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Defeating Heisenberg's Uncertainty Principle
eldavojohn writes "As we strive closer and closer to quantum computing, physics may need to be improved. A paper released in Nature Physics suggests that the limit defined by Heisenberg's Uncertainty Principle can be beaten with quantum memory. From the article, 'The cadre of scientists behind the current paper realized that, by using the process of entanglement, it would be possible to essentially use two particles to figure out the complete state of one. They might even be able to measure incompatible variables like position and momentum. The measurements might not be perfectly precise, but the process could allow them to beat the limit of the uncertainty principle.' Will we find out that Heisenberg was shortsighted in limiting the power of quantum physics or will the scientists be surprised to find that such a theoretical scenario — once conducted — performs unexpectedly in Heisenberg's favor?"
1st and second post simoultaneously :-)
America, Home of the Brave.
the uncertainty principle is in doubt.
Taking guns away from the 99% gives the 1% 100% of the power.
Why do people have to pitch stories like this as ego conflicts? If they get around the uncertainty principle, they'll be erasing it no more than classical mechanics. It's like Relativity is just a more accurate description, an improvement, a super theory on top of Newtonian physics. It doesn't 'defeat' Newton. We use his work as a valid framework all the time anyways. And when we need to go beyond it, we take up Einstein's work. Similarly, getting around the uncertainty principle won't really 'defeat' Heisenberg's work, it'll just build on it. These writers just sound stupid when they frame stories as simple binary conflicts.
They demoed the Ferrari 458, and one of the "features" that Jeremy Clarkson highlighted was that the dash mounted display could show you either the speedo or the GPS Sat Nav, but not at the same time - my immediate thought was "Heh, someone at Ferrari has a sense of humour and knows what the Heisenbergs Uncertainty Principle is." :)
Heisenberg's uncertainty principle is still a fundamental cornerstone in quantum physics. Incompatible observables remain incompatible. What the article says isn't that the principle is wrong, but that there is a work-around for a technical problem which the principle was causing. Much the same way the invention of airplanes did not imply gravity is wrong.
That's all I can say without seeing some math.
GAAH! MY PRINTER IS ON FIRE!!! PUT IT OUT! PUT IT OUT!
That's all I can say without seeing some math.
2+2=5. There you've seen some math, now please continue.
For those interested, the preprint of the Nature article can be found at: http://arxiv.org/abs/0909.0950
:).
However, I don't really see what the fuzz is about. What they are in fact demonstrating is a relationship between conditional von Neumann entropies, which they claim is a measure of "uncertainty" (it is in a specific meaning of the word "uncertainty"). However, there is a difference between von Neumann entropy and the variance of a physical observable as used in the Heisenberg uncertainty principle. On the other hand, if you label a physical property such as entropy "uncertainty" and demonstrate a relationship between those entropies, then you can indeed call that an "uncertainty relation" but that's just a cheap way of attracting attention.
Also, I am not sure if it is possible to obtain the Heisenberg uncertainty relation from their equation. I would expect that, for example by entering pure, disentangled states in their equation, that Heisenberg should be recoverable (because of course, Heisenberg also applies to pure states). I don't immediately see how that can happen since the von Neumann entropy for a pure state is zero. Perhaps I am just missing something and perhaps my QM is a bit rusty
The uncertainty principle originally made statements about what can be known about position and velocity. You cannot measure both position and velocity simultaneously above a certain degree of accuracy. The more accurate your measure of velocity, the less you know about position, and visa versa. Since most purists will hold this up as the true test of any theory wanting to refute the uncertainty principle, the theorists felt the need to suggest that this, theoretically could be invalidated, as well. Hence the mention of momentum.
The fly in the ointment seems to be this part of the theory:
...maximally entangling a particle with a quantum memory, meaning all states and all degrees of freedom in the particle would be tied to all of the quantum memory's states.
I'm not sure how many states and degrees of freedom would be required. The total is infinite for both, but perhaps these can be limited to ranges. Still the size of the quantum memory would be huge, I think. Also, there is the non-trivial trick of entangling the particle's states and degrees of freedom with the quantum memory states...
I don't think Heisenberg will be turning over in his grave very soon...
For example the uncertainty in momentum multiplied by the uncertainty in position for a particle must be greater than or equal to h/4pi. Breaking that limit would break Heisenberg, even if the results still weren't totally totally certain, accurate and precise.
Breaking that limit would break the mathematics of quantum physics, not just Heisenberg. The momentum and position wavefunctions are simply the Fourier transforms of each other. If position is precisely known, then the position function is an impulse, and the momentum function must be a wave that extends throughout all space. This is simply the nature of the Fourier transform. If the uncertainty relation between momentum and position did not hold, then it would mean that the momentum and position wavefunctions are NOT the Fourier transforms of each other, and that would mean that all of quantum mechanics is wrong.
What's been demonstrated here is, very clearly, not that.