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Physicists Discover a Way Around Heisenberg's Uncertainty Principle
Hugh Pickens writes writes "Science Daily Headlines reports that researchers have applied a recently developed technique to directly measure the polarization states of light overcoming some important challenges of Heisenberg's famous Uncertainty Principle and demonstrating that it is possible to measure key related variables, known as 'conjugate' variables, of a quantum particle or state directly. Such direct measurements of the wave-function had long seemed impossible because of a key tenet of the uncertainty principle — the idea that certain properties of a quantum system could be known only poorly if certain other related properties were known with precision. 'The reason it wasn't thought possible to measure two conjugate variables directly was because measuring one would destroy the wave-function before the other one could be measured,' says co-author Jonathan Leach. The direct measurement technique employs a 'trick' to measure the first property in such a way that the system is not disturbed significantly and information about the second property can still be obtained. This careful measurement relies on the 'weak measurement' of the first property followed by a 'strong measurement' of the second property. First described 25 years ago, weak measurement requires that the coupling between the system and what is used to measure it be, as its name suggests, 'weak,' which means that the system is barely disturbed in the measurement process. The downside of this type of measurement is that a single measurement only provides a small amount of information, and to get an accurate readout, the process has to be repeated multiple times and the average taken. Researchers passed polarized light through two crystals of differing thicknesses: the first, a very thin crystal that 'weakly' measures the horizontal and vertical polarization state; the second, a much thicker crystal that 'strongly' measures the diagonal and anti-diagonal polarization state. As the first measurement was performed weakly, the system is not significantly disturbed, and therefore, information gained from the second measurement was still valid. This process is repeated several times to build up accurate statistics. Putting all of this together gives a full, direct characterization of the polarization states of the light."
So, is the damned cat dead or alive?
This is old news.
It doesn't violate the uncertainty principle.
Are you sure?
And no.
I thought the premise behind QKD was that you couldn't measure the polarization of one of a pair of entangled photons on two different bases at the same time, so once you perform the measurement in either basis, you're stuck with it and can't recreate that photon to forward it to the receiver (you'll only get the right basis half of the time). If this means you can get information about the photon on both the horizontal/vertical and diagonal bases, doesn't that mean you can MITM QKD?
certain properties of a quantum system could be known only poorly if certain other related properties were known with precision.
This careful measurement relies on the 'weak measurement' of the first property followed by a 'strong measurement' of the second property.
Weak measurements are not precise. They can become statistically significant with a large data set, but on an individual event basis, they give you effectively nothing. There's no violation of the Uncertainty Principle here.
Short answer: No.
Slightly more details: this technique could only "break" quantum encryption when the sender helpfully decides to send the same message over and over again --- effectively returning to the classical limit of large numbers of quanta, hence self-defeating the "quantumness" of the encryption. Used properly, the quantum encrypted signal (a series of photons sent with pre-set polarizations) is only sent once, so the large uncertainties in single "weak" measurements assure that anyone intercepting the message still gets a garbled, uninformative result (and the end receiver does too, so they know their security was compromised).
What they are doing is assuming that their light source is broadly uniform and averaging over the double-measurement (which is clever, no doubt). So we still haven't learned anything about a particular photon that violates the uncertainty principle, only something about the entire population. If we assume that the population is uniformly polarized (which is reasonable in this case) then we can conclude that the average reflects the properties of the individual photons. If the population was not uniform, however, then the average tells us very little about the properties of the individual photons.
And before someone too clever tries to argue that you can take a single input photon and make multiple copes and send them through this process to get results about that one photon, there is the No Clone Theorem to here to prevent that maneuver.
So really they haven't gone around Heisenberg (which talked only about individual wave-functions) but used multiple compound measurements and an assumption about the properties of the group to infer something that Heisinberg says they can't measure directly -- which is quite clever but Herr Doctor's principle still stands quite strong.
Like many non-rigorous descriptions, the summary makes the mistake of describing the uncertainty principle as if it is a measurement problem, where the lack of precision somehow arises from inadequate measurement technology. This is not a correct statement of the uncertainty principle. The fundamental issue is that the conjugate variable values are linked on a quantum level, such that there is a certain amount of natural, inherent uncertainty in their collective values due to the statistical/wavelike nature of the quantum particle. With perfect measurement, there is still uncertainty in the pair of values for any conjugate variables because the uncertainty lies in the actual values themselves. Position and momentum are the quintessential conjugate pair. The Heisenberg uncertainty principle is sometimes framed as the idea that you cannot know the speed and position of a particle at the same time. But it's more correct to say that a particle does not HAVE an exact speed and position at the same time. This weak measurement technique is certainly useful and interesting since it allows some observations of wavefunctions without collapse, but it does not actually allow the measurement of conjugate variables more precisely than the uncertainty principle allows - because the values themselves do not exist more precisely than that.
*This description is based one one of the multiple interpretations of quantum mechanics, and probably does not accurately represent physical reality, only our human understanding of a part of reality that we have not really figured out completely yet.
I am a geek attorney, but not your geek attorney unless you've already retained me. This is not legal advice.
all experiments are subject to error...
But the HUP is made for a case of a single strong measurement. This describes using multiple weak measurements which was proposed back in 1993. Good to see it is finally coming to light as a useful tool.
This:
Physicists Discover a Way Around Heisenberg's Uncertainty Principle
versus this:
The downside of this type of measurement is that a single measurement only provides a small amount of information, and to get an accurate readout, the process has to be repeated multiple times and the average taken.
(my emphasis)
/. editors at their best again </sarcasm>
Indeed, the quality of the senders/receivers equipment determines how much redundant data they have to "leak" beyond the theoretical limits --- and a sender/receiver using crude technology might be vulnerable to an attacker with far more sensitive equipment. Fortunately, once the sender/receiver's equipment gets "good enough," they can be mathematically certain that there isn't enough leaked data to sneakily reconstruct the message even if an attacker had theoretically "perfect" technology. While the "expected range of errors" with one current lab setup might have been broad enough to allow sneaky snooping, further technology development might squeeze this range down to exclude this possibility.
As in false: not true. It isn't just distorted or exaggerated. It's wrong.
Warning: this article may contain humor, sarcasm, parody, and perhaps even irony. Read at your own risk.
No, Heisenberg bounds the product of the errors in the measurements of the two by means of a Schwartz inequality: i.e. if you measure one very precisely, you will get a big error in your measurement of the other one.
Turns out it's a standard parlor trick. The cat has a twin sibling.
The rest is all mirrors ... and ball bearings.
------ The best brain training is now totally free : )
OK, sometimes.
Get thee glass eyes, and, like a scurvy politician, seem to see things thou dost not.--King Lear
You can only know the qualities sampled on a discrete digital grid to certain resolution due the limits of the grid. Take a Fourier transform of quanity sampled on that grid. You can only reliable compute frequencies with wavelengths two grid points wide. Else "aliasing" allows you fit an arbitrary number of smaller wavelengths to the same sample points.
In nature the Planck unit of action discretizes the universe into the smallest quantities you can resolve.
Dead. Starvation, because kept trying to measure it instead of feeding it.
---- Teach Peace. It's Cheaper Than War.
...and after that it's turtles all the way down.
There is no cat.