title: Quantum noise limited and entanglement-assisted magnetometry.
abstract: We study experimentally the fundamental limits of sensitivity of an atomic radio-frequency magnetometer. First, we apply an optimal sequence of state preparation, evolution, and the backaction evading measurement to achieve a nearly projection noise limited sensitivity. We furthermore experimentally demonstrate that Einstein-Podolsky-Rosen entanglement of atoms generated by a measurement enhances the sensitivity to pulsed magnetic fields. We demonstrate this quantum limited sensing in a magnetometer utilizing a truly macroscopic ensemble of 1.5x10(12) atoms which allows us to achieve subfemtotesla/square root(Hz) sensitivity.
authors.affiliation: Niels Bohr Institute, Danish Quantum Optics Center QUANTOP, Copenhagen University, Blegdamsvej 17, 2100 Copenhagen, Denmark.
authors.forename: W
authors.initials: W
authors.lastname: Wasilewski
authors.forename: K
authors.initials: K
authors.lastname: Jensen
authors.forename: H
authors.initials: H
authors.lastname: Krauter
authors.forename: J J
authors.initials: JJ
authors.lastname: Renema
authors.forename: M V
authors.initials: MV
authors.lastname: Balabas
authors.forename: E S
authors.initials: ES
authors.lastname: Polzik