Carlton Caves Abstract

Quantum-limited metrology: Dynamics vs. entanglement

Carlton M. Caves

Department of Physics and Astronomy, University of New Mexico

Questions about quantum limits on measurement precision were once viewed from the perspective of how to reduce or avoid the effects of the quantum noise that is a consequence of the uncertainty principle. With the advent of quantum information science came a paradigm shift to proving rigorous bounds on measurement precision. These bounds have been interpreted as saying, first, that the best achievable sensitivity scales as 1/N, where N is the number of particles one has available for a measurement and, second, that the only way to achieve this Heisenberg-limited sensitivity is to use quantum entanglement. I will review these results and introduce a new perspective based on using nonlinear quantum dynamics to improve sensitivity. Using quadratic couplings of N particles to a parameter to be estimated, one can achieve sensitivities that scale as 1/N^2 if one uses entanglement, but even in the absence of any entanglement at any time during the measurement protocol, one can achieve a super-Heisenberg scaling of 1/N^{3/2}. Such sensitivity scalings might be achieved in Bose-Einstein condensates or in nanomechanical resonators.