Uncertainty Sets Limits On Quantum Nonlocality

An anonymous reader writes "Research in today's issue of the journal Science helps explain why quantum theory is as weird as it is, but not weirder. Ex-hacker Stephanie Wehner and physicist Jonathan Oppenheim showed that the Heisenberg Uncertainty Principle sets limits on Einstein's 'spooky action at a distance.' Wired reports that the discovery was made by 'thinking of things in the way a hacker might' to uncover a fundamental link between the two defining properties of quantum physics (abstract, supplement). Oppenheim describes how uncertainty and nonlocality are like coding problems, enabling us to make a quantitative link between two of the cornerstones of quantum theory."

Comments

Re:Define 'observe'

[MozeeToby]

I think the closest plain English definition would be: has an interaction with something. More accurate, but more confusing might be: things are undefined until something happens that requires them to be defined in order for that something to happen. An electron doesn't have a position or a momentum until something occurs which require the electrons position and momentum to be known in order to determine the outcome. That might be a human being with an incredibly complex apparatus measuring the properties of an individual electron, or it might be a chemical reaction that is sweeping through the entire sample of whatever the electron is a part of.

Re:Define 'observe'

[gsliepen]

The best definition I have heard is this: suppose we have an observer O in state A, and a system S which is in the superposition of the states 1 and 2. When the observer observers the system, the state of S does not collapse, rather the observer and system become one, say OS, and is in a superposition of the states A1 and A2.

You can interpret this in various ways; one could say that this means the observer, or even the whole universe for that matter, branches all the time, and/or all possible states of the observer/universe exist simultaneously, however that again is just a description, not what might really be the case.

Disclaimer: I am a physicist.

Wired article completely misleading

[DrJimbo]

The Wired article "explains" entanglement by talking about Bob predicting what Alice did even though Alice is far away from Bob. This is the fundamental misunderstanding of quantum entanglement and has led to all sorts of wacky (and false) speculations and "theories".

The actual paper correctly says:

Non-locality can be exhibited when performing measurements on two or more distant quantum systems – the outcomes can be correlated in way that defies any local classical description. This is why we know that quantum theory will never by superceded by a local classical theory. Nevertheless, even quantum correlations are restricted to some extent – measurement results cannot be correlated so strongly that they would allow signalling between two distant systems.

Quantum entanglement (QE) provides a correlation not a communication. What this means is that not only can't you use QE to pass signals (or any information) between Alice and Bob, you actually need some other form of after-the-fact communication between them to detect the correlation in order to determine if QE happened at all. If QE was a method of communication then you could verify it by sending Bob a "cheat cheat" of what Alice was going to do or transmit. Instead, you need to look at the outcome of a series of measurements taken by Alice's and the outcome of a series of measurments taken by Bob just to see if QE actually happened.

Correlation is not communication.