arXiv subjects

Xuedong Hu

Publications and source records attributed to Xuedong Hu.

134 records · Page 8Linked to original sources

arXiv2001

Spin Electronics and Spin Computation

We review several proposed spintronic devices that can provide new functionality or improve available functions of electronic devices. In particular, we discuss a high mobility field effect spin transistor, an all-metal spin transistor, and our recent proposal of an all-semiconductor spin transistor and a spin battery. We also address some key issues in spin-polarized transport, which are relevant to the feasibility and operation of hybrid semiconductor devices. Finally, we discuss a more radical aspect of spintronic research--the spin-based quantum computation and quantum information processing.

cond-mat↗

arXiv2001

Effect of an inhomogeneous external magnetic field on a quantum dot quantum computer

We calculate the effect of an inhomogeneous magnetic field, which is invariably present in an experimental environment, on the exchange energy of a double quantum dot artificial molecule, projected to be used as a 2-qubit quantum gate in the proposed quantum dot quantum computer. We use two different theoretical methods to calculate the Hilbert space structure in the presence of the inhomogeneous field: the Heitler-London method which is carried out analytically and the molecular orbital method which is done computationally. Within these approximations we show that the exchange energy J changes slowly when the coupled dots are subject to a magnetic field with a wide range of inhomogeneity, suggesting swap operations can be performed in such an environment as long as quantum error correction is applied to account for the Zeeman term. We also point out the quantum interference nature of this slow variation in exchange.

cond-mat.mes-hall↗

arXiv2001

Theoretical issues in spin-based quantum dot quantum computation

We review our recent work addressing various theoretical issues in spin-based quantum dot quantum computation and quantum information processing. In particular, we summarize our calculation of electron exchange interaction in two-electron double quantum dots and multi-electron double dots, and discuss the physical implication of our results. We also discuss possible errors and how they can be corrected in spin-based quantum dot quantum computation. We critically assess the constraints and conditions required for building spin-based solid state quantum dot quantum computers.

cond-mat.mes-hall↗

arXiv2000

Interplay between Zeeman Coupling and Swap Action in Spin-based Quantum Computer Models: Error Correction in Inhomogeneous Magnetic Fields

We consider theoretically the interplay between Zeeman coupling and exchange-induced swap action in spin-based quantum dot quantum computer models in the presence of inhomogeneous magnetic fields, which are invariably present in real systems. We estimate quantitatively swap errors caused by the inhomogeneous field, establishing that error correction would, in principle, be possible in the presence of non-uniform magnetic fields in realistic structures.

cond-mat.mes-hall↗

arXiv2000

Issues, Concepts, and Challenges in Spintronics

We review from a theoretical perspective the emerging field of spintronics where active control of spin transport and dynamics in electronic materials may provide novel device application possibilities. In particular, we discuss the quantum mechanical principles underlying spintronics applications, emphasizing the formidable challenges involving spin decoherence and spin injection facing any eventual device fabrication. We provide a critical assessment of the current status of the field with special attention to possible device applications.

cond-mat↗

arXiv2000

Hilbert space structure of a solid state quantum computer: two-electron states of a double quantum dot artificial molecule

We study theoretically a double quantum dot hydrogen molecule in the GaAs conduction band as the basic elementary gate for a quantum computer with the electron spins in the dots serving as qubits. Such a two-dot system provides the necessary two-qubit entanglement required for quantum computation. We determine the excitation spectrum of two horizontally coupled quantum dots with two confined electrons, and study its dependence on an external magnetic field. In particular, we focus on the splitting of the lowest singlet and triplet states, the double occupation probability of the lowest states, and the relative energy scales of these states. We point out that at zero magnetic field it is difficult to have both a vanishing double occupation probability for a small error rate and a sizable exchange coupling for fast gating. On the other hand, finite magnetic fields may provide finite exchange coupling for quantum computer operations with small errors. We critically discuss the applicability of the envelope function approach in the current scheme and also the merits of various quantum chemical approaches in dealing with few-electron problems in quantum dots, such as the Hartree-Fock self-consistent field method, the molecular orbital method, the Heisenberg model, and the Hubbard model. We also discuss a number of relevant issues in quantum dot quantum computing in the context of our calculations, such as the required design tolerance, spin decoherence, adiabatic transitions, magnetic field control, and error correction.

quant-ph↗

arXiv2000

Spintronics: electron spin coherence, entanglement, and transport

Prospect of building spintronic devices in which electron spins store and transport information has attracted strong attention in recent years. Here we present some of our representative theoretical results on three fundamental aspects of spintronics: spin coherence, spin entanglement, and spin transport. In particular, we discuss our detailed quantitative theory for spin relaxation and coherence in electronic materials, resolving in the process a long-standing puzzle of why spin relaxation is extremely fast in Al (compared with other simple metals). In the study of spin entanglement, we consider two electrons in a coupled GaAs double-quantum-dot structure and explore the Hilbert space of the double dot. The specific goal is to critically assess the quantitative aspects of the proposed spin-based quantum dot quantum computer architecture. Furthermore, we discuss our theory of spin-polarized transport across a semiconductor-metal interface. In particular, we study Andreev reflection, which enables us to quantify the degree of carrier spin polarization and the strength of interfacial scattering.

cond-mat.mes-hall↗

arXiv2000

Theoretical Perspectives on Spintronics and Spin-Polarized Transport

Selected problems of fundamental importance for spintronics and spin-polarized transport are reviewed, some of them with a special emphasis on their applications in quantum computing and coherent control of quantum dynamics. The role of the solid-state environment in the decoherence of electron spins is discussed. In particular, the limiting effect of the spin-orbit interaction on spin relaxation of conduction electrons is carefully examined in the light of recent theoretical and experimental progress. Most of the proposed spintronic devices involve spin-polarized transport across interfaces in various hybrid structures. The specific example discussed here, of a magnetic semiconductor/superconductor interface, displays many intricacies which a complex spin-dependent interface introduces in the spin-polarized transport. It is proposed that pairs of entangled electrons in a superconductor (Cooper pairs) can be transfered to a non-superconducting region, and consequently separated for a transport study of the spin entanglement. Several important theoretical proposals for quantum computing are based on electronic and nuclear spin entanglement in a solid. Physical requirements for these proposals to be useful are discussed and some alternative views are presented. Finally, a recent discovery of optical control of nuclear spins in semiconductors is reviewed and placed in the context of a long-standing search for electronic control of nuclear dynamics.

cond-mat.mes-hall↗