Scientists provide a measure of uncertainty
What would Heisenberg’s position be?
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A group of Canadian PhD researchers claim to have obtained information beyond the “Heisenberg limit” using a technique called “weak measurement”.
Heisenberg’s Uncertainty Principle limits the amount of information that can be known at the quantum level: the more you know about the position of an object, the less you can know about its momentum. As this article at Phys.org puts it, “any attempt to measure a particle’s position must randomly change its speed”.
The University of Toronto researchers have looked at measurements of a single property – polarization – and how to obtain information about polarization without disturbing it.
The answer is to use a technique described as “weak measurement”, in which the quantum system is probed with a very small interaction, so as to obtain information about it with a minimum of change. The idea is to try and work around the problem Heisenberg originally framed, the “observer effect”.
Lee Rozema, a University of Toronto PhD student and lead author of the study, designed an apparatus to measure a photon’s polarization, making a weak measurement of the photon before it was sent to the apparatus. That pre-measurement, they found, induced less disturbance than predicted by Heisenberg’s precision-disturbance relation.
By repeating the experiment many times, the researchers say, they were able to “get a very good idea about how much the photon is disturbed”.
As the American Physical Society notes, while Heisenberg’s statement of the minimum uncertainty any quantum system must possess is “rigorously proven”, doubt has been cast on his calculation of the observer effect in recent years.
There is a practical application to all of this. Systems like quantum cryptography use the precision-disturbance relation to decide whether or not a communication has been “sniffed” (so to speak). Work such as Rozema’s suggests that the mathematics used to decide whether or not a quantum channel is secure might have to be revised. ®
COMMENTS
A Non and Francis are right
Physics simplified for us non-physicists often gets a bit confused in the process, and that's what seems to have happened here. Apparently, there are *two* distinct Heisenbergian relations that have been conflated (starting in paragraph two of the article):
#1 is the famous Uncertainty Principle, which states that there is a limit to the precision with which certain pairs of physical properties of a particle can be simultaneously known, and specifies that limit. This has been proven true and experimentally confirmed to be so. This relation is not based on measurements disturbing a particle, and has nothing to do with the Rozema paper.
#2 is called the "observer effect" and involves a lower limit to the degree of disturbance of a particle by a "measurement" of that particle. It is often confused with #1, especially by non-physicists.
The confusion is not surprising, as Heisenberg himself originally approached relation #1 by thinking about measurements affecting particles' properties. In fact, for a very specialized case, he found that (measured position precision)(momentum distubance) <= 1/2 Planck's constant (that is, he found that Relation #2 = Relation #1 for one special case).
Many assume that Heisenberg's relation #1 is generally correct for relation #2. Not only has this not been proven, it has been shown that relation #2 usually does NOT equal relation #1. In particular, the Ozawa (2003) paper proposed a more involved formula as the correct limit for relation #2. Drawing on the ideas of the Lund (2010) paper as to how to test Ozawa's formula, the current Rozema paper reports experimental results verifying that:
a) Heisenberg's formula for the #2 limit is wrong and
b) Ozawa's formula for the #2 limit seems to be correct and
c) Ozawa's relation #2 limit is (at least often) less than that from Heisenberg's formula (couldn't access article behind its paywall, so I'm inferring this from secondary sources)
Why we care: quantum cryptosystems that assume higher natural uncertainty (from Heisenberg's incorrect formula) than actually exists (Ozawa's correct limit for relation #2) may miss evidence of tampering that increases uncertainty above Ozawa's limit but keeps it below the level of Heisenberg's formula).
Useful sources used for the above:
Wikipedia's entry on "Uncertainty principle" (surprisingly unbad)
The Lund(2010) article which inspired the recent work (http://iopscience.iop.org/1367-2630/12/9/093011/)
One time Hieseberg was cruising down the highway in his Cadillac Eldorado when a patrolman pulls him over. The cop says "Dr. Heisenberg, do you have any idea how fast you were going?" He replies, "No, but I know exactly where I am!"
"What would Heisenberg’s position be?"
There or thereabouts.
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