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 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.
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
The preparation of a coherent heavy-hole spin via ionization of a spin-polarized electron-hole pair in an InAs/GaAs quantum dot in a Voigt geometry magnetic field is experimentally investigated. For a dot with a typical bright-exciton fine-structure splitting of 17 μeV, the fidelity of the spin preparation is limited to 0.75, with optimum preparation occurring when the effective fine structure of the bright exciton matches the in-plane hole Zeeman energy. In principle, higher fidelities can be achieved by minimizing the bright-exciton fine-structure splitting.
Recently, longitudinal acoustic phonons have been identified as the main source of the intensity damping observed in Rabi rotation measurements of the ground-state exciton of a single InAs/GaAs quantum dot. Here we report experiments of intensity damped Rabi rotations in the case of detuned laser pulses. The results have implications for the coherent optical control of both excitons and spins using detuned laser pulses.
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