Students are taught that quantum uncertainty is always in the eye of the beholder, but that principle might have been proven false by a new experiment that measured a quantum system which doesn’t necessarily introduce uncertainty. It overthrows a common classroom explanation of quantum mechanics, but the fundamental limit of what is knowable at the smallest scales remains unchanged.
Scientists published their findings in the journal Physical Review Letters. The Heisenberg Uncertainty Principle states that there is a fundamental limit to what is knowable about a quantum system. The more precisely the observer knows a particle’s position, the less he or she can now about its momentum, and vice versa. The limit is usually expressed as a simple equation.
Heisenberg explains this principle by exposing how a scientist trying to take a photograph of an electron, has to bounce a light particle off its surface. This reveals its position, but imparts energy as well causing it to move.
Aephraim Steinberg of the University of Toronto in Canada and his team of researchers have performed measurements of photons and showed that the act of measuring can introduce less uncertainty that is required by Heisenberg’s principle.
The group didn’t measure position and momentum, but its polarization states. The polarization state along one plane is intrinsically linked to the polarization along the other. By Heisenberg’s principle, there’s a limit to the certainty to which both states can be simultaneously known.
The researchers compared weak measurements with strong measurements multiple times. They found that one measurement of one polarization did not always disturb the other state as much as the uncertainty principle predicted. In the strongest case, the induced fuzziness was little as half of what is predicted by the principle.
There’s still no way that you can know both quantum states accurately at the same time, states Steinberg. However, the experiment shows that the act of measurement doesn’t always cause the uncertainty.
Reference: “Violation of Heisenberg’s Measurement-Disturbance Relationship by Weak Measurements” by Lee A. Rozema, Ardavan Darabi, Dylan H. Mahler, Alex Hayat, Yasaman Soudagar and Aephraim M. Steinberg, 6 September 2012, Physical Review Letters.