Summary form only given. Over the last two years much effort has been devoted to experimentally explore spin electronics in semiconductors. These experiments also support the hope of implementing quantum information devices based on spins using the mature technology of semiconductors. A basic requirement for quantum information processing with spins is the control of the mutual interaction between single spins. We extend here the investigation of spin coherence to the case of exchange coupled electron hole spins within a single exciton. The sample under investigation is an intrinsic 3 nm GaAs/(AlGa)As multiple quantum well (MQW) with a large Stokes shift of 9 meV.
Summary form only given. Over the last two years much effort has been devoted to experimentally explore spin electronics in semiconductors. These experiments also support the hope of implementing quantum information devices based on spins using the mature technology of semiconductors. A basic requirement for quantum information processing with spins is the control of the mutual interaction between single spins. We extend here the investigation of spin coherence to the case of exchange coupled electron hole spins within a single exciton. The sample under investigation is an intrinsic 3 nm GaAs/(AlGa)As multiple quantum well (MQW) with a large Stokes shift of 9 meV.
Summary form only given. The storage of the quantum bit, also called “qubit”, is a critical issue in the field of quantum information. As a promising candidate of being a quantum device, semiconductor quantum dots (QDs) are expected to play a central role in quantum information networks. Therefore, the study of quantum information storage in the QDs is really important. In this work, we employ the quantum master equation [1-2] to simulate the dynamics of the bright exciton, which is also used as a qubit, in coupled two QDs. In our model (shown in Fig. 1(a)), each QD is theoretically treated as a three-level subsystem and the exciton in this two-QD system is affected by the intrinsic electron-hole exchange interaction δ'ehxcand the dipole-dipole interactions for different exciton transfer δ'dd(change exciton spin after the transfer) and δdd(do not change exciton spin after the transfer). We assume that the incident photon is absorbed by the QD1, which creates an exciton. However, the initial exciton spin state will be disturbed by the intrinsic electron-hole exchange interaction, repeatedly changing the status from spin up to spin down during the exciton lifetime.
Systematic investigation of the spin dephasing times in GaAs/(Al,Ga)As quantum wells and its depending on parameters like temperature, magnetic fields and doping will be shown in Chapter 3.In Chapter 4 are presented results of investigation of the nuclear spin systems in quantum well heterostructures.The chapter 5 presents the results of studying the carrier spin dynamics in a n-doped (In,Ga)As/GaAs quantum wells by time-resolved Faraday rotation and ellipticity techniques in the temperature range down to 430 mil-liKelvin.In Chapter 6 is devoted to experimental and theoretically studying of optical control of the spin coherence of quantum well electrons by short laser pulses with circular or linear polarization.For that purpose the coherent electron spin dynamics in a n-doped CdTe/(Cd,Mg)Te quantum well structure was measured by time-resolved pump-probe Kerr rotation, using resonant excitation of the negatively charged exciton (trion) state.on the eigenstates |s z ; ↑ and |s z ; ↓ yields the states themselves independent of time.Along the field axis (longitudinal direction) these quantum-mechanical spin states are thus stationary.Any loss of coherence is induced by coupling of those states with the environment.Applying the time-evolution operator on the spin states perpendicular
Single spins in semiconductor quantum dots form a promising platform for solid-state quantum information processing. The spin-up and spin-down states of a single electron or hole, trapped inside a quantum dot, can represent a single qubit with a reasonably long decoherence time. The spin qubit can be optically coupled to excited (charged exciton) states that are also trapped in the quantum dot, which provides a mechanism to quickly initialize, manipulate and measure the spin state with optical pulses, and to interface between a stationary matter qubit and a 'flying' photonic qubit for quantum communication and distributed quantum information processing. The interaction of the spin qubit with light may be enhanced by placing the quantum dot inside a monolithic microcavity. An entire system, consisting of a two-dimensional array of quantum dots and a planar microcavity, may plausibly be constructed by modern semiconductor nano-fabrication technology and could offer a path toward chip-sized scalable quantum repeaters and quantum computers. This article reviews the recent experimental developments in optical control of single quantum dot spins for quantum information processing. We highlight demonstrations of a complete set of all-optical single-qubit operations on a single quantum dot spin: initialization, an arbitrary SU(2) gate, and measurement. We review the decoherence and dephasing mechanisms due to hyperfine interaction with the nuclear-spin bath, and show how the single-qubit operations can be combined to perform spin echo sequences that extend the qubit decoherence from a few nanoseconds to several microseconds, more than 5 orders of magnitude longer than the single-qubit gate time. Two-qubit coupling is discussed, both within a single chip by means of exchange coupling of nearby spins and optically induced geometric phases, as well as over longer-distances. Long-distance spin-spin entanglement can be generated if each spin can emit a photon that is entangled with the spin, and these photons are then interfered. We review recent work demonstrating entanglement between a stationary spin qubit and a flying photonic qubit. These experiments utilize the polarization- and frequency-dependent spontaneous emission from the lowest charged exciton state to single spin Zeeman sublevels.
Abstract Electron spins confined in semiconductor quantum dots (QDs) are one of potential candidates for physical implementation of scalable quantum information processing technologies. Tunnel coupling based inter exchange interaction between QDs is crucial in achieving single-qubit manipulation, two-qubit gate, quantum communication and quantum simulation. This review first provides a theoretical perspective that surveys a general framework, including the Helter−London approach, the Hund−Mulliken approach, and the Hubbard model, to describe the inter exchange interactions between semiconductor quantum dots. An electrical method to control the inter exchange interaction in a realistic device is proposed as well. Then the significant achievements of inter exchange interaction in manipulating single qubits, achieving two-qubit gates, performing quantum communication and quantum simulation are reviewed. The last part is a summary of this review.
This chapter discusses some experiments on coherent control of carrier densities, angular momenta, electron spin and exciton polarisations. The use of direct gap semiconductors as the active medium in all-optical switching devices is favoured by their large optical nonlinearities. The presence of spin polarised electrons in the sample can also be seen in the longer timescale dynamics after coherent carrier control. Selection of angular momentum is achieved by using shorter optical pulses which have a broad enough spectral content to excite both the heavy and light hole exciton states. Selected coherent pulse shaping techniques are introduced and used to demonstrate novel phenomena produced by coherent control in semiconductor heterostructures. New features in the interaction of light with semiconductor heterostructures are resolved, highlighting nonlinear mechanisms that can be of future use, as well as significantly modifying the picture of how light interacts with such samples.
Coherent optically driven semiconductor quantum dot systems are proposed for the implementation of quantum information processing devices. Fundamental quantum computation units, the quantum bits (qubits), are realized and optically manipulated in the exciton and spin bases. In this thesis, consecutive phase-sensitive rotations of a single qubit and their importance in the measurement of the full physical density matrix have been demonstrated experimentally in the exciton system in neutral GaAs/Al0:3Ga0:7As interface fluctuation quantum dots. The experimental work done on the exciton system serves as a foundation for consequent work on the long coherence lifetime spin-based qubits in charged quantum dots. In the spin-based system in the charged GaAs/Al0:3Ga0:7As interface fluctuation quantum dots, we demonstrate the selective excitation from the two spin ground states to the charged exciton (trion) state through phase sensitive control of the spin coherence in an ensemble of three-level spin-trion Lambda systems. This controlled excitation utilized the optically coupled and decoupled states of the electron spin in the presence of coherent transient excitation of the spin ground states in singly charged quantum dots. This progress lays the ground work for achieving complete ultrafast spin rotations.