We show that by illuminating an InGaAs/GaAs self-assembled quantum dot with circularly polarized light, the nuclei of atoms constituting the dot can be driven into a bistable regime, in which either a threshold-like enhancement or reduction of the local nuclear field by up to 3 Tesla can be generated by varying the intensity of light. The excitation power threshold for such a nuclear spin "switch" is found to depend on both external magnetic and electric fields. The switch is shown to arise from the strong feedback of the nuclear spin polarization on the dynamics of spin transfer from electrons to the nuclei of the dot.The hyperfine interaction in solids [1] arises from the coupling between the magnetic dipole moments of nuclear and electron spins. It produces two dynamical effects: (i) inelastic relaxation of electron spin via the "flip-flop" process ( Fig.1a) and (ii) the Overhauser shift of the electron energy [2]. Recently, the hyperfine interaction in semiconductor quantum dots (QDs) has attracted close attention [3,4,5,6,7,8,9,10,11,12,13,14] fuelled by proposals for QD implementation in quantum information applications [15]. The full quantization of the electron states in QDs is beneficial for removing decoherence mechanisms present in extended systems [16,17]. However, the electron localization results in a stronger (than in a bulk material) overlap of its wave-function with a large number of nuclei (N ∼ 10 4 in small selfassembled InGaAs/GaAs dots and up to 10 5 ÷ 10 6 in electrostatically-defined GaAs QDs), and the resulting hyperfine interaction with nuclear spins has been found to dominate the decoherence [3,4,5,12,13,14] and life-time [9] of the electron spin at low temperatures.In this Letter, we report the observation of a pronounced bistable behaviour of nuclear spin polarisation, S, in optically pumped self-assembled InGaAs/GaAs dots. In our experiments, spin-polarized electrons are introduced one-by-one into an individual InGaAs dot at a rate w x (see Fig.1b) by the circularly polarized optical excitation of electron-hole pairs 120 meV above the lowest QD energy states. Due to hole spin-flip during its energy relaxation, both bright and dark excitons can form in the dot ground state. The former will quickly recombine radiatively with a rate w rec ≈ 10 9 sec −1 , whereas the dark exciton can recombine with simultaneous spin transfer to a nucleus via a spin "flip-flop" process (as in Fig.1a) at the rate w rec N p hf [12,18]. Here N is the number of nuclei interacting with the electron and p hf is the probability of a "flip-flop" process, which from our perturbation theory treatment is given by:(1) Here γ is the exciton life-time broadening, h hf is the strength of the hyperfine interaction of the electron with a single nucleus and E eZ is the electron Zeeman splitting. E eZ is strongly dependent on the effective nuclear magnetic field B N generated by the nuclei. This provides a feedback mechanism between the spin transfer rate and the degree of nuclear polarization (B N ∝ S) in the dot [19]. Th...

An optical spin orientation is achieved for a Mn atom localized in a semiconductor quantum dot using quasiresonant excitation at zero magnetic field. Optically created spin-polarized carriers generate an energy splitting of the Mn spin and enable magnetic moment orientation controlled by the photon helicity and energy. The dynamics and the magnetic field dependence of the optical pumping mechanism show that the spin lifetime of an isolated Mn atom at zero magnetic field is controlled by a magnetic anisotropy induced by the built-in strain in the quantum dots.

We propose and demonstrate the sequential initialization, optical control, and readout of a single spin trapped in a semiconductor quantum dot. Hole spin preparation is achieved through ionization of a resonantly excited electron-hole pair. Optical control is observed as a coherent Rabi rotation between the hole and charged-exciton states, which is conditional on the initial hole spin state. The spin-selective creation of the charged exciton provides a photocurrent readout of the hole spin state. DOI: 10.1103/PhysRevLett.100.197401 PACS numbers: 78.67.Hc, 42.50.Hz, 71.35.Pq The ability to sequentially initialize, control, and readout a single spin is an essential requirement of any spin based quantum information protocol [1]. This has not yet been achieved for promising schemes based on the optical control of semiconductor quantum dots [2]. These schemes seek to combine the picosecond optical gate speeds of excitons [3][4][5][6], with the potential for millisecond coherence times of quantum dot spins [7][8][9], by optically manipulating the spin via the charged exciton. This results in a system where the potential number of operations before coherence loss could be extremely high, in the range 10 4-9 , and in a system compatible with advanced semiconductor device technologies. A number of important milestones have recently been reached, but these focus on the continuous initialization of an electron [10,11] or hole spin [12], detection of a single quantum dot spin [13,14], or optical control of ensembles of 10 6-7 spins [15,16]. In this Letter, we demonstrate sequential triggered ondemand preparation, optical manipulation, and picosecond time-resolved detection of a single hole spin confined to a quantum dot, thus demonstrating an experimental framework for the fast optical manipulation of single spins. This is achieved using a single self-assembled InGaAs quantum dot embedded in a photodiode structure. The hole spin is prepared by ionizing an electron-hole pair created by resonant excitation. A second laser pulse then drives a coherent Rabi oscillation between the hole and positive trion states, which due to Pauli blocking is conditional on the initial hole spin state, key requirements for the optical control of a spin via the trion transition. Because of Pauli blockade, creation of the charged exciton provides a photocurrent readout of the hole spin state.First we will describe the principle of operation. The qubit is represented by the spin states of the heavy hole (J Figure 1 shows an idealized quantum dot, embedded in an n-i-Schottky diode structure. An electric field is applied, such that the electron tunneling rate is much faster than the hole tunneling rate. The experiments use a sequence of two circularly polarized, timeseparated laser pulses, with a time duration shorter than the electron tunneling time, labeled the ''preparation'' and ''control'' pulses. Figure 1 illustrates the steps (a)-(d) involved in the preparation and readout of the hole spin.Preparation.-(a) The circularly polarized prepar...

We have investigated the spin preparation efficiency by optical pumping of individual Mn atoms embedded in CdTe/ZnTe quantum dots. Monitoring the time dependence of the intensity of the fluorescence during the resonant optical pumping process in individual quantum dots allows to directly probe the dynamics of the initialization of the Mn spin. This technique presents the convenience of including preparation and readout of the Mn spin in the same step. Our measurements demonstrate that Mn spin initialization, at zero magnetic field, can reach an efficiency of 75% and occurs in the tens of nanoseconds range when a laser resonantly drives at saturation one of the quantum-dot transition. We observe that the efficiency of optical pumping changes from dot-to-dot and is affected by a magnetic field of a few tens of millitesla applied in Voigt or Faraday configuration. This is attributed to the local strain distribution at the Mn location which predominantly determines the dynamics of the Mn spin under weak magnetic field. The spectral distribution of the spin-flip-scattered photons from quantum dots presenting a weak optical pumping efficiency reveals a significant spin relaxation for the exciton split in the exchange field of the Mn spin.