http://physics.aps.org/synopsis-for/10.1103/PhysRevLett.109.100404

Synopsis: The Certainty of Uncertainty

L. A. Rozema et al. Phys. Rev. Lett. (2012)
Violation of Heisenberg’s Measurement-Disturbance Relationship by Weak Measurements

Lee A. Rozema, Ardavan Darabi, Dylan H. Mahler, Alex Hayat, Yasaman Soudagar, and Aephraim M. Steinberg
Phys. Rev. Lett. 109, 100404 (2012)
Published September 6, 2012

When first taking quantum mechanics courses, students learn about Heisenberg’s uncertainty principle, which is often presented as a statement about the intrinsic uncertainty that a quantum system must possess. Yet Heisenberg originally formulated his principle in terms of the “observer effect”: a relationship between the precision of a measurement and the disturbance it creates, as when a photon measures an electron’s position. Although the former version is rigorously proven, the latter is less general and—as recently shown—mathematically incorrect. In a paper in Physical Review Letters, Lee Rozema and colleagues at the University of Toronto, Canada, experimentally demonstrate that a measurement can in fact violate Heisenberg’s original precision-disturbance relationship.

If the observer affects the observed, how can one even make such a measurement of the disturbance of a measurement? Rozema et al. use a procedure called “weak” quantum measurement: if one can probe a quantum system by means of a vanishingly small interaction, information about the initial state can be squeezed out with little or no disturbance. The authors use this approach to characterize the precision and disturbance of a measurement of the polarizations of entangled photons. By comparing the initial and final states, they find that the disturbance induced by the measurement is less than Heisenberg’s precision-disturbance relation would require.

While the measurements by Rozema et al. leave untouched Heisenberg‘s principle regarding inherent quantum uncertainty, they expose the pitfalls of its application to measurements’ precision. These results not only provide a demonstration of the degree of precision achievable in weak-measurement techniques, but they also help explore the very foundations of quantum mechanics. – David Voss

http://arxiv.org/abs/1208.0034

Violation of Heisenberg's Measurement-Disturbance Relationship by Weak Measurements

Lee A. Rozema, Ardavan Darabi, Dylan H. Mahler, Alex Hayat, Yasaman Soudagar, Aephraim M. Steinberg
(Submitted on 31 Jul 2012 (v1), last revised 15 Aug 2012 (this version, v2))

While there is a rigorously proven relationship about uncertainties intrinsic to any quantum system, often referred to as "Heisenberg's Uncertainty Principle," Heisenberg originally formulated his ideas in terms of a relationship between the precision of a measurement and the disturbance it must create. Although this latter relationship is not rigorously proven, it is commonly believed (and taught) as an aspect of the broader uncertainty principle. Here, we experimentally observe a violation of Heisenberg's "measurement-disturbance relationship", using weak measurements to characterize a quantum system before and after it interacts with a measurement apparatus. Our experiment implements a 2010 proposal of Lund and Wiseman to confirm a revised measurement-disturbance relationship derived by Ozawa in 2003. Its results have broad implications for the foundations of quantum mechanics and for practical issues in quantum mechanics.