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    Heisenberg's Uncertainty Principle Only Mostly Uncertain?
    By News Staff | July 27th 2010 09:31 AM | 15 comments | Print | E-mail | Track Comments

    Werner Heisenberg's 'Uncertainty Principle'(1927) is a fundamental concept in quantum physics, basically saying you can be increasingly accurate in position or momentum (mass X velocity), but not both(1).  

    This can be an important feature rather than a defect in something like quantum cryptography, where information is transmitted in the form of quantum states such as the polarization of particles of light.

    A group of scientists from LMU and the ETH in Zurich say they have shown that position and momentum can be predicted more precisely than Heisenberg's Uncertainty Principle states - if the recipient makes use of a quantum memory that employs ions or atoms. They say their results show that the magnitude of the uncertainty depends on the degree of correlation ("entanglement") between the quantum memory and the quantum particle.

    "The result not only enhances our understanding of quantum memories, it also provides us with a method for determining the degree of correlation between two quantum particles", says Professor Matthias Christandl. "Moreover, the effect we have observed could yield a means of testing the security of quantum cryptographic systems." (Nature Physics online, July 25, 2010)

    Unlike classical computers, quantum computers operate not with bits, but with quantum bits or qubits, quantum mechanical states of particles. The crucial feature of qubits is that they can exist in different states at once, not just 0 or 1, but also as a superposition of 0 and 1. The ability to exploit superposition states is what makes quantum computers potentially so powerful.

    "The goal of our research is to work out how quantum memories, i.e. memory systems for qubits, might be utilized in the future and how they affect the transmission of quantum bits", explains Christandl.

    Heisenberg's Uncertainty Principle plays a central role in quantum computing because it sets a fundamental limit to the accuracy with which a quantum state can be determined. Quantum mechanics also tells us that the measurement of a parameter can itself perturb the state of a particle. If, for example, one were to measure the position of a particle with infinite precision, the particle's momentum would become completely uncertain. Quantum cryptography uses this effect to encrypt data, for instance by entangling two quantum particles in a way that the probability with which the measurement of one particle yields a certain value depends on the state of the other particle. Eavesdropping can thus easily be uncovered, because any measurement will change the state of the particle measured.

    The teams at LMU and the ETH Zurich say they have now shown that the result of a measurement on a quantum particle can be predicted with greater accuracy if information about the particle is available in a quantum memory. Atoms or ions can form the basis for such a quantum memory. The researchers have, for the first time, derived a formula for Heisenberg's Principle, which takes account of the effect of a quantum memory.

    In the case of entangled particles, whose states are very highly correlated (i.e. to a degree that is greater than that allowed by the laws of classical physics), the uncertainty can disappear.

     According to Christandl, this can be roughly understood as follows "One might say that the disorder or uncertainty in the state of a particle depends on the information stored in the quantum memory. Imagine having a pile of papers on a table.  Often these will appear to be completely disordered -- except to the person who put them there in the first place.

    "Our results not only improve our understanding of quantum memories, they also give us a way of measuring entanglement.  The effect could also help us to test the security of quantum cryptographic systems."

    One can picture the method as a game in which player B transmits a particle to player A. A then performs a measurement on the particle, introducing an uncertainty. A subsequent measurement by B will only yield the value determined by A with an uncertainty given by Heisenberg's Principle.

     "But if B uses a quantum memory", says Christandl, "he can determine the correct value and win the game."

    Citation: “The Uncertainty Principle in the Presence of Quantum Memory“, M. Berta, M. Christandl, R. Colbeck, J.M. Renes, R. Renner, Nature Physics, July 25th 2010


    NOTES:

    To see it expressed mathematically, go here.
    For a thought experiment, go here.

    Comments

    So what we have concluded is that the uncertainty principle is uncertain but we don't know how uncertain? Haha!

    Hank
    That about sums it up.    What happened to the good old days, when uncertainty was more predictable?   Now we have dead cats, live cats, and then cats in different phases.  Madness!
    Garth Sundem
    This is a great post! At least, I'm pretty sure it is. Okay, maybe I'm not so certain. Oh gosh, what did I just read? (Were you able to increasingly calculate my typing speed?)

    Garth Sundem, TED speaker, Wipeout loser and author of Brain Trust

    Hfarmer
    So if I understand correctly they have shown that if you have a quantum memory and you entangle it with a particle.  Then you observe say the position of the particle and the momentum indicated by the quantum memory you get around the uncertainty principle. 
    Most discussions of uncertainty have one system and one observer.   What they have shown is that with two entangled systems the particle and the quantum memory, and two entangled observers one can get around the uncertainty principal by comparing the measurements of the two observers.  (Remember observers, us are also quantum mechanical systems and to take a measurement we end up entangling ourself with the very thing we wish to measure.  This entanglement is what lay behind the uncertainty principal in the first place.) 
    Science advances as much by mistakes as by plans.
    Garth Sundem
    Boy, I might be missing something (or a lot of somethings), but isn't it the fact of MEASUREMENT and not OBSERVATION that affects uncertainty? I know that's splitting semantic hairs, but it's the measurement that affects the situation---by measuring position, you change speed, right? (Or vice versa?) So it wouldn't really matter if you had one observer or sixty--the first observer who measures position would change the speed; and the first observer who measured speed would change the position, right? So if you went to compare MEASUREMENTS from two OBSERVERS, you'd be comparing data on apples and oranges, right?

    Hmmm, I need some tutelage.

    Garth Sundem, TED speaker, Wipeout loser and author of Brain Trust

    Hank
    It was the big debate in the early days of quantum mechanics and the gist of the whole Copenhagen 'interpretation' but at some point it becomes quasi-philosophical(to us).   It takes a photon to observe, which may be a 'disturbance' in the same way a measurement is but they basically settled for the idea that, like everything else at the very small level, words like 'observer' and 'measurement' have to be redefined.

    That's why they put a friend of Wigner in the box with the cat.   To Wigner and his friend they were entirely different wave functions but for the rest of us it caused the whole objective reality versus subjective probability mess to get worse.   It seems we all need some tutelage but no one can do the instructing yet.
    Bonny Bonobo alias Brat
    It seems to me that 50 years ago Hugh Everett III explained much of what is now being explained in this blog. Quote "A group of scientists from LMU and the ETH in Zurich say they have shown that position and momentum can be predicted more precisely than Heisenberg's Uncertainty Principle states - if the recipient makes use of a quantum memory that employs ions or atoms. They say their results show that the magnitude of the uncertainty depends on the degree of correlation ("entanglement") between the quantum memory and the quantum particle. " Quote "The Universal Wavefunction or Universal Wave Function is a term introduced by Hugh Everett in his Princeton PhD thesis The Theory of the Universal Wave Function, and forms a core concept in the relative state interpretation or many-worlds interpretation of quantum mechanics." See wikipedia at http://en.wikipedia.org/wiki/Universal_wavefunction Quote from Hugh Everett “If we try to limit the applicability so as to exclude the measuring apparatus, or in general systems of macroscopic size, we are faced with the difficulty of sharply defining the region of validity. For what n might a group of n particles be construed as forming a measuring device so that the quantum description fails? And to draw the line at human or animal observers, i.e., to assume that all mechanical aparata obey the usual laws, but that they are not valid for living observers, does violence to the so-called principle of psycho-physical parallelism.” Anyone interested can also find a lot more information about Hugh Everett III and his parallel universe and universal wave theories and why the uncertainty principle is not quite so uncertain at http://www.relativitycalculator.com/
    My article about researchers identifying a potential blue green algae cause & L-Serine treatment for Lou Gehrig's ALS, MND, Parkinsons & Alzheimers is at http://www.science20.com/forums/medicine
    Bonny Bonobo alias Brat
    I should have also mentioned (see http://en.wikipedia.org/wiki/Hugh_Everett) Quote “Everett was unable to communicate the core idea that the universe is describable, in theory, by an objectively existing universal wave function (which does not "collapse"); this was simply heresy to Bohr and the others at Copenhagen. The conceptual gulf between their positions was simply too wide to allow any meeting of minds; Léon Rosenfeld, one of Bohr's followers, talking about Everett's visit, described Everett as being "undescribably stupid and could not understand the simplest things in quantum mechanics". Everett later described this experience as "hell...doomed from the beginning.” Quote “Discouraged by the "scorn" other physicists heaped on MWI, Everett left physics after completing his Ph.D. Afterwards, he developed the use of generalized Lagrange multipliers in operations research and applied this commercially as a defense analyst and a consultant. He was married to Nancy Everett née Gore. They had two children: Elizabeth Everett and Mark Oliver Everett, frontman of the band Eels.” As I once asked in an earlier post on Science 2.0, if Einstein was alive today would he be sufficiently qualified and conformist enough to receive a research grant or be allowed to publish a controversial peer reviewed paper? I think the scientific community needs to have a good long look at itself and its often sheep-like mentality. Safeguards need to be put in place to allow non mainstream scientists to receive grants and get published without being ridiculed simply for having alternative views. Everett died in his sleep of a heart attack, aged 51, after apparently drinking and smoking himself to death. His daughter Elizabeth had attempted suicide a few weeks earlier and succeeded later in life. This also fits in with my own personal theories about genius often being associated with Bipolar Disorder or ‘manic-depression’, an illness where the sufferers often desperately try to self-medicate with alcohol, nicotine and other drugs. Eric springs to mind. Here is Hugh Everett’s son Mark Everett singing ‘This Could be your Lucky Day in Hell’ http://www.youtube.com/watch?v=V1ED9ky-ojQ
    My article about researchers identifying a potential blue green algae cause & L-Serine treatment for Lou Gehrig's ALS, MND, Parkinsons & Alzheimers is at http://www.science20.com/forums/medicine
    Bonny Bonobo alias Brat
    According to Mark Everret in this interview, his father Hugh Everett the physicist sent a letter to Einstein when he was only 13 and they used to communicate for several years arguing about their different theories. I would love to see those letters. See http://www.youtube.com/watch?v=eWsKsZR7B6k&feature=fvw
    My article about researchers identifying a potential blue green algae cause & L-Serine treatment for Lou Gehrig's ALS, MND, Parkinsons & Alzheimers is at http://www.science20.com/forums/medicine
    Hfarmer
    What does the measuring?   Some thing that has to interact with the system thereby becoming entangled with the system.   That some thing is an "observer".  I like to keep that term because real life is relativistic quantum mechanics in which observers can become way way more important, suddenly what frame of reference you work in can matter.  (At least then it comes time to compute what will be seen at an accelerator laboratory.)
    Science advances as much by mistakes as by plans.
    News is silver, explanation is gold.

    And observation is mercury.

    I think that this was a direct outcome of the EPR experiment;
    Einstein wanted to attack quantum entanglement in order to defeat quantum theory completeness, and he did not attack the uncertainty principle, but since entanglement is a fact then there is a way to circumvent the uncertainty principle.

    http://e1saman.wordpress.com/2010/08/30/heisenbergs-uncertainty-principl...

    Its possible to predict for the one who can observe the universe without disturbing it..right

    Its possible to predict for the one who can observe the universe without disturbing it..right