title: Ultrasensitive Magnetometer using a Single Atom.
abstract: Precision sensing, and in particular high precision magnetometry, is a central goal of research into quantum technologies. For magnetometers, often trade-offs exist between sensitivity, spatial resolution, and frequency range. The precision, and thus the sensitivity of magnetometry, scales as 1/sqrt[T_{2}] with the phase coherence time T_{2} of the sensing system playing the role of a key determinant. Adapting a dynamical decoupling scheme that allows for extending T_{2} by orders of magnitude and merging it with a magnetic sensing protocol, we achieve a measurement sensitivity even for high frequency fields close to the standard quantum limit. Using a single atomic ion as a sensor, we experimentally attain a sensitivity of 4.6  pT/sqrt[Hz] for an alternating-current magnetic field near 14 MHz. Based on the principle demonstrated here, this unprecedented sensitivity combined with spatial resolution in the nanometer range and tunability from direct current to the gigahertz range could be used for magnetic imaging in as of yet inaccessible parameter regimes.
authors.affiliation: Department Physik, Naturwissenschaftlich-Technische Fakultät, Universität Siegen, 57068 Siegen, Germany.
authors.forename: I
authors.initials: I
authors.lastname: Baumgart
authors.affiliation: School of Physics, Huazhong University of Science and Technology, Wuhan 430074, China.
authors.forename: J-M
authors.initials: JM
authors.lastname: Cai
authors.affiliation: Racah Institute of Physics, The Hebrew University of Jerusalem, Jerusalem 91904, Givat Ram, Israel.
authors.forename: A
authors.initials: A
authors.lastname: Retzker
authors.affiliation: Institut für Theoretische Physik, Universität Ulm, 89069 Ulm, Germany.
authors.forename: M B
authors.initials: MB
authors.lastname: Plenio
authors.affiliation: Department Physik, Naturwissenschaftlich-Technische Fakultät, Universität Siegen, 57068 Siegen, Germany.
authors.forename: Ch
authors.initials: Ch
authors.lastname: Wunderlich