Measurement beyond the standard quantum limit

Physics needs measurements which are more and more precise, but any measurement in physics is marred by noise: imperfect preparation of the system to be measured, thermal disturbance, etc. At the most extreme scales this occurs at the level of particles such as photons, and this noise is thus quantum by nature. This is what is known as the Heisenberg limit, a limit which cannot be overcome and which represents the fact that no measurement can be of an absolute and infinite precision. But there are easier ways than others to get closer to this limit. Daniel Braun, of the University of Toulouse, and John Martin, of the University of Liège, have just conceived of a theoretical system which will doubtless aid experimenters and favour applications in numerous scientific disciplines. Their work has just been the subject of a publication in Nature Communications (1).

The research by Daniel Braun, Professor at the theoretical physics laboratory at the University of Toulouse III, and John Martin, lecturer and head of the quantum optics group at the ULg, enters the context of precision measurements whose sensitiveness can be improved in using quantum specificities, the properties of microscopic objects such as particles or light, properties which have no equivalence in classical physics.

The article they have just published presents a general formalism which they have applied to the particular case of measuring the length of a cavity. By a cavity is here meant two mirrors between which light can be trapped. The idea is to be able to measure variations in the length of the cavity over the course of time with the greatest precision possible. The length can in effect vary due to different factors: external perturbations, temperature variations, etc.

measure cavity
(1) Braun, D. and Martin, J. Heisenberg-limited sensitivity with decoherence-enhanced measurements. Nat. Commun. 2:223 doi: 10.1038/ncomms1220 (2011).

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