8 April 2013

A University of Queensland (UQ) physicist has been able to clarify the limits imposed by the famous uncertainty principle of quantum theory.

Dr Cyril Branciard, a researcher in the ARC Centre of Excellence for Engineered Quantum Systems based in the School of Mathematics and Physics, has published his latest research in the Proceedings of the National Academy of Sciences.

Heisenberg's uncertainty principle tells us that it is in general impossible to measure, for instance, both the location and speed of a quantum object with perfect accuracy.

“Measuring one necessarily disturbs the other, all the more so as the accuracy of the measurement is increased,” Dr Branciard said.

Even 85 years after Heisenberg formulated his principle, how large this disturbance must be according to quantum theory has never been determined precisely, until now.

“Interestingly, the disturbance may not necessarily be as important as Heisenberg – and many physicists after him – thought,” Dr Branciard said.

“This has been realised some time ago, and demonstrated experimentally only last year. But it remained unclear what the ultimate limit imposed by quantum theory was.”

“I have been able to answer that question by deriving new ‘uncertainty relations’ for the joint measurement of incompatible quantities, like location and speed: these tell exactly what the minimal disturbance must be, for a given precision of the measurement.”

UQ Professor Andrew White, an expert in experiments testing the foundations of quantum theory, said it was “highly desirable” for physicists to clarify this aspect of the uncertainty principle.

“This is because experiments are now able to demonstrate quantum measurements with minimal possible disturbance,” Professor White said.

Dr Branciard's study opens the door to possible experiments to uncover the limits of quantum theory.

Such an experiment is under way in Professor White’s lab in the ARC Centres of Excellence for Engineered Quantum Systems (EQuS) and for Quantum Computation and Communication Technology (CQC2T).

“Clarifying the limits imposed by quantum theory, and determining what can and cannot be done quantum mechanically is essential for the development of quantum technologies,” Dr Branciard said.

Uncertainty relations similar to those Dr Branciard derived could be used, for instance, to prove the security of quantum cryptography, where quantum systems are used to transmit secret information.

The relations could tell how much information an eavesdropper could obtain on the communication, by monitoring the disturbance introduced.

Such studies on the limits of quantum theory also provide crucial insights on quantum foundations.

“I hope that further investigations will give hints on the profound reasons why the theory imposes such limits, and why nature follows such puzzling rules,” Dr Branciard said.

“They will certainly offer a new perspective to address these metaphysical questions, which have been challenging physicists and philosophers since the invention of quantum theory.”

For more information about Research at EQuS visit equs.org or contact Lynelle Ross (lynelle.ross@uq.edu.au or 07 3346 7931) or Cyril Branciard (c.branciard@physics.uq.edu.au or +61 7 3365 7415).

About Dr Branciard

Dr Branciard is a researcher in the Australia Research Council Centre of Excellence for Engineered Quantum Systems (EQuS).

EQuS aims to initiate the Quantum Era in the 21st century by engineering designer quantum systems.

Through focused and visionary research, EQuS will deliver new scientific insights and fundamentally new technical capabilities across a range of disciplines.

Impacts of this work will improve the lives of Australians and people all over the world by producing breakthroughs in physics, engineering, chemistry, biology and medicine.


C. Branciard, “Error-tradeoff and error-disturbance relations for incompatible quantum measurements”, the Proceedings of the National Academy of Sciences, April 2013 (http://www.pnas.org/content/early/2013/04/04/1219331110.).