We study optically driven Rabi rotations of a quantum dot exciton transition between 5 and 50 K, and for pulse areas of up to 14π. In a high driving field regime, the decay of the Rabi rotations is nonmonotonic, and the period decreases with pulse area and increases with temperature. By comparing the experiments to a weak-coupling model of the exciton-phonon interaction, we demonstrate that the observed renormalization of the Rabi frequency is induced by fluctuations in the bath of longitudinal acoustic phonons, an effect that is a phonon analogy of the Lamb shift.

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 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.

The four-level exciton/biexciton system of a single semiconductor quantum dot acts as a two-qubit register. We experimentally demonstrate an exciton-biexciton Rabi rotation conditional on the initial exciton spin in a single InGaAs/GaAs dot. This forms the basis of an optically gated two-qubit controlled rotation ͑CROT͒ quantum-logic operation where an arbitrary exciton spin is selected as the target qubit using the polarization of the control laser.

We demonstrate fast initialization of a single hole spin captured in an InGaAs quantum dot with a fidelity F>99% by applying a magnetic field parallel to the growth direction. We show that the fidelity of the hole spin, prepared by ionization of a photogenerated electron-hole pair, is limited by the precession of the exciton spin due to the anisotropic exchange interaction.A single spin trapped in a semiconductor quantum dot is a potential qubit with long coherence times 1 and the possibility of picosecond optical control 2 . Recently, there has been considerable interest in the use of hole spins as qubits due to the p-type Bloch functions of the valence band resulting in a suppressed contact hyperfine interaction 3,4 which is the main source of dephasing for electron spins. The initialization of a qubit is a key ingredient of any quantum information processing protocol. Successful approaches of single spin initialization in quantum dots include optical pumping 5,6,7 , coherent population trapping 1,8 and the ionization of an electron-hole pair 9,10,11 . Although fidelities F>99.8% have been reported by optical pumping 9 , there have been no reports of such fidelities with preparation times comparable to the picosecond gate times used in coherent control experiments.Previously, we demonstrated a scheme for the fast initialization of a single hole spin (with F=81%), by the ionization of a spin-polarized electron-hole pair 12 . In this letter, we study the dependence of the fidelity on applied magnetic and electric fields. We show that by applying a magnetic field in the growth direction (Faraday geometry), we can achieve near unit fidelity of hole spin preparation, by suppressing the spin mixing generated by the neutral exciton fine structure splitting. We also find that an increased electric-field at B=0 also improves the fidelity by reducing the time available for this spin mixing.The sample was mounted in a helium bath magneto-cryostat (T=4.2K, B≤5T) and consists of a single layer of InGaAs self assembled quantum dots embedded in the intrinsic region of an n-i-Schottky diode. Details of the layer structure of the wafer can be found in ref. 13. Importantly, in the reverse bias regime, the electron tunnelling rate  e~3 0 ps -1 (V bias =0.8V) is much greater than the rates of hole tunnelling  h~1 ns -1 , radiative recombination  r ~1 ns -1 and the fine structure splitting  fs~2 /225 ps -1 . The slow hole tunnelling rate is due to a hole blocking tunnelling barrier. Therefore if we resonantly excite the neutral exciton transition, the electron quickly tunnels out of the quantum dot, to leave a spin polarized hole.Before discussing the experimental results, we introduce the principle of operation for the preparation of the single hole spin. Figure 1