QUANTUM MECHANICS: Weak measurements on photons push past Heisenberg’s precision limit

University of Toronto scientists have gathered the most direct experimental evidence that Werner Heisenberg’s original formulation of his uncertainty principle is wrong. The group used many weak measurements on entangled photons, an approach that surpasses the precision of the more conventional, single strong measurement.

(It should be noted that this does not disprove the rigorously proven generalized equations of the Heisenberg Uncertainty Principle; what it does disprove is the commonly used, looser interpretation of the Heisenberg Uncertainty Principle in which it is interpreted as a “measurement-disturbance relationship” describing the relationship between the precision of a measurement and the disturbance it ultimately creates.)

The results were published online in the journal Physical Review Letters in September and the researchers presented their findings for the first time at the Optical Society’s (OSA) Annual Meeting, Frontiers in Optics (presentation FW4J.4), held Oct. 14–18, 2012, in Rochester, NY.

The Toronto team set up an apparatus to measure the polarization of a pair of entangled photons. The different polarization states of a photon, like the location and momentum of an electron, are complementary physical properties and so subject to the generalized Heisenberg uncertainty relationship. The main goal was to quantify how much the act of measuring the polarization disturbed the photons, accomplished by observing the light particles both before and after measurement. However, if the “before shot” disturbed the system, the “after shot” would be tainted.

The researchers found a way around this by using techniques from quantum-measurement theory to sneak no disruptive peeks of the photons before their polarization was measured. “If you interact very weakly with your quantum particle, you won’t disturb it very much,” explains Lee Rosemary, a PhD candidate in quantum optics research at the University of Toronto, and lead author of the study. Weak interactions, however, yield very little information about the particle. “If you take just a single measurement, there will be a lot of noise in that measurement,” says Rosemary. “But if you repeat the measurement many, many times, you can build up statistics and can look at the average.”

Comparing thousands of “before” and “after” views of the photons determined that the precise measurements disturbed the system much less than predicted by the original formula. This provides the first direct experimental evidence that a new measurement-disturbance relationship, mathematically computed in 2003 by physicist Masanao Ozawa at Japan’s Nagoya University, is more accurate.