Trapped atomic ions have been successfully used for demonstrating basic elements of universal quantum information processing (QIP) [1]. Nevertheless, scaling up of these methods and techniques to achieve large scale universal QIP, or more specialized quantum simulations [2][3][4][5] remains challenging. The use of easily controllable and stable microwave sources instead of complex laser systems [6,7] on the other hand promises to remove obstacles to scalability. Important remaining drawbacks in this approach are the use of magnetic field sensitive states, which shorten coherence times considerably, and the requirement to create large stable magnetic field gradients. Here, we present theoretically a novel approach based on dressing magnetic field sensitive states with microwave fields which addresses both issues and permits fast quantum logic. We experimentally demonstrate basic building blocks of this scheme to show that these dressed states are long-lived and coherence times are increased by more than two orders of magnitude compared to bare magnetic field sensitive states. This changes decisively the prospect of microwave-driven ion trap QIP and offers a new route to extend coherence times for all systems that suffer from magnetic noise such as neutral atoms, NV-centres, quantum dots, or circuit-QED systems. arXiv:1105.1146v1 [quant-ph] 5 May 20112 Introduction -Using laser light for coherent manipulation of qubits gives rise to fundamental issues, notably, unavoidable spontaneous emission which destroys quantum coherence [8,9]. The difficulty in cooling a collection of ions to their motional ground state and the time needed for such a process in the presence of spurious heating of Coulomb crystals limits the fidelity of quantum logic operations in laser-based quantum gates, and thus hampers scalability. This limitation is only partially removed by the use of 'hot' gates [10,11]. Technical challenges in accurately controlling the frequency and intensity of laser light as well as delivering a large number of laser beams of high intensity to trapped ions are further obstacles for scalability.These issues associated with the use of laser light for scalable QIP have lead to the development of novel concepts for performing conditional quantum dynamics with trapped ions that rely on radio frequency (rf) or microwave (mw) radiation instead of laser light [6,7,[12][13][14][15]. Rf or mw radiation can be employed for quantum gates through the use of magnetic gradient induced coupling (MAGIC) between spin states of ions [16], thus averting technical and fundamental issues of scalability that were described above. Furthermore, the sensitivity to motional excitation of ions is reduced in such schemes. A drawback of MAGIC is the necessity to use magnetic field sensitive states for conditional quantum dynamics, thus making qubits susceptible to ambient field noise and shortening their coherence time. This issue is shared with some optical ion trap schemes for QIP that usually rely on magnetic field sensitive states for cond...

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/ √ 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 / √ 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.Introduction -High precision measurements often have played a pivotal role for new discoveries in physics. Today, detecting electromagnetic fields with extreme sensitivity and spatial resolution is particularly important in condensed matter physics and in biochemical sciences. State-of-theart magnetometers reach their best sensitivity in a limited frequency band or do not work at all (for all practical purposes) outside a certain frequency range. They often require a cryogenic and/or a carefully shielded environment. Also, their limited spatial resolution often makes them unsuitable for the applications mentioned above. Here, we introduce and demonstrate a novel method for sensing magnetic fields at the standard quantum limit, based on the use of a single atom as a sensor that is confined to a nanometer-sized region in space. The sensor can be tuned to a desired frequency where a signal shall be measured and is not affected by magnetic disturbances. Also, the magnetometer is essentially immune against amplitude fluctuations of the microwave fields that decouple the sensor from environmental disturbances.Before introducing this novel magnetometer scheme and describing the experimental procedure, we briefly outline state-of-the-art magnetometry by means of a few examples. Magnetic field sensitivities in the range of femto-or even subfemtotesla

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