Original URL: http://www.theregister.co.uk/2012/09/10/measure_on_uncertainty/

Scientists provide a measure of uncertainty

What would Heisenberg’s position be?

By Richard Chirgwin

Posted in Science, 10th September 2012 23:16 GMT

A group of Canadian PhD researchers claim to have obtained information beyond the “Heisenberg limit” using a technique called “weak measurement”.

Heisenberg’s Uncertainty Principle limits the amount of information that can be known at the quantum level: the more you know about the position of an object, the less you can know about its momentum. As this article at Phys.org puts it, “any attempt to measure a particle’s position must randomly change its speed”.

The University of Toronto researchers have looked at measurements of a single property – polarization – and how to obtain information about polarization without disturbing it.

The answer is to use a technique described as “weak measurement”, in which the quantum system is probed with a very small interaction, so as to obtain information about it with a minimum of change. The idea is to try and work around the problem Heisenberg originally framed, the “observer effect”.

Lee Rozema, a University of Toronto PhD student and lead author of the study, designed an apparatus to measure a photon’s polarization, making a weak measurement of the photon before it was sent to the apparatus. That pre-measurement, they found, induced less disturbance than predicted by Heisenberg’s precision-disturbance relation.

By repeating the experiment many times, the researchers say, they were able to “get a very good idea about how much the photon is disturbed”.

As the American Physical Society notes, while Heisenberg’s statement of the minimum uncertainty any quantum system must possess is “rigorously proven”, doubt has been cast on his calculation of the observer effect in recent years.

There is a practical application to all of this. Systems like quantum cryptography use the precision-disturbance relation to decide whether or not a communication has been “sniffed” (so to speak). Work such as Rozema’s suggests that the mathematics used to decide whether or not a quantum channel is secure might have to be revised. ®