Session J1: Toward Single Spin Electronics

Imaging and Manipulating Single and Interacting Spins on Surfaces: Towards Atomic-Scale Spin Devices

Spin-Polarized Scanning Tunneling Microscopy (SP-STM) provides new insight into spin structures at a length scale and a sensitivity level which are inaccessible by other magnetic-sensitive measurement techniques [1]. The combination of atomic resolution in direct space, single spin sensitivity, and high energy resolution nowadays offers unique possibilities for probing spin-dependent states and interactions in natural or artificially created nanostructures [2]. The ultimate goal has been the combination of spin-resolved imaging with atomic resolution and magnetometry at the single-atom level in order to probe spin states and magnetic interactions of individual adatoms and nanostructures at solid surfaces quantitatively and in a most direct way. This challenging goal has been achieved by operating a SP-STM system at temperatures below 1 Kelvin and in external magnetic fields up to several Tesla. The new method of single-atom magnetometry with an unprecedented degree of magnetization measurement sensitivity is applicable to metallic [3, 4] as well as to semiconducting [5] and molecular systems [6]. The combination of single-atom manipulation techniques and single-atom magnetometry has recently led to the first demonstration of atomic-scale spin logic devices based solely on spin- rather than charge-transport for realizing computation and information transmission at the atomic level. \\[4pt] [1] R. Wiesendanger, Rev. Mod. Phys. 81, 1495 (2009).\\[0pt] [2] D. Serrate, et al., Nature Nanotechnology 5, 350 (2010). \\[0pt] [3] F. Meier, et al., Science 320, 82 (2008). \\[0pt] [4] L. Zhou, et al., Nature Physics 6, 187 (2010). \\[0pt] [5] A. A. Khajetoorians, et al., Nature 467, 1084 (2010). \\[0pt] [6] J. Brede, et al., Phys. Rev. Lett. 105, 047204 (2010).   

All-electric control of single atom spin states

The quantum state of a single spin is a great candidate for forming a qubit. Spin systems in various forms are considered for the task, ranging from electrons trapped in artificial quantum dots to magnetic dopants in semiconductors and diamond. In this talk I will review recent progress towards controlling the spins of individual atoms on a surface through local access with an STM probe tip: an intriguing approach in view of the possibility to rearrange the atoms at will so as to build multi-atom structures. Magnetic d-metal atoms, separated from a metal substrate by a thin decoupling layer, are studied through inelastic electron tunneling spectroscopy (IETS): a tool by which transition energies of the spin state can be accurately followed. By addressing the atoms with a spin-filtered probe tip, controlled excitations or de-excitations can be made, effectively pumping the spin into a magnetization direction of choice. In a more recent experiment, spin pumping is performed in short pulses, opening up ways to control atomic spins in the time domain. I will discuss avenues to further develop this technique, eventually leading to coherent control of an atomic spin qubit.   

Quantum control and nanoscale placement of single spins in diamond

Diamond is a unique solid state platform for fundamental studies of spintronics and quantum information science that has recently enabled control, readout, and storage of quantum states at the single spin level. Nitrogen-vacancy (NV) center spins can be individually addressed and have remarkably long spin coherence times at room temperature. We show that the spin of single NV centers in both the orbital ground\footnote {G. D. Fuchs, V. V. Dobrovitski, D. M. Toyli, F. J. Heremans, and D. D. Awschalom, \emph{Science} \textbf{326}, 1520 (2009).} and excited state\footnote{G. D. Fuchs, V. V. Dobrovitski, D. M. Toyli, F. J. Heremans, C. D. Weis, T. Schenkel, and D.D. Awschalom, \emph{Nat. Phys.} \textbf{6}, 668 (2010).} can be controlled on sub-nanosecond time scales using intense microwave fields. Moreover, coherent light-matter interactions enable non-destructive spin measurement and localized single spin manipulation with near-resonant light.\footnote{B. B. Buckley, G. D. Fuchs, L. C. Bassett, and D. D. Awschalom,\emph {Science Express} (DOI: 10.1126/science.1196436)} An associated quantum memory is also demonstrated using the intrinsic nuclear spin of nitrogen.\footnote{G. D. Fuchs, G. Burkard, P. Klimov, and D. D. Awschalom, in preparation.} Scaling these findings toward a spin network is a key challenge - to this end we present a simple method for patterning NV center formation on 50 nm length scales.\footnote {D. M. Toyli, C. D. Weis, G. D. Fuchs, T. Schenkel, and D. D. Awschalom,\emph{NanoLett.} \textbf{10}, 3168 (2010).} These results represent progress toward control, coupling, and scaling of single spins for future spin and photon based quantum information processing.   

Exploring the quantum frontier of spin dynamics

Our familiar classical concept of a \textit{spin} is that of a system characterized by the \textit{direction} in which the spin is \textit{pointing.} In this picture, we may think of the dynamics of a spin as the motion of a \textit{classical gyroscope}, wich we can aptly describe the spin dynamics as the motion of a point on a sphere. This classical description of the spin dynamics, formalized in the Landau-Lifshits-Gilbert equation, has proved extremely successful in the field micro- and nanomagnetism. However, as the size of the system is further decreased (e.g., when considering molecular magnets such as the Fe$_{8}$ or Mn$_{12}$ systems, which have a spin $S$=10), \textit{quantum} effects such as tunneling, interference, entanglement, coherence, etc., play an essential role, and one must adopt a fully quantum mechanical description of the spin system. The landscape in which the system evolves is then no longer a mere sphere, but rather it is the projective Hilbert space (wich is the projective complex space $\le $P$^{2S}$ for a spin $S)$, as space of considerably greater richness and complexity than the sphere of classical spin dynamics. A very appealing tool to describe a quantum spin system is Majorana's stellar representation, which is the extension for a spin $S$ of the Bloch sphere description of a spin $\raise.5ex\hbox{$\scriptstyle 1$}\kern-.1em/ \kern-.15em\lower.25ex\hbox{$\scriptstyle 2$} $. I shall discuss how this representation can help us in improving our understanding of fundamental quantum processes and concept such as Landau-Zener transitions, Rabi oscillations, Berry phase, diabolical points and illustrate this on the example of spin dynamics of molecular magnets.