Quantum metrology with collective atomic spins

Quantum metrology uses quantum features such as entanglement and squeezing to improve the sensitivity of quantum-limited measurements. Long established as a valuable technique in optical measurements such as gravitational-wave detection, quantum metrology is increasingly being applied to atomic instruments such as matter-wave interferometers, atomic clocks, and atomic magnetometers. Several of these new applications involve dual optical/atomic quantum systems, presenting both new challenges and new opportunities. I will describe an optical magnetometry system based on cold rubidium-87 in an optical trap, which achieves both shot-noise- and projection-noise-limited performance, allowing study of optical magnetometry in a fully-quantum regime [1,2]. The versatility of this system allows us to design both linear and non-linear atom- light couplings, and to apply dynamical decoupling and quantum non-demolition measurement, for application in quantum-enhanced magnetometry [3]. As an example, we have recently developed a method for generating metrologically-advantageous optical nonlinearities and performed the first interaction-based quantum-noise-limited measurements of atomic magnetisation [4]. With this technique we implement a non-linear metrology scheme proposed by Boixo et al. with the surprising feature of precision scaling better than the 1/N ``Heisenberg limit'' [5]. \\[4pt] [1] M. Koschorreck, M. Napolitano, B. Dubost, and M. W. Mitchell. Sub-projection-noise sensitivity in broadband atomic magnetometry. Phys. Rev. Lett., 104(9):093602, 2010. \\[0pt] [2] M. Koschorreck, M. Napolitano, B. Dubost, and M. W. Mitchell. Quantum nondemolition measurement of large-spin ensembles by dynamical decoupling. Phys. Rev. Lett., 105 (9):093602, 2010. \\[0pt] [3] S R de Echaniz, M Koschorreck, M Napolitano, M Kubasik, and MWMitchell. Hamiltonian design in atom-light interactions with rubidium ensembles: A quantum-information toolbox. Phys. Rev. A, 77(3):032316, 2008. \\[0pt] [4] M Napolitano and M W Mitchell. Nonlinear metrology with a quantum interface. New J. Phys., 12(9):093016, 2010. \\[0pt] [5] Sergio Boixo, Animesh Datta, Matthew J Davis, Steven T Flammia, Anil Shaji, and Carlton M Caves. Quantum metrology: dynamics versus entanglement. Phys. Rev. Lett., 101(4):040403, 2008. \\[0pt] [6] M. Napolitano, M. Koschorreck, B. Dubost, N. Behbood, R. J. Sewell, and M. W. Mitchell. Interaction-based quantum metrology showing scaling beyond the Heisenberg limit. Nature (in press) arXiv:1012.5787v1, 2011.