Spins In Semiconductor Quantum Dots Research Articles

Control and measurement of electron spins in semiconductor quantum dots

AbstractWe present an overview of experimental steps taken towards using the spin of a single electron trapped in a semiconductor quantum dot as a spin qubit [Loss and DiVincenzo, Phys. Rev. A 57, 120 (1998)]. Fabrication and characterization of a double quantum dot containing two coupled spins has been achieved, as well as initialization and single‐shot read‐out of the spin state. The relaxation time T 1 of single‐spin and two‐spin states was found to be on the order of a millisecond, dominated by spin–orbit interactions. The time‐averaged dephasing time T 2*, due to fluctuations in the ensemble of nuclear spins in the host semiconductor, was determined to be on the order of several tens of nanosceconds. Coherent manipulation of single‐spin states can be performed using a microfabricated wire located close to the quantum dot, while two‐spin interactions rely on controlling the tunnel barrier connecting the respective quantum dots [Petta et al., Science 309, 2180 (2005)]. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Few-body spin couplings and their implications for universal quantum computation

Electron spins in semiconductor quantum dots are promising candidates for theexperimental realization of solid-state qubits. We analyse the dynamics of a system of threequbits arranged in a linear geometry and a system of four qubits arranged in a squaregeometry. Calculations are performed for several quantum dot confining potentials. In thethree-qubit case, three-body effects are identified that have an important quantitativeinfluence upon quantum computation. In the four-qubit case, the full Hamiltonian is foundto include both three-body and four-body interactions that significantly influence thedynamics in physically relevant parameter regimes. We consider the implications of theseresults for the encoded universality paradigm applied to the four-electron qubit code; inparticular, we consider what is required to circumvent the four-body effects in an encodedsystem (four spins per encoded qubit) by the appropriate tuning of experimentalparameters.

Coherent Manipulation of Coupled Electron Spins in Semiconductor Quantum Dots

We demonstrated coherent control of a quantum two-level system based on two-electron spin states in a double quantum dot, allowing state preparation, coherent manipulation, and projective readout. These techniques are based on rapid electrical control of the exchange interaction. Separating and later recombining a singlet spin state provided a measurement of the spin dephasing time, T2*, of approximately 10 nanoseconds, limited by hyperfine interactions with the gallium arsenide host nuclei. Rabi oscillations of two-electron spin states were demonstrated, and spin-echo pulse sequences were used to suppress hyperfine-induced dephasing. Using these quantum control techniques, a coherence time for two-electron spin states exceeding 1 microsecond was observed.

Indirect coupling between spins in semiconductor quantum dots

The optically induced indirect exchange interaction between spins in two quantum dots is investigated theoretically. We present a microscopic formulation of the interaction between the localized spin and the itinerant carriers including the effects of correlation, using a set of canonical transformations. Correlation effects are found to be of comparable magnitude as the direct exchange. We give quantitative results for realistic quantum dot geometries and find the largest couplings for one dimensional systems.

Optically programmable electron spin memory using semiconductor quantum dots

The spin of a single electron subject to a static magnetic field provides a natural two-level system that is suitable for use as a quantum bit, the fundamental logical unit in a quantum computer. Semiconductor quantum dots fabricated by strain driven self-assembly are particularly attractive for the realization of spin quantum bits, as they can be controllably positioned, electronically coupled and embedded into active devices. It has been predicted that the atomic-like electronic structure of such quantum dots suppresses coupling of the spin to the solid-state quantum dot environment, thus protecting the 'spin' quantum information against decoherence. Here we demonstrate a single electron spin memory device in which the electron spin can be programmed by frequency selective optical excitation. We use the device to prepare single electron spins in semiconductor quantum dots with a well defined orientation, and directly measure the intrinsic spin flip time and its dependence on magnetic field. A very long spin lifetime is obtained, with a lower limit of about 20 milliseconds at a magnetic field of 4 tesla and at 1 kelvin.

Electronic coupling of vertically coupled quantum dots: effect of the time-dependent magnetic field

The fast development of the new classes nanostructures such as self-organized quantum dots leads to the possibility of controlling their shape, their dimensions, the structure of energy levels and the number of confined electrons (from one to 1000). A fascinating idea of great promise is the possibility of utilizing electronic spins in semiconductor quantum dots as quantum bits (qubits) for a new generation of computers. In this work, we consider two vertically Ga 0.68In 0.32As coupled-quantum dots embedded in GaAs. We study the dynamical response of two electrons in these two vertically tunnel-coupled quantum dots with a time dependent magnetic field and including the Coulomb interaction between the electrons. We find that, for a certain frequency ranges, a sinusoidal perturbation acts like an attractive effective interaction between electrons. Including the Zeeman splitting on the spins of the electrons we discuss the dynamical evolution of the magnetization.

Coherent dynamics and manipulation of electron spins in nanostructures

With present-day technology, spins in semiconductor quantum dots can be created, coherently manipulated, and measured. The ability of controlling spins in semiconductor nanostructures has new and exciting applications in electronics and information processing, including quantum computing and quantum communication. We give an overview over some of the theoretical work motivated by the potential use of electron spins in quantum dots as qubits for quantum information processing. Quantum gate mechanisms in laterally and vertically tunnel-coupled quantum dots and methods for single-spin manipulation and measurements are presented. Entanglement acts as a fundamental resource for many known quantum information processing schemes. We discuss recently proposed schemes to detect the entanglement of electrons in normal and superconducting wires via transport and noise measurements.