Contextual Quantum Mechanics

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This is a post I made at another site hoping to get a discussion started but, without success. I believe Quantum Darwinism, which has also had some experimental success recently and is awaiting varification, is a good example of Contextual QM. This seems to me the first real breakthrough since Bell's Inequality and is very deserving of discussion.

Evidently a new experiment at Innsbruck Institute in Austria has provided the first proof that quantum indeterminacy is dependent upon the context in which measurements are taken, that is, whether or not other measurements are taken at the same time. Supposidly this rules out the possibility that quantum weirdness can be explained in a purely causal manner and, therefore, places further constraints on the number of possible theoretical explanations for the phenomenon. Because Eurekalert quickly removes it's webpages in a day or two I will publish the short article here and welcome any discussion on the topic.

Eurekalert.org said:
Quantum measurements: Common sense is not enough
Experimental physicists refute non-contextual quantum models
This press release is available in German.

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 quantum mechanics by classical models of hidden variables, thereby excluding randomness, which is omnipresent in quantum theory. 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 Quantum Optics and Quantum Information (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).

http://www.eurekalert.org/pub_releases/2009-07/uoi-qmc072009.php
 

Answers and Replies

  • #2
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The operational interpretation of qm makes quantum theory a theory of measurements. I would be surprized to see evidence pointing the other way - that it isn't contextual. There is another discussion going on this, where i said exactly the same thing, that objects existing with definite properties in space and time is a misnomer.

Here is the thread:

https://www.physicsforums.com/showthread.php?t=414568
 
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I agree, I would have been very surprised to see evidence pointing the other way. However, this is a milestone in that it is the first direct experimental evidence. There is also experimental evidence to support Quantum Darwinism, but that is awaiting verification.

I'm particularly interested in exploring the basic contextual theories such as Quantum Darwinism, Relational Quantum Mechanics, etc. and that other thread seems more dedicated to describing superposition in particular and doesn't even use the terminology of "contextualism". Even the term "operational" seems strange to me and not quite as broad.
 
  • #4
ConradDJ
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I'm particularly interested in exploring the basic contextual theories such as Quantum Darwinism, Relational Quantum Mechanics, etc.

This is also a particular interest of mine, so I'm curious about where you're coming from. I have the papers by Rovelli and Zurek, but I have a feeling you may be referring to something else when you say the “terminology of contextualism".

I started a thread awhile back with some thoughts about Relational QM --

https://www.physicsforums.com/showthread.php?t=314441"
 
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  • #5
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Thank you for the link. It will take some time to read but I must say I am already enjoying the clarity and conciseness of your writing.

I'm actually what you might call a "Pragmatic Taoist" and my interest is in exploring the subject using a rudamentary Functional Contextualist approach. This has a high degree of relativism and can be applied to the terminology used to describe QM. Essentially, the idea is that words only have demonstrable meaning according to their function in a given context. If quanta are contextual and our language used to describe them is also contextual, then we have the means of describing them in terms of the observer and their context.
 

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