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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.
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?
Who knows? Maybe both?
I want to delete my account but Slashdot doesn't allow it.
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." :)
Nature:Physics is pay-for, but this appears to be the same paper on arXiv:
http://arxiv.org/abs/0909.0950
It's just like Thunderdome - except two man enter, one man may or may not leave.
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 have both 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.
Is that the one where you don't know whether or not your will get shot for killing meth dealers that work for your boss or something?
Now you know know where the dimension full of evil people with goatees comes from.
So, perhaps if you took two entangled particles, and measured the momentum of one, and the position of the other, you could extrapolate the total state of one of them. JUST LIKE THE SUMMARY SAYS
No folly is more costly than the folly of intolerant idealism. - Winston Churchill
It's more like this:
I want to know the name and date of birth of a particular person, but I'm only allowed to ask one question per person. If that person has a twin, I can ask the person i'm curious about their name, and the twin their age, thus circumventing the one question limit in this case.
It's sort of a cheat, it probably can't be used to get arbitrarily accurate information regarding a particle of interest, and it may only work on a small subset of particles (they need to have entangled partners).
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
Generally entangled particles are created at the same place. For example, certain decay processes will fire off two complementary particles in opposite directions, at the same speed, in order to conserve momentum. If you measure the location of one of those particles, you know that the other must be the same distance from the origin point, in the opposite direction, so you know it's position as well.
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...
The Heisenberg uncertainty principle puts a well-defined, quantitative lower limit on the certainty for certain pairs of variables. 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.
No kidding!!! What do you say at this point?
Physics is not merely an evolutionary process; occasionally, the models are simply wrong, and must be replaced. For example, consider epicycles. For the purpose of calculation, they were adequate, if expensive. However, a simpler and better theory was found, and they were replaced entirely. Unfortunately, history has shown us that most people will bitterly defend the accepted theory, rather than consider possible alternatives.
As Feynman once said, "If I were forced to sum up in one sentence what the Copenhagen interpretation says to me, it would be 'Shut up and calculate!'." Anyone with common sense would consider the currently accepted interpretation of quantum mechanics to be nonsense. Useful nonsense perhaps, but certainly not a suitable description of the world that just needs some refinement.
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.
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 are based on the way they appear...
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...