In classical physics, a measurement can in principle be carried out with unlimited precision without affecting the system being measured. In quantum physics, by contrast, every measurement that reveals information about a quantum system necessarily exerts a back-action on the system; this effect is also known as the collapse of the wavefunction. However, most measurements performed in the laboratory lead to a much larger back-action than is imposed by quantum theory.
On page 210 of this issue, Volz et al.1 describe how an optical cavity can allow a single atom to be measured with essentially only the back-action required by quantum theory. Their result not only deepens our understanding of the boundary between quantum and classical physics, but is also a step towards making atom-based quantum-information processing a reality2.