## Defeating Heisenberg's Uncertainty Principle 160

Posted
by
CmdrTaco

from the schrodinger's-cat-is-next dept.

from the schrodinger's-cat-is-next dept.

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?"*
## EPR (Score:3, Informative)

Wasn't this the whole basis for the EPR paradox? Using two different measurements of location and momentum with entangled particles to build a complete state?

If not, what am I missing?

## ArXiv link (Score:2, Informative)

Nature:Physics is pay-for, but this appears to be the same paper on arXiv:

http://arxiv.org/abs/0909.0950

## Physics (Score:2, Informative)

IANAP, but I've read quite a bit over the years, and my understanding was that the uncertainty principle wasn't a limitation in our "measurements" per se, but rather how the world itself works. To take the classic example of momentum and position, for example: the problem isn't that we can't measure both the speed and position of an electron (like our tools aren't "fine" enough or something), but rather that an electron

doesn't haveboth speed and position in the sense that we think about it. If we attempt to measure one of those two aspects, by that measurement we define the electron in a particular way and therefore blur the meaning of the other measurement.My money is on Heisenberg, but then, I'm just a caveman.

## Bad choice of names? (Score:5, Informative)

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

## Re:EPR (Score:3, Informative)

Yes, it was. The point being that after you do any measure your state is no more correlated and the second measure does not project the state of the first.

I read the arxiv version of the paper (later I will have to go down to the library to get the journal one) and it seems that they simply reframe a lot of common knowledge in a different terminology. It is not like they show incompatible observables measured at the same time. Measuring position and momentum

of different particlesis not a problem since they do commute.P.S. The article defines Paul Dirac as "another physicist". Just look at his page on Wikipedia for Landau's sake.

## Re:Afty0r (Score:5, Informative)

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.

## Re:Perhaps I'm wrong on this... (Score:3, Informative)

No, it was initially based on our ability to measure. This measurement uncertainty has certain implications (or, at least, seems to). It's these implications that have led some theorist to draw conclusions about the way things

arebased on the way theyappear...Of course, not everyone agrees with Bohr's interpretation. Feynman was one of the first to speak up... well, at least one of the first to speak up and be heard (Feynman was no slouch). These days it's Lee Smolin and the quantum gravity crowd that are denouncing the importance of the uncertainty principle.

Actually, they are saying that the whole idea of particles and wave functions as "things" is just wrong. We should be thinking of these observable and measurable phenomena as holographic manifestations of things we are unable to perceive in only 4 dimensions...

## Re:Someone assfuck the writer please (Score:1, Informative)

No, it's F=dp/dt, which is even correct in relativity, since momentum p = m*v. Writing it out fully: F = (dm*dv)/dt; now to capture relativity (at least for systems with constant rest mass), just define m(v) = m_0 * 1/sqrt(1-v^2/c^2). (Remember that velocity is also a function of time, which is why this works out even tough we're only taking the time derivative.)

You can bitchslap Newton for doing alchemy, but you can't fault him for his mechanics. They worked pretty well for most stuff, also predicted light bending near massive stuff, such as your mother, even tough he off by a factor of 2. He also had Mercury's orbit almost right.

(Captcha: realists. Science isn't realistic, considering that it only models reality, therefore, scientists are models. Or modelists?)