Probing Single-Electron Spin Decoherence in Quantum Dots using Charged Excitons

We propose to use optical detection of magnetic resonance (ODMR) to measure the decoherence time T 2 of a single-electron spin in a semiconductor quantum dot. The electron is in one of the spin 1/2 states and a circularly polarized laser can only create an optical excitation for one of the electron spin states due to Pauli blocking. An applied electron spin resonance (ESR) field leads to Rabi spin flips and thus to a modulation of the photoluminescence or, alternatively, of the photocurrent. This allows one to measure the ESR linewidth and the coherent Rabi oscillations, from which the electron spin decoherence can be determined. We study different possible schemes for such an ODMR setup, including cw or pulsed laser excitatio

Molecular spintronics: Coherent spin transfer in coupled quantum dots

Time-resolved Faraday rotation has recently demonstrated coherent transfer of electron spin between quantum dots coupled by conjugated molecules. Using a transfer Hamiltonian ansatz for the coupled quantum dots, we calculate the Faraday rotation signal as a function of the probe frequency in a pump-probe setup using neutral quantum dots. Additionally, we study the signal of one spin-polarized excess electron in the coupled dots. We show that, in both cases, the Faraday rotation angle is determined by the spin transfer probabilities and the Heisenberg spin exchange energy. By comparison of our results with experimental data, we find that the transfer matrix element for electrons in the conduction band is of order 0.08 eV and the spin transfer probabilities are of order 10%.Comment: 13 pages, 6 figures; minor change

High fidelity optical preparation and coherent Larmor precession of a single hole in an InGaAs quantum dot molecule

We employ ultrafast pump-probe spectroscopy with photocurrent readout to directly probe the dynamics of a single hole spin in a single, electrically tunable self-assembled quantum dot molecule formed by vertically stacking InGaAs quantum dots. Excitons with defined spin configurations are initialized in one of the two dots using circularly polarized picosecond pulses. The time-dependent spin configuration is probed by the spin selective optical absorption of the resulting few Fermion complex. Taking advantage of sub-5 ps electron tunneling to an orbitally excited state of the other dot, we initialize a single hole spin with a purity of >96%, i.e., much higher than demonstrated in previous single dot experiments. Measurements in a lateral magnetic field monitor the coherent Larmor precession of the single hole spin with no observable loss of spin coherence within the ~300 ps hole lifetime. Thereby, the purity of the hole spin initialization remains unchanged for all investigated magnetic fields

Probing Single-Electron Spin Decoherence in Quantum Dots using Charged Excitons

We propose to use optical detection of magnetic resonance (ODMR) to measure the decoherence time T_{2} of a single electron spin in a semiconductor quantum dot. The electron is in one of the spin 1/2 states and a circularly polarized laser can only create an optical excitation for one of the electron spin states due to Pauli blocking. An applied electron spin resonance (ESR) field leads to Rabi spin flips and thus to a modulation of the photoluminescence or, alternatively, of the photocurrent. This allows one to measure the ESR linewidth and the coherent Rabi oscillations, from which the electron spin decoherence can be determined. We study different possible schemes for such an ODMR setup, including cw or pulsed laser excitation.Comment: 8 pages, 7 figures. Proceedings of the PASPS3 conference, Santa Barbara, CA (USA). To appear in the Journal of Superconductivit

Enhanced sequential carrier capture into individual quantum dots and quantum posts controlled by surface acoustic waves

Individual self-assembled Quantum Dots and Quantum Posts are studied under the influence of a surface acoustic wave. In optical experiments we observe an acoustically induced switching of the occupancy of the nanostructures along with an overall increase of the emission intensity. For Quantum Posts, switching occurs continuously from predominantely charged excitons (dissimilar number of electrons and holes) to neutral excitons (same number of electrons and holes) and is independent on whether the surface acoustic wave amplitude is increased or decreased. For quantum dots, switching is non-monotonic and shows a pronounced hysteresis on the amplitude sweep direction. Moreover, emission of positively charged and neutral excitons is observed at high surface acoustic wave amplitudes. These findings are explained by carrier trapping and localization in the thin and disordered two-dimensional wetting layer on top of which Quantum Dots nucleate. This limitation can be overcome for Quantum Posts where acoustically induced charge transport is highly efficient in a wide lateral Matrix-Quantum Well.Comment: 11 pages, 5 figure

Vanishing electron g factor and long-lived nuclear spin polarization in weakly strained nanohole-filled GaAs/AlGaAs quantum dots

GaAs/AlGaAs quantum dots grown by in situ droplet etching and nanohole in-filling offer a combination of strong charge confinement, optical efficiency, and high spatial symmetry advantageous for polarization entanglement and spin-photon interface. Here, we study experimentally electron and nuclear spin properties of such dots. We find nearly vanishing electron g factors (ge<0.05), providing a potential route for electrically driven spin control schemes. Optical manipulation of the nuclear spin environment is demonstrated with nuclear spin polarization up to 65% achieved. Nuclear magnetic resonance spectroscopy reveals two distinct types of quantum dots: with tensile and with compressive strain along the growth axis. In both types of dots, the magnitude of strain εb<0.02% is nearly three orders of magnitude smaller than in self-assembled dots: On the one hand, this provides a route for eliminating a major source of electron spin decoherence arising from nuclear quadrupolar interactions, and on the other hand such strain is sufficient to suppress nuclear spin diffusion leading to a stable nuclear spin bath with nuclear spin lifetimes exceeding 500 s. The spin properties revealed in this work make this new type of quantum dot an attractive alternative to self-assembled dots for the applications in quantum information technologies

Recipes for spin-based quantum computing

Technological growth in the electronics industry has historically been measured by the number of transistors that can be crammed onto a single microchip. Unfortunately, all good things must come to an end; spectacular growth in the number of transistors on a chip requires spectacular reduction of the transistor size. For electrons in semiconductors, the laws of quantum mechanics take over at the nanometre scale, and the conventional wisdom for progress (transistor cramming) must be abandoned. This realization has stimulated extensive research on ways to exploit the spin (in addition to the orbital) degree of freedom of the electron, giving birth to the field of spintronics. Perhaps the most ambitious goal of spintronics is to realize complete control over the quantum mechanical nature of the relevant spins. This prospect has motivated a race to design and build a spintronic device capable of complete control over its quantum mechanical state, and ultimately, performing computations: a quantum computer. In this tutorial we summarize past and very recent developments which point the way to spin-based quantum computing in the solid-state. After introducing a set of basic requirements for any quantum computer proposal, we offer a brief summary of some of the many theoretical proposals for solid-state quantum computers. We then focus on the Loss-DiVincenzo proposal for quantum computing with the spins of electrons confined to quantum dots. There are many obstacles to building such a quantum device. We address these, and survey recent theoretical, and then experimental progress in the field. To conclude the tutorial, we list some as-yet unrealized experiments, which would be crucial for the development of a quantum-dot quantum computer.Comment: 45 pages, 12 figures (low-res in preprint, high-res in journal) tutorial review for Nanotechnology; v2: references added and updated, final version to appear in journa

Quantum computation and the production of entangled photons using coupled quantum dots

We review recent theoretical progress on the use of electron spins as qubits in coupled semiconductor quantum dots for quantum information processing. We discuss the spin exchange mechanism and its microscopic origin in both laterally and vertically tunnel-coupled quantum dots and explain how it can be used to implement the quantum XOR gate which, in combination with single spin rotations, allows to perform arbitrary quantum computations. In addition to their functionality as a quantum gate, coupled quantum dots can act as a source for photon pairs in entangled polarization states which are useful for quantum communication. We describe a mechanism for the production of such entangled photon pairs via a biexciton state in tunnel-coupled quantum dots

Dynamics of coupled qubits interacting with an off-resonant cavity

We study a model for a pair of qubits that interact with a single off-resonant cavity mode and, in addition, exhibit a direct interqubit coupling. Possible realizations for such a system include coupled superconducting qubits in a line resonator as well as exciton states or electron spin states of quantum dots in a cavity. The emergent dynamical phenomena are strongly dependent on the relative energy scales of the interqubit coupling strength, the coupling strength between qubits and cavity mode, and the cavity mode detuning. We show that the cavity mode dispersion enables a measurement of the state of the coupled-qubit system in the perturbative regime. We discuss the effect of the direct interqubit interaction on a cavity-mediated two-qubit gate. Further, we show that for asymmetric coupling of the two qubits to the cavity, the direct interqubit coupling can be controlled optically via the ac Stark effect