We demonstrate coherent optical control of a single hole spin confined to an InAs/GaAs quantum dot. A superposition of hole-spin states is created by fast (10-100 ps) dissociation of a spin-polarized electron-hole pair. Full control of the hole spin is achieved by combining coherent rotations about two axes: Larmor precession of the hole spin about an external Voigt geometry magnetic field, and rotation about the optical axis due to the geometric phase shift induced by a picosecond laser pulse resonant with the hole-trion transition.

Preparation of a specific quantum state is a required step for a variety of proposed practical uses of quantum dynamics. We report an experimental demonstration of optical quantum state preparation in a semiconductor quantum dot with electrical readout, which contrasts with earlier work based on Rabi flopping in that the method is robust with respect to variation in the optical coupling. We use adiabatic rapid passage, which is capable of inverting single dots to a specified upper level. We demonstrate that when the pulse power exceeds a threshold for inversion, the final state is independent of power. This provides a new tool for preparing quantum states in semiconductor dots and has a wide range of potential uses.Preparation of a specific quantum state in a semiconductor quantum system is a required step for quantum computation 1,2 , generation of single photons 3 and entangled photon pairs 4 , and studies of Bose-Einstein condensation 5 . A two-level quantum system, such as that of an exciton in a single quantum dot, can be driven into a specified state by use of a coherent interaction between the system and a tuned optical field. Previously, the interaction used to invert a two-level system in semiconductor quantum dots has driven the system with a resonant transform-limited light field. In this case, in the Bloch sphere representation the Bloch vector precesses about a field vector which lies in the equatorial plane, and so the optical pulse rotates the Bloch vector from its initial position at the south pole (ground state) through an angle θ = π to the north pole (inversion). The angle θ = (µ·E) h dt is defined as the pulse area in a Rabi rotation where µ is the dipole moment describing the twolevel system and E(t) is the envelope of the optical field. Coherent resonant interaction has been shown to be capable of generating several such Rabi cycles, and permits readout of the state of the system optically 6-8 , or electrically by ionisation of the optical excitation and extraction of a current 9 . The Rabi approach requires precise control over the integrated pulse area (determined by the temporal field profile and the dipole coupling strength) to achieve an inversion angle of π as shown schematically in Fig. 1a.Here we show experimentally that state preparation is also possible by adiabatic rapid passage (ARP), which has the advantage that it is largely unaffected by variation in the dipole coupling, which is a normal feature of dot systems, and likewise insensitive to variation in the optical field which typically arises from laser fluctuation or positional variation in arrays of dots 10 . Several theoretical proposals have recognized the potential of ARP excitation to create entanglement between locally separated electron spins for robust two-qubit quan-FIG. 1: Schematic representation of the dynamics of the twolevel quantum system in time in the (a) Rabi excitation regime with a transform-limited pulse and (b) the ARP regime with a chirped pulse. The curves are the eigenenergies versus time of the two ...

Solid state quantum emitters have shown strong potential for applications in quantum information, but spectral inhomogeneity of these emitters poses a significant challenge. We address this issue in a cavity-quantum dot system by demonstrating cavity-stimulated Raman spin flip emission. This process avoids populating the excited state of the emitter and generates a photon that is Raman shifted from the laser and enhanced by the cavity. The emission is spectrally narrow and tunable over a range of at least 125 GHz, which is two orders of magnitude greater than the natural linewidth. We obtain the regime in which the Raman emission is spin-dependent, which couples the photon to a long-lived electron spin qubit. This process can enable an efficient, tunable source of indistinguishable photons and deterministic entanglement of distant spin qubits in a photonic crystal quantum network.Controlled absorption and emission of single photons by quantum emitters are essential processes for quantum information technologies. Single photons can be used to transfer quantum information from one stationary qubit to another as part of a quantum network 1-4 , or they can be used as a qubit for photonic quantum computing 5 or secure communication 6 . Currently the largest challenge in achieving these goals is in scaling up the number of qubits. A promising approach is the integration of solid state quantum emitters into a photonic architecture [7][8][9] . Candidate materials include quantum dots 7,8 (QDs), QD molecules 10-13 , nitrogen-vacancy centers in diamond 9,14 , and other impurities or defects in solids 15,16 . These materials can take advantage of nanofabrication technologies to produce monolithic integrated structures that simplify the scaling-up problem 7,9 . Unfortunately, solid state quantum emitters suffer from spectral inhomogeneity, which greatly limits their usefulness for protocols that involve identical photons 5 or that involve the exchange of a photon between two qubits 1-4 .Here we demonstrate for the first time in a solid state system a cavity-stimulated Raman process [17][18][19] that can be used to overcome spectral inhomogeneity. We do this by coupling a negatively charged InAs/GaAs quantum dot (QD) that acts as a -type quantum emitter to a photonic crystal defect cavity 20 . Most previous work on QDs in cavities involves the coupling of 2 a 2-level exciton system to a cavity 7,[21][22][23] . In contrast, the three level -type system here provides a long-lived ground state electron spin coherence 24,25 , with ultrafast optical gates [25][26][27][28] and cavityenhanced initialization and readout 20 . The key feature of the Raman process (see Fig. 1a) is that the frequency of the emitted Raman photon is determined by the laser photon energy and the Zeeman energy, not by the excited state energy of the quantum emitter. The cavity strongly enhances this process when the Raman photon is resonant with the cavity mode. We measure cavity-stimulated Raman photons detuned from the QD over a range of at least 0.5 me...

We present a magneto-photoluminescence study on neutral and charged excitons confined to InAs/GaAs quantum dots. Our investigation relies on a confocal microscope that allows arbitrary tuning of the angle between the applied magnetic field and the sample growth axis. First, from experiments on neutral excitons and trions, we extract the in-plane and on-axis components of the Landé tensor for electrons and holes in the s-shell. Then, based on the doubly negatively charged exciton magneto-photoluminescence we show that the p-electron wave function spreads significantly into the GaAs barriers. We also demonstrate that the p-electron g-factor depends on the presence of a hole in the s-shell. The magnetic field dependence of triply negatively charged excitons photoluminescence exhibits several anticrossings, as a result of coupling between the quantum dot electronic states and the wetting layer. Finally, we discuss how the system evolves from a KondoAnderson exciton description to the artificial atom model when the orientation of the magnetic field goes from Faraday to Voigt geometry.

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