Daniel Braun, Université Paul Sabatier, Toulouse
Abstract
One of the most promising spin-offs of quantum information theory for science is the idea of using quantum information processing in order to increase the sensitivity of precision measurements. The major goal of these "Quantum Enhanced Measurements" (QEM) is to achieve the Heisenberg limit (HL) - a scaling of the sensitivity as 1/N with the number N of quantum resources. This would represent a major improvement over the standard quantum limit (SQL) in which the sensitivity scales as the inverse square root of N. However, despite about 30 years of efforts, the SQL has been surpassed only by very few experiments so far, and only for small values of N. Indeed, the standard protocols of QEM require highly entangled states that are typically very prone to decoherence, and are therefore unlikely to scale up to the large numbers of N required before QEM can compete with classical precision measurements.
In this talk I show that, under certain conditions, decoherence itself can be used as a signal that allows one to achieve the HL by using only an initial product state. After exploring the effect at the example of the measurement of the length of a cavity using superradiance, I put these new type of "decoherence-enhanced measurements" into a broader context, and present a general theory that shows the feasibility of a much larger class of precision measurements that can achieve the HL with only a initial product state.
[1] Decoherence-enhanced measurements, D. Braun and J. Martin,
arXiv:0902.1213
[2] Collectively enhanced quantum measurements at the Heisenberg limit,
Daniel Braun, John Martin, arXiv:1005.4443v1