Extracting order from a quantum measurement finally shown experimentally

QUANTUM TECHNOLOGY:

In physics, it is essential to be able to show a theoretical assumption in actual, physical experiments. For more than a hundred years, physicists have been aware of the link between the concepts of disorder in a system, and information obtained by measurement. However, a clean experimental assessment of this link in common monitored systems, that is systems which are continuously measured over time, was missing so far.

But now, using a “quantum drum”, a vibrating, mechanical membrane, researchers have realized an experimental setup that shows the physical interplay between the disorder and the outcomes of a measurement. A collaboration of experimentalists from the Niels Bohr Institute, University of Copenhagen and theorists at Queen’s University Belfast, and the University of Sao Palo, could show how to extract order from this largely disordered system, providing a general tool to engineer the state of the system, essential for future quantum technologies, like quantum computers. The result is now published in as an Editors’ Suggestion in Physical Review Letters.

Measurements will always introduce a level of disturbance of any system it measures. In the ordinary, physical world, this is usually not relevant, because it is perfectly possible for us to measure, say, the length of a table without noticing that disturbance. But on the quantum scale, the consequences of the disturbance made by measurements are huge. These large disturbances increase the entropy, or disorder, of the underlying system,  and apparently preclude to extract any order from the measurement. But before explaining how the recent experiment realized this, the concepts of entropy and thermodynamics need a few words…