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Optical control of the spin of individual magnetic atoms L. Besombes1, C. Le Gall1, A. Brunetti1, C.L. Cao1, 2, H. Boukari1, J. Fernandez-Rossier2,3 1 Institut Néel, CNRS, 25 av. des Martyrs, 38042 Grenoble, France. 2 Departamento de Fisica, Universidad de Alicante, San Vicente del Raspeig, 03690 Spain. 3 International Iberian Nanotechnology Laboratory, Av. Jose Veiga, 4715-330 Braga, Portugal. E-mail: [email protected] The decrease of the structure size in semiconductor electronics and magnetic information storage devices has dramatically reduced the number of atoms necessary to process and store one bit of information: An individual magnetic atom would represent the ultimate size limit for storing and processing information. With semiconductor quantum dots (QDs) doped with Mn atoms, the probing of a single atomic spin in a solid state environment become possible using optical techniques. The state of a photon emitted or absorbed by a II-VI semiconductor QD containing a Mn atom is directly related to the spin state of the magnetic atom (S=5/2): the exciton acts as an effective magnetic field along the QD growth axis that lifts up the degeneracy between the six (2S+1) Mn spins states. Depending on the Mn spin orientation, the recombination of an injected exciton emits a photon with a given energy and polarization. We already demonstrated that a Mn atom embedded in a QD may act as an optically addressable spin based memory [1]. The next steps would be to perform a fast coherent control of an individual atom and to tune the coupling between such ultimate memories: a carrier mediated interaction between two Mn spins inserted in the same QD is one step towards these challenging goals. For a QD containing two Mn atoms, the optical spectra are in general quite complex (see figure), nevertheless each collective state of the two Mn spins can be optically addressed. Left: AFM image of a CdTe layer deposited on a ZnTe substrate and schematic view of a QD containing an individual Mn atom and an exciton. Right: Photoluminescence (PL) spectra of a CdTe QD containing 1 (X, 1Mn) or 2 (X, 2Mn) Mn atoms. The PL intensity maps present the emission of a QD containing 2 Mn atoms versus the power (bottom panel) of a single mode laser on resonance with the high energy state |Sz1=+5/2;Sz2=+5/2;J=+1> (Szi and J are the angular momentum of the Mn atom i and exciton respectively) and the emission of the same QD when a single mode laser is scanned around this high energy state E1 (top panel). One observes a power dependent and detuning dependent splitting of the low energy state |Sz1=+5/2;Sz2=+5/2;J=-1> showing that the Mn spins state |Sz1=+5/2;Sz2=+5/2> is dressed by the resonant laser field. We have shown that the resonant optical injection of spin polarized carriers can be used to initialize these localized spins [2]. We then demonstrated that under a strong resonant optical field, the energy of any spin state of one or two Mn atoms can be independently tuned using the optical Stark effect [3]. The strong coupling with a resonant laser field mixes the states associated with a given Mn spin orientation in the presence (X, Mn) or absence (Mn alone) of the exciton. At the resonance, hybrid matter-field states are created. As the laser detuning increases, the optically active transitions asymptotically approach the original excitonic transitions, where the remaining energy offset is the optical Stark shift (PL intensity map in the figure). In the ground state of the QD (one or two Mn without exciton), the laser induced shift modifies the Mn fine and hyperfine structure. This optically controlled energy shifts affects the spin dynamics of the magnetic atoms. It will be used in future experiments for a coherent manipulation of the coupled electronic and nuclear spins of individual Mn atoms. References: [1] Phys. Rev. Lett. 102, 127402 (2009). [2] Phys. Rev. B 81, 245315 (2010). [3] Phys. Rev. Lett. 107, 057401 (2011).