The third part of the review of hybrid semiconductor nanostructures concerns the problems of their use as elements for a full-scale solid-state quantum computer. We consider the main versions of implementation of quantum computers with the use of the exciton and spin degrees of freedom of a quantum dot and the phonons that propagate in the quantum network with quantum dots. The theoretical schemes and experimental data illustrating operation of the available prototypes of such devices are reported.
The third part of the review of hybrid semiconductor nanostructures concerns the problems of their use as elements for a full-scale solid-state quantum computer. We consider the main versions of implementation of quantum computers with the use of the exciton and spin degrees of freedom of a quantum dot and the phonons that propagate in the quantum network with quantum dots. The theoretical schemes and experimental data illustrating operation of the available prototypes of such devices are reported.
A review of semiconductor-based schemes for the realization of quantum information processing devices is presented. After recalling the fundamentals of quantum information/computation theory, we shall discuss potential implementation schemes based on charge and/or spin degrees of freedom in semiconductor nanostructures. More specifically, we shall present an all-optical implementation scheme of quantum information processing with semiconductor macroatoms/molecules, where the computational degrees of freedom are interband optical transitions (excitonic states) manipulated/controlled by ultrafast sequences of multicolor laser pulses. We shall primarily focus on implementation schemes dealing with charge excitations in GaAs as well as GaN quasi-zero-dimensional structures. We shall finally discuss the possibility to combine charge and spin degrees of freedom, thus, allowing for fast-quantum gates, which do not translate into fast-decoherence times.
A review of semiconductor-based schemes for the realization of quantum information processing devices is presented. After recalling the fundamentals of quantum information/computation theory, we shall discuss potential implementation schemes based on charge and/or spin degrees of freedom in semiconductor nanostructures. More specifically, we shall present an all-optical implementation scheme of quantum information processing with semiconductor macroatoms/molecules, where the computational degrees of freedom are interband optical transitions (excitonic states) manipulated/controlled by ultrafast sequences of multicolor laser pulses. We shall primarily focus on implementation schemes dealing with charge excitations in GaAs as well as GaN quasi-zero-dimensional structures. We shall finally discuss the possibility to combine charge and spin degrees of freedom, thus allowing for fast quantum gates which do not translate into fast decoherence times.
Any physical implementation of quantum information processing is known to be extremely demanding. One should be able to perform long coherent quantum manipulations (gating), precise quantum-state synthesis, and detection as well. Ever since the very beginning it has been recognized that the major obstacle arises from the unavoidable open character of any realistic quantum system. The coupling with external (i.e. non-computational) degrees of freedom spoils the unitary structure of quantum evolution, which is the crucial ingredient in quantum computation (QC). The only existing experimental realizations of quantum processors are based on quantum optics and atomic/molecular systems. It is however generally believed that future applications of quantum information may hardly be realized in terms of such systems, which do not permit the large-scale integration of existing microelectronics technology. In contrast, in spite of the serious difficulties related to the fast decoherence times, a solid-state implementation of QC seems to be the only way to benefit synergistically from the recent progress in ultrafast optoelectronics as well as in nanostructure fabrication and characterization. Following this spirit, we shall propose and discuss potential implementations of quantum computation/information based on semiconductor nanostructures. More specifically, as originally envisioned Zanardi and Rossi, 1 we shall present an all-optical implementation of QC based on semiconductor macroatoms. 2 In this context we shall discuss a number of excitonic encoding schemes based on charge and/or spin degrees of freedom. In particular, we shall review a novel implementation scheme 3 which merges ideas from both the fields of spintronics and optoelectronics and sets the stage for an all optical semiconducting spin based quantum information processing.
The computer industry has seen an immense development in the last decades. \nPersonal computers have become available for everybody living in industrialized \ncountries with rapidly increasing performance in terms of speed and storage capacities. \nHowever, the performance of nowadays' computers is fundamentally limited by the \nlaws of classical physics: a classical bit can only take on either of the two distinct \nvalues `0' or `1'. \nIn contrast, a quantum computer could, in principle, make direct use \nof quantum phenomena, such as state superpositions \n-- a quantum bit can be in both states `0' and `1' simultaneously --, \nto perform complex computational tasks much faster than any classical computer. \n \nThe idea of building computers that work according to the laws of quantum physics \nhas opened various fields of research, one of which is the search for the best \nphysical system to use as a quantum bit (qubit). One important criterion for determining \nthe optimal qubit system is the lifetime of state superpositions. \nTypically, once initialized, such superpositions are destroyed on remarkably short \ntimescales due to interactions with the environment -- a process which is referred to \nas decoherence --, posing the question which physical qubit candidate system might show a high-enough robustness against the \ninfluence of the `outside world' to allow for viable quantum computation. \n \nIn this thesis, we will consider three particular realizations of one specific and \nvery promising type of qubit candidate system: \nan electron (or hole) confined to a quantum dot -- a nanoscale structure \nwithin a (typically semiconducting) material --, where the spin states `down' \nand `up' of our electron (or hole) encode the logical states `0' and `1'. \nOur task will be to study the decay of spin-state superpositions in such quantum-dot \nsystems. The main objective of this thesis is to understand the most important physical \nprocesses that lead to spin decoherence and to show ways to suppress this undesirable effect. \nIt turns out that at low temperatures, the main source of decoherence \nis the coupling of the electron (hole) to the surrounding nuclear spins. \n \nThis thesis is divided into three logical parts, corresponding to the three qubit candidate \nsystems under consideration. First we will study electron-spin qubits in III-V semiconductor \nquantum dots, where the electron spin interacts with the nuclear spins of the semiconducting \nhost material \nvia the isotropic Fermi contact hyperfine interaction. Second we consider quantum-dot-confined \nheavy holes and the decoherence of their (pseudo-)spin states due to anisotropic interactions with the \nnuclear spins. Third and last, we study electron-spin qubits made from carbon-nanotube and \ngraphene quantum dots. Quantum dots made of carbon have the advantage of a low abundance \nof spin-carrying nuclear isotopes, therefore reducing decoherence effects significantly. \n \nFor each of the systems under consideration, we will carry out analytical calculations on \nthe nuclear-spin interactions and the spin dynamics of the qubit. Although one main goal of \nthis thesis is to show ways to extend spin decoherence times, we will also focus on physically more \nfundamental questions. Not only the timescale of the decay is relevant for \nthe system's applicability as a qubit, but also the form of the decay which can vary \nsignificantly from system to system. For example, the decay of spin-state superpositions \ncan follow an exponential, super-exponential or power-law decay, and can even pass through \nvarious stages. This is not only of academic interest, but also important for practical \npurposes, such as the implementation of quantum error-correction schemes in a potential \nquantum computer. \n
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.