Physicists Discover a Way Around Heisenberg's Uncertainty Principle 153
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."
Nothing to see here (Score:5, Insightful)
This is old news.
It doesn't violate the uncertainty principle.
Re:So... Quantum cryptography is doomed ? (Score:5, Insightful)
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).
Re:Nothing to see here (Score:4, Insightful)
Yup, they just measured a little of one and a lot of the other. Still falls under the h.u.p.
It's the fault of the stupid haedline (Score:5, Insightful)
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>
Re:This is not a way *around* Heisenberg (Score:5, Insightful)
Except that the no hidden variables results suggest that the photon really doesn't have both those properties at the same time. You can measure the average, but that's all it is - it doesn't tell you anything you shouldn't know about the state of a single photon, even if they are all quantum mechanically "identical." So Heisenberg gets to be right in the strong sense, as well as the weak.
Re:Schrodinger would be happy (Score:5, Insightful)
That's more a matter of the way the brain selectively ignores and forgets things which would lead to inconsistency.
Or perhaps it's simply due to the fact that our brains evolved only to cope with severely limited range of environments. We can't imagine complicated local geodetics because we didn't evolve near a black hole. We can't imagine the weird effects of special relativity because we haven't evolved at relativistic speeds. We can't grok the fractal-like nature of subatomic world and physics because we aren't molecule-sized in order to notice it. Perhaps those "inconsistencies" are no more inconsistent than, say, the hydrostatic "paradox" is paradoxical. (In fact, the very existence of the word "paradox" seems to suggest that we just get all too often confused by perfectly normal things that are simply outside the realm of our daily experience.)
Re:Schrodinger would be happy (Score:5, Insightful)
Re:This is not a way *around* Heisenberg (Score:5, Insightful)
Re:Schrodinger would be happy (Score:5, Insightful)