Violation of Heisenberg's Uncertainty Principle

mbone writes "A very interesting paper (PDF) has just hit the streets (or, at least, Physics Review Letters) about the Heisenberg uncertainty relationship as it was originally formulated about measurements. The researchers find that they can exceed the uncertainty limit in measurements (although the uncertainty limit in quantum states is still followed, so the foundations of quantum mechanics still appear to be sound.) This is really an attack on quantum entanglement (the correlations imposed between two related particles), and so may have immediate applications in cracking quantum cryptography systems. It may also be easier to read quantum communications without being detected than people originally thought."

Nobody with a clue is surprised

[gweihir]

Quantum "encryption" was never that. It is only quantum "modulation" and its "security" is pure conjecture, not anything actually provable in the mathematical sense as you get with real encryption. That does not hinder a log of gullible fools to hail it as the new thing. (It does have a lot of other fundamental and unsolved problems, even if it should be secure.)

Re:Magic

[History's Coming To]

Actually, it's the equivalent of finding socks in the dark. If two photons are produced by an interaction of spin zero then the two photons will have spin up and spin down, although you can't know which is which without measuring one. What you DO know is that they have opposite spin, so by measuring one you instantly receive information about the other, however far away it is. There are several "pairs" of information which each particle/photon can have, such as momentum/location, the more accurately you measure one the less accurately you know the other, what these guys are proposing (as far as I can tell, it's at the limit of my understanding) is that they can use entanglement properties to discover information beyond Heisenberg's original limit.

Re:Nobody with a clue is surprised

[doshell]

Except for the one-time pad [wikipedia.org].

Re:Not magic

[TapeCutter]

Some things in Physics are "magical", or "miraculous" if you prefer. The most obvious are the fundamental forces, space, and time. Currently Physics gives us a very useful description of how these things behave and interact, but it is more or less clueless as to why they exist in the first place [youtube.com].

Re:Magic

[N7DR]

Bearing in mind that it's generally an error to try to summarise anything about quantum mechanics in a paragraph or two:

Actually, it's the equivalent of finding socks in the dark. If two photons are produced by an interaction of spin zero then the two photons will have spin up and spin down, although you can't know which is which without measuring one. .

I'm sorry,. but the way you write that makes it seem that they have spin up and spin down, and then you measure them to find out which is which. If that's indeed what you meant, I'm afraid that's fundamentally incorrect.

The whole point about the weirdness of quantum entanglement is that the quanta are NOT in a state where one is up and one is down prior to the measurement. Only when you make the measurement does this happen. Prior to the measurement, quantum mechanics says that they are both in a state that is BOTH up and down at the same time.

In other words, quanta are not like socks. We can be reasonably sure that socks' measurable properties are fixed before we actually look at them. Not so with quanta.

You can think of this in this way: when you make a measurement on one of the quanta, it flips a coin that tells it whether to be up or down. Its twin quantum is then bound to give the opposite result. But prior to the coin toss, neither quantum knows how it will respond to a measurement. The most that can be said is that whatever the result of measuring one, the other will give the opposite result.

Re:Magic

[aBaldrich]

This has been tested experimentally. http://en.wikipedia.org/wiki/Bell_test_experiments [wikipedia.org]

Re:I have the fix

[feepness]

It's *decouple*. And now I'm disgusted by both of us.

Re:Argh science journalism.

[SoftwareArtist]

While the article is terrible, the actual paper is very clear about this. There are two different things that are commonly referred to as "the Heisenberg uncertainty principle". One refers to the intrinsic properties of a wavefunction and the impossibility of being in an eigenstate of two noncommuting observables. The other - which is what Heisenberg originally proposed - refers to the fact that performing a measurement alters the state of the thing being measured. Many people, including the authors of quantum mechanics textbooks, frequently talk about these as if they were equivalent, but they aren't.

Here's the first paragraph of the paper, which lays all this out very clearly:

The Heisenberg Uncertainty Principle is one of the cornerstones of quantum mechanics. In his original paper on the subject, Heisenberg wrote “At the instant of time when the position is determined, that is, at the instant when the photon is scattered by the electron, the electron undergoes a discontinuous change in momentum. This change is the greater the smaller the wavelength of the light employed, i.e., the more exact the determination of the position” [1]. Here Heisenberg was following Einstein’s example and attempting to base a new physical theory only on observable quantities, that is, on the results of measurements. The modern version of the uncertainty principle proved in our textbooks today, however, deals not with the precision of a measurement and the disturbance it introduces, but with the intrinsic uncertainty any quantum state must possess, regardless of what measurement (if any) is performed [2–4]. These two readings of the uncertainty principle are typically taught side-by-side, although only the modern one is given rigorous proof. It has been shown that the original formulation is not only less general than the modern one – it is in fact mathematically incorrect [5]. Recently, Ozawa proved a revised, universally valid, relationship between precision and disturbance [6], which was indirectly validated in [7]. Here, using tools developed for linear-optical quantum computing to implement a proposal due to Lund and Wiseman [8], we provide the first direct experimental characterization of the precision and disturbance arising from a measurement, violating Heisenberg’s original relationship.