatt realized an almost error-free quantum gate with a fidelity of 99 %. With this technological headstart, the scientists have now proven comprehensively in an experiment for the first time that the experimentally observed phenomena cannot be described by non-contextual models with hidden variables. The result is independent of the quantum state - it was tested in ten different states. Possible measurement disturbances could be ruled out by the experimental physicists with the help of theoreticians Otfried Gühne and Matthias Kleinmann from the group led by Prof. Hans Briegel at the IQOQI in Innsbruck.

Randomness cannot be excluded

In 1935 already, Albert Einstein, Boris Podolsky and Nathan Rosen questioned whether quantum mechanics theory is complete in the sense of a realistic physical theory - a criticism that is now well know in the scientific world as the EPR paradox. In the mid 1960s, John Bell showed that quantum theory cannot be a real and at the same time local theory, which, in the meantime, has also been proven experimentally.

Kochen and Specker’s results exclude other theoretical models but until now it was difficult to provide a convincing experimental proof. Following a proposition by the Spaniard Adán Cabello, the Innsbruck scientists have now successfully proven this point and produced unambiguous results experimentally.

The physicists are supported by the Austrian Science Funds (FWF), the European Union, the Federation of Austrian Industry Tyrol, and Intelligence Advanced Research Projects Activity (IARPA).

More information: State-independent experimental test of quantum contextuality. G. Kirchmair, F. Zähringer, R. Gerritsma, M. Kleinmann, O. Gühne, A. Cabello, R. Blatt, and C. F. Roos. Nature. July 23, 2009, DOI: 10.1038/nature08172

Provided by Austrian Academy of Sciences

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Jul 22, 2009
What exactly was discovered here? What exactly was discussed here? I read this article twice and still I am at a loss.

Jul 22, 2009
I believe they've confirmed that the phenomena observed in quantum systems are context dependent, and can't be made to fit in a big all-encompassing scheme of things.
Basically, God is, in fact, playing dice, or at least that's the only way we can see it. Think Godel's theorems applied to quantum theories.
I could be quite wrong, though.

Jul 22, 2009
Godel's theorem had to do with the incompleteness of a mathematical system.

The above experimental proof, like the Bell inequality tests, show that QM is complete as formulated, ... that this, that one cannot find a way of rationizing it using concepts of everyday experience.

The reason is, is that the conceptual structure of our knowledge of reality (i.e. phenomenal reality) is dependant on mind, and is added as it were to experience in the intellectual process of the understanding. In other words, concepts like time, space, and causality, (all of which fail to rationalize QM), are, with respect to noumenal Reality, an artificial coherence due to a functioning mind. The above concepts are not discoverable independantly of their use, they are not entities 'out there', they are forms in which the mind understands,.. they are the subjective component of phenomenal reality, and Reality proper is not oblidged to be tamed by them.

Jul 22, 2009
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Jul 22, 2009
sounds like emptiness

Jul 23, 2009
For me, quantum physics is the epitome of applied mathematics. From a purely intuitive point of view, Planck units seem like a perfect artifice to make the universe 'mathematical', i.e. perfectly measurable or mappable, while the uncertainty principle restrains how much we can actually measure, much like Godel's theorems do with pure mathematics.
Furthermore, wouldn't any set of mathematical theories and axioms, regardless of their origin(real-world measurements, mental experiments), be subject to Godel's incompleteness theorems?

Jul 23, 2009
The press release expounds, perhaps confusingly and specifically erroneously, on some non-main stream ideas.

First, QM doesn't conflict with classical physics but is known from its inception to converge to classicality in large systems.

Second, QM phenomena and theory are eminently causal, it's the very idea behind the theory.

Third, QM has, again since its conception, been known to have context dependency as regards observers, today expressed as decoherence (formerly, "collapse of the wavefunction").

Fourth, again the very idea behind the theory, it expresses both realism and locality so compatible with special relativity. What it does clash with is counterfactual definitiveness, the "common sense" idea that observables exist outside of the observations that effect them. (As opposed to QM states or classically wave functions that are persistent, affected but not effected by observation.)

Really, all of this is so mainstream that even old Wikipedia is a good resource.

Jul 23, 2009
"For me, quantum physics is the epitome of applied mathematics."

Little in physics is mathematics, applied or especially formal. That is why we need testing to throw out incorrect theories, and use other means to choose the likeliest of the correct. (Say, parsimony consistent with bayesian likelihoods.)

Specifically for QM, and a good test of the model of science above, I believe nobody has managed to formally derive 2nd quantization from any axiom set.

"much like Godel's theorems do with pure mathematics."

Gödel's theorems hasn't anything to say about physics today, see above. And in the future, even if, say, string theory one day would be found to be the theory that can predict basic physics there remains scores of emergent phenomena that can't be derived from it.

Also, even string theory can't derive our physics entirely from formalism, as it has 10^500 vacuums to choose between. The way to bet is that all theories of basic physics will be thus, have vacuum as free parameters. So take your pick, all physics theories are "context dependent" to use the article's terms, with a free non-axiomatizable sector.

In any case it is a mistake to think that Gödel prohibit exploration in math, when they do exactly the opposite. By them it is known that we can always extend math by new axioms to capture ever more powerful models, say the tiny islands of formal parts and even the majestic sea of algorithmic parts of physics.

Jul 23, 2009
"the "common sense" idea that observables exist outside of the observations that effect them."

D'oh! Better formulated, and as can be read from the term, _conterfactual_ [I blame my dumb spell checker! :-)] definiteness is "the ability to speak meaningfully about the definiteness of the results of measurements, even if they were not performed."

Jul 23, 2009
'Theory of Everything' implies complete knowledge. But there would need to be an infinity of New Theories of Everything to supercede each old one. Godel's Incompleteness theory does mean that we have to keep broadening our framework to advance knowledge as any particular framework is deficient within itself for proving everything hypothesised within it.

Similarly, physicists seem to need more (11 and rising?) dimensions to explain the four common sense ones. This parallels the use of broader frameworks to overcome Godel's theory. Common sense ought to be mistrusted. What we take in with our senses is very limited. The article uses the word "real" ["cannot be a real and at the same time local theory"] although it might be being used in a technical sense that I am not familiar with. If it is not being used in a technical sense then it does not mean much. I imagine that matter/energy would be very rational and commonsensical if viewed in 11 dimensions using super senses. Or would it? Would 11-D beings need to postulate 20 dimensions to overcome more oddities in behaviour?

There is no end to our ignorance. But that is good news as it allows infinite advancement. Perhaps "Theory of Quite a Lot" would be better title?

Jul 24, 2009
"Little in physics is mathematics, applied or especially formal."
Then how come a physics student's curriculum is mostly maths? Maths is the language of science and like common languages, it ultimately limits what we can know, or observe in its terms.
To quote Noumenon, "our knowledge of reality (i.e. phenomenal reality) is dependant on mind", to which I add the requirement of communication. Whatever view of the universe, or theory, prediction model or whatever anyone would come up with would have to be communicated to someone else, in virtue of the scientific method. Communication occurs via languages, maths is the language of science...Does anyone see my point here?

Jul 24, 2009
@ Kasen,

I think what Torbjorn is saying is that fundamentally physics is inductive rather than deductive. So in effect you don't need 'Godel's incompleteness theorem' to find an upper limit as it were, for science,.... the process of induction is already a limit, since it is based on an accumulation of observed events, so that the truth of conclusions drawn are never guaranteed.

Jul 24, 2009
... that's not to say I agree with Torbjorn's four points above, but is why I suggest qm interpretations should be epistemological.

[(1) Yes it does, Statistical convergence doesn't imply fundamental compatibility. 2) acausal, science is not purely mathematical as you state yourself (Schrodinger function causal) 4) Not so fast, either locality OR definiteness according to Bell. (Entanglement != locality)).]

Jul 26, 2009
Imagine that we have two rather special Cheshire cats. Say that mine has its body here while its

disembodied smile is (somehow) taken to Mars. My friend goes with his disembodied cat to Mars and

leaves the disembodied smile of his cat with me.

I bring a photo of a snarling dog to the attention of my cat. And its smile, on Mars, changes to a

frown (I am trying to avoid deliberately standing on its tail, which would be more sure of an instant

effect.) Say Mars is four light minutes away. When my friend notes that the smile has changed to a

frown, he too shows his cat a photo of the snarling dog, and the disembodied smile with me changes to

a frown.

If non-locality (I think that is the right term) holds then all this could take place within a few

seconds. If non-locality was not operating then the whole process would take at least eight minutes.

If there are extra hidden dimensions, could the signals involved have resulted from local processes

which appear to us to be non-local? For example, a very long quasi-neuron connecting the cat's body

with its smile and the quasi-neuron being channelled through a hidden dimension. Much folding of our

three dimensions would be necessary? For example, a 2-D sheet of paper can be folded in the third

dimension to shortcut distances. But we cannot expect such drastic folding of our spacetime to

conveniently occur as and when we need it to explain every non-local occurrence? (Though I expect

that our spacetime framework is thrashing about and vibrating wildly on a larger scale than required


If I placed both hands wide apart on a sheet of paper, it might appear to 2-D dwellers in the paper

that I am simultaneously in two places at the same time. And my left and right hands would appear to

them to be entangled, and indeed they would be connected through the extra third dimension. But if

they somehow applied a shock to my left hand. The signal would have to pass up my arm through my

nervous system and it would be some time before my right hand flinched. Not an instantaneous effect.

Nor non-local? Generally, if you use extra dimensions in an explanation it ought to take more time

rather than less to send a signal, even for a connected body, assuming you don't make up convenient


Language interferes with understanding as I think of my right and left hand as mine, but my left hand

is not the same as my right hand. Shocking one is not the same as shocking the other. "I" am in two

places at the same time, on the sheet of paper, but only because at any instant I take up rather a

lot of space.

Jul 30, 2009
I don't know, I look at 'extra dimensions', as more duck tape, or to pile on more degrees of freedom to duck tape 'the thing' in place for us. As long as it allows for predictions not otherwise possible,.. but to save the 'classical understanding' it's no longer meaningful.

Jul 30, 2009
Quantum mechanics is what happens when we humans peer too closely at the machine code running the universe...like trying to derive the entire computer operating system then the computer hardware from a memory dump of notepad.exe...

Aug 01, 2009
Isn't the hypothesising of extra dimensions a way of trying to make a common sense understanding? That is, an observation that does not seem to make sense in 3 space dimensions (or in 4-D spacetime) can seem to make sense in 4 dimensions. Accepting more dimensions in ones argument is a sign that one already accept a lack of sense within 4-D? And expresses a hope that the observation makes sense in 4 dimensions.

Aug 01, 2009
Word editor is playing up. The fourth line should have read "make sense in 4 dimensions ...".
(The plus sign was dropped somehow during the paste operation. I don't understand how.)

Aug 01, 2009
Try again.
Word editor is playing up. The fourth line should have read "make sense in 4 PLUS dimensions ...".
(The plus sign was dropped somehow during the paste operation. I don't understand how.)

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