A doubt on the uncertainty principle

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U of T scientists cast doubt on the uncertainty principle

Werner Heisenberg was too pessimistic, researchers say

Sean Bettam | Posted Friday, September 7, 2012

Researchers at the University of Toronto have demonstrated that theoretical physicist Werner Heisenberg was too pessimistic in 1927 when formulating his famous uncertainty principle.

The Heisenberg uncertainty principle is one of the cornerstones of quantum mechanics.  In its most familiar form, it says that it is impossible to measure anything without disturbing it. For instance, any attempt to measure a particle’s position must randomly change its speed.

The principle has bedeviled quantum physicists for nearly a century, until recently, when researchers at U of T demonstrated the ability to directly measure the disturbance.

“We designed an apparatus to measure a property – the polarization – of a single photon. We then needed to measure how much that apparatus disturbed that photon,” says Lee Rozema, a PhD candidate in Professor Aephraim Steinberg’s quantum optics research group at U of T, and lead author of a study published this week in Physical Review of Letters.

“To do this, we would need to measure the photon before the apparatus but that measurement would also disturb the photon,” Rozema says.

In order to overcome this hurdle, Rozema and his colleagues employed a technique known as weak measurement wherein the action of a measuring device is weak enough to have an imperceptible impact on what is being measured. Before each photon was sent to the measurement apparatus, the researchers measured it weakly and then measured it again afterwards, comparing the results. They found that the disturbance induced by the measurement is less than Heisenberg’s precision-disturbance relation would require.

“Each shot only gave us a tiny bit of information about the disturbance, but by repeating the experiment many times we were able to get a very good idea about how much the photon was disturbed,” says Rozema.

The findings build on recent challenges to Heisenberg’s principle by scientists the world over. Nagoya University physicist Masanao Ozawa suggested in 2003 that Heisenberg’s uncertainty principle does not apply to measurement, but could only suggest an indirect way to confirm his predictions. A validation of the sort he proposed was carried out last year by Yuji Hasegawa’s group at the Vienna University of Technology. In 2010, Griffith University scientists Austin Lund and Howard Wiseman showed that weak measurements could be used to characterize the process of measuring a quantum system. However, there were still hurdles to clear as their idea effectively required a small quantum computer, which is difficult to build.

“In the past, we have worked experimentally both on implementing weak measurements, and using a technique called ‘cluster state quantum computing’ to simplify building quantum computers.  The combination of these two ideas led to the realization that there was a way to implement Lund and Wiseman’s ideas in the lab,” says Rozema.

It is often assumed that Heisenberg’s uncertainty principle applies to both the intrinsic uncertainty that a quantum system must possess, as well as to measurements. These results show that this is not the case and demonstrate the degree of precision that can be achieved with weak-measurement techniques.

“The results force us to adjust our view of exactly what limits quantum mechanics places on measurement,” says Rozema. “These limits are important to fundamental quantum mechanics and also central in developing ‘quantum cryptography’ technology, which relies on the uncertainty principle to guarantee that any eavesdropper would be detected due to the disturbance caused by her measurements.”

“The quantum world is still full of uncertainty, but at least our attempts to look at it don’t have to add as much uncertainty as we used to think!”

The findings are reported in the paper “Violation of Heisenberg’s Measurement-Disturbance Relationship by Weak Measurements”. The research is supported by funding from Natural Sciences and Engineering Research Council of Canada and the Canadian Institute for Advanced Research.

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RESEARCH ARTICLE (requires subscription)

Phys. Rev. Lett. 109, 100404 (2012) [5 pages]

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

APS » Journals » Phys. Rev. Lett. » Volume 109 » Issue 10

Lee A. Rozema, Ardavan Darabi, Dylan H. Mahler, Alex Hayat, Yasaman Soudagar, and Aephraim M. Steinberg
Centre for Quantum Information & Quantum Control and Institute for Optical Sciences, Department of Physics, 60 St. George Street, University of Toronto, Toronto, Ontario, Canada M5S 1A7

 Received 4 July 2012; published 6 September 2012

See accompanying Physics Synopsis

Synopsis: The Certainty of Uncertainty

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

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

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U of T scientists cast doubt on the uncertainty principle | U of T News.

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Sean Bettam (2012).
U of T scientists cast doubt on the uncertainty principle
University of Toronto News

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