Towards Quantum Information Processing Using Spins
The basic idea of spin-optoelectronics is to explicitly utilize the spin in addition to the charge of electrons and holes in optoelectronic devices. Possible major future applications of such devices comprise the area of quantum information processing (QIP) but also devices like fast low-power spin transistors and spin-polarized single photon sources. Information processing relies on quantum mechanical systems (like two-level spin systems) which form the building blocks of QIP devices: quantum bits (qubits) and quantum registers. Basic gate operations are given by logical and coherent operations on individual qubits (single-qubit operations) and controlled coherent interactions between two qubits (two-qubit operations). The realization of a full scale quantum computer is in principle feasible but puts strict requirements on the quantum system as has been formulated by Loss and DiVincenco: scalability, ability to be initialized, sufficient coherence, realization of a universal set of gates, addressability of the qubits, interconversion of stationary and flying qubits and faithful exchange of flying qubits.
Qubits Based on InGaAs Quantum Dots
Quantum dots based on III–V compound semiconductors show features which make them most promising for the future development of QIP. Semiconductor technology is highly developed and (self-organized) quantum structures are readily available. The relevant compounds like GaAs and InAs are the most important ones for modern optoelectronic devices based on the fact that photons easily couple to their electronic states. This property is also favorable for an easy convertibility of flying (photons) and stationary (electronic spins) qubits. Various groups have reported long coherence times for electron spins in QDs, basic qubit operations have been performed and cavity QED (i.e., strong coupling between photons and electronic states) has been demonstrated.
Electrical Spin Injection into QDs
The aim of project A2 is to advance a spin-based optoelectronics utilizing spin states in InGaAs QDs. In particular we address the topics of initialization of spin states with high fidelity as well as their storage, addressability and manipulation. Our approach is based on an electrical injection of electron spins into the QDs which are situated in the active region of a light-emitting diode (spin-LED) or a transistor-type structure. Our collaboration has demonstrated single QD spin-LEDs with the highest spin injection efficiency worldwide proving that concurrent and repeatable initialization of spin-polarized electrons in several InAs/GaAs quantum dots with fidelity near unity is possible by electrical injection (see subproject A2.3: Spatially Resolved and Magneto-Optical Spectroscopy). The same design has recently also been shown to be useful as electrically operated light source for single photons with well-defined helicity. Furthermore, electrical spin injection into QDs has been shown to lead to an efficient, purely electrical driven dynamic nuclear spin polarization (via the hyperfine interaction) that might be interesting for QIP schemes based on nuclear spins. The experiments are supported by our comprehensive theoretical analysis of spin relaxation in QDs and semi-magnetic materials.
Growth and Characterization of Quantum-Dot Structures
Electron microscopy is used to characterize InAs/GaAs quantum-dot (QD) as well as group-III-nitride structures (see A2.5: Structural and Chemical Properties of Quantum Dot Structures). This subproject is very successful in developing new imaging and image-evaluation techniques. Based on the feedback by electron microscopy the QD growth by molecular-beam epitaxy is optimized in the ‘young scientist group’ of D. Schaadt (see A2.6: Optimized Quantum Dots for Spin Devices and Optical Resonator Structures). One goal is the well-defined lateral positioning of the QDs. A second goal is to implement group-III Nitride structures into spintronic devices (see A2.7: Growth of Nitride Spin Devices).
Towards Optical Coupling of Spin States
Our approach to realize 2-qubit operations is an optical coupling of QD states via resonant modes in cavity structures. Such cavities are currently investigated in A2.8: Optical Microcavities. In particular coupled pillar-type Bragg structures and pyramidal microcavities have been developed. The later have also a large potential for application as single-photon sources.