Spin polarization in parallel double dots with spin-orbit interaction

Spin polarization in parallel double quantum dots embedded in arms of Aharonov-Bohm interferometer is investigated. The spin-orbit interaction exists in quantum dots. We find that the spin polarization is quite large even with a weak spin-orbit interaction. The direction and the strength of the spin polarization are well controllable and manipulatable by simply varying the strength of spin-orbit interaction or the energy levels in quantum dots. Moreover, electron-electron interaction strengthens the spin accumulation when the energy levels of the two quantum dots are identical. As the energy levels are unequal, electron-electron interaction cannot increase the spin accumulation. It is worth mentioning that the device is free of a magnetic field or a ferromagnetic material and it can be easily realized with present technology.     

Adiabatic passage schemes in coupled semiconductor nanostructures

Adiabatic passage schemes in coupled semiconductor quantum dots are discussed. For optical control, a doped double-dot molecule is proposed as a qubit realization. The quantum information is encoded in the carrier spin, and the flexibility of the molecular structure allows to map the spin degrees of freedom onto the orbital ones and vice versa, which opens the possibility for high-finesse quantum gates by means of stimulated Raman adiabatic passage. For tunnel-coupled dots, adiabatic passage of two correlated electrons in three coupled quantum dots is shown to provide a robust and controlled way of distilling, transporting and detecting spin entanglement, as well as of measuring the rate of spin disentanglement. Employing tunable interdot coupling the scheme creates, from an unentangled two-electron state, a superposition of spatially separated singlet and triplet states, which can be discriminated through a single measurement. Finally, we discuss phonon-assisted dephasing in quantum dots, and present control strategies to suppress such genuine solid-state decoherence losses.     

Mesoscopic one-way channels for quantum state transfer via the quantum Hall effect

We show that the one-way channel formalism of quantum optics has a physical realization in electronic systems. In particular, we show that magnetic edge states form unidirectional quantum channels capable of coherently transporting electronic quantum information. Using the equivalence between one-way photonic channels and magnetic edge states, we adapt a proposal for quantum state transfer to mesoscopic systems using edge states as a quantum channel, and show that it is feasible with reasonable experimental parameters. We discuss how this protocol may be used to transfer information encoded in number, charge, or spin states of quantum dots, so it may prove useful for transferring quantum information between parts of a solid-state quantum computer.     

Quantum Cryptography in Spin Networks

In this paper we propose a new scheme of long-distance quantum cryptography based on spin networks with qubits stored in electron spins of quantum dots. By＂ conditional Faraday- rotation, single photon polarization measurement, and quantum state transfer, maximal-entangled Bell states for quantum cryptography between two long-distance parties are created. Meanwhile, efficient quantum state transfer over arbitrary＂ distances is obtained in a spin chain by＂ a proper choice of coupling strengths and using spin memory- technique improved. We also analyse the security＂ of the scheme against the cloning-based attack which can be also implemented in spin network and discover that this spin network cloning coincides with the optimal fidelity- achieved by＂ an eavesdropper for entanglement-based cryptography.     

Nitrogen vacancy and oxygen impurity in AlN: spintronic quantum dots

Point defects with non-zero spin are prototypical spintronic quantum dots. Here two anion-site defects in AlN are studied intensively in terms of their spin and related properties. The charged states of the nitrogen vacancy and substitutional oxygen impurity, in both ground and spin-flip states, are analyzed. The theoretical analysis includes optical absorption and emission, diffuse excited states, spin densities and local mode force constants. The relevance to spintronic quantum dots in semiconductors is discussed.     

Spin manipulation in semiconductor quantum dots qubit

Thirty years of effort in semiconductor quantum dots has resulted in significant developments in the control of spin quantum bits(qubits). The natural two-energy level of spin states provides a path toward quantum information processing. In particular, the experimental implementation of spin control with high fidelity provides the possibility of realizing quantum computing. In this review, we will discuss the basic elements of spin qubits in semiconductor quantum dots and summarize some important experiments that have demonstrated the direct manipulation of spin states with an applied electric field and/or magnetic field. The results of recent experiments on spin qubits reveal a bright future for quantum information processing.     

Spin relaxation and decoherence of holes in quantum dots

We investigate heavy-hole spin relaxation and decoherence in quantum dots in perpendicular magnetic fields. We show that at low temperatures the spin decoherence time is 2 times longer than the spin relaxation time. We find that the spin relaxation time for heavy holes can be comparable to or even longer than that for electrons in strongly two-dimensional quantum dots. We discuss the difference in the magnetic-field dependence of the spin relaxation rate due to Rashba or Dresselhaus spin-orbit coupling for systems with positive (i.e., GaAs quantum dots) or negative (i.e., InAs quantum dots) g factor.     

基于应变锗空穴双量子点的可调耦合和自旋阻塞研究

应变锗空穴量子点是实现超大规模量子计算最有前景的平台之一.由于锗空穴不受超精细相互作影响，有着较长的自旋弛豫时间和量子退相干时间，且锗中本征的强旋轨道耦合和空穴载流子的低有效质量，使得全电场操控空穴自旋量子比特得以实现，极大地降低了器件加工难度，增加了量子点的可扩展性.本文介绍了一种使用应变锗异质结制备重叠栅空穴双量子点器件的方法，完成了应变锗异质结性质测量，空穴双量子点器件制作，单量子点输运性质和双量子点输运性质研究，双量子点耦合可研究调节性研究，以及外磁场存在下的漏电流性质研究和泡利自旋阻塞解除机制的研究.这些工作为未来实现高质量自旋量子比特制备和高保真度量子逻辑门操控提供了实验平台和基本参数.     

Entanglement of spin-orbit qubits induced by Coulomb interaction

Spin-orbit qubit (SOQ) is the dressed spin by the orbital degree of freedom through a strong spin-orbit coupling (SOC). We show that Coulomb interaction between two electrons in quantum dots located separately in two nanowires can efficiently induce quantum entanglement between two SOQs. But to achieve the highest possible value for two SOQs concurrence, strength of SOC and confining potential for the quantum dots should be tuned to an optimal ratio. The physical mechanism to achieve such quantum entanglement is based on the feasibility of the SOQ responding to the external electric field via an intrinsic electric dipole spin resonance.     

Quantum computation with two-dimensional graphene quantum dots

We study an array of graphene nano sheets that form a two-dimensional S=1/2 Kagome spin lattice used for quantum computation. The edge states of the graphene nano sheets are used to form quantum dots to confine electrons and perform the computation. We propose two schemes of bang-bang control to combat decoherence and realize gate operations on this array of quantum dots. It is shown that both schemes contain a great amount of information for quantum computation. The corresponding gate operations are also proposed.     

Electron spins in an array of tunnel-coupled quantum dots

Mechanisms of electron spin relaxation in semiconductor arrays with tunnel-coupled quantum dots are reviewed. The contribution for anisotropic exchange interaction is shown for asymmetrical quantum dots having no inversion axis relative to their plane. The configuration of vertically coupled double Ge/Si quantum dots is found where anisotropic exchange coupling does not contribute to spin decoherence. It could be a basic configuration of spin-based quantum computation schemes.     

Voltage-selective bidirectional polarization and coherent rotation of nuclear spins in quantum dots

We propose and demonstrate that the nuclear spins of the host lattice in GaAs double quantum dots can be polarized in either of two opposite directions, parallel or antiparallel to an external magnetic field. The direction is selected by adjusting the dc voltage. This nuclear polarization manifests itself by repeated controlled electron-nuclear spin scattering in the Pauli spin-blockade state. Polarized nuclei are also controlled by means of nuclear magnetic resonance. This Letter confirms that the nuclear spins in quantum dots are long-lived quantum states with a coherence time of up to 1 ms, and may be a promising resource for quantum-information processing such as quantum memories for electron spin qubits.     

Spin interactions,relaxation and decoherence in quantum dots

We review recent studies on spin decoherence of electrons and holes in quasi-two-dimensional quantum dots, as well as electron-spin relaxation in nanowire quantum dots. The spins of confined electrons and holes are considered major candidates for the realization of quantum information storage and processing devices, provided that sufficiently long coherence and relaxation times can be achieved. The results presented here indicate that this prerequisite might be realized in both electron and hole quantum dots, taking one large step towards quantum computation with spin qubits.     

Quantum phase transition and Coulomb blockade effect in triangular quantum dots with interdot capacitive and tunnel couplings

The quantum phase transition and the electronic transport in a triangular quantum dot system are investigated using the numerical renormalization group method.We concentrate on the interplay between the interdot capacitive coupling V and the interdot tunnel coupling t.For small t,three dots form a local spin doublet.As t increases,due to the competition between V and t,there exist two first-order transitions with phase sequence spin-doublet-magnetic frustration phase-orbital spin singlet.When t is absent,the evolutions of the total charge on the dots and the linear conductance are of the typical Coulomb-blockade features with increasing gate voltage.While for sufficient t,the antiferromagnetic spin correlation between dots is enhanced,and the conductance is strongly suppressed for the bonding state is almost doubly occupied.     

Entanglement swapping between electromagnetic field modes and matter qubits

Scalable quantum networks require the capability to create, store and distribute entanglement among distant nodes (atoms, trapped ions, charge and spin qubits built on quantum dots, etc.) by means of photonic channels. We show how the entanglement between qubits and electromagnetic field modes allows generation of entangled states of remotely located qubits. We present analytical calculations of linear entropy and the density matrix for the entangled qubits for the system described by the Jaynes-Cummings model. We also discuss the influence of decoherence. The presented scheme is able to drive an initially separable state of two qubits into an highly entangled state suitable for quantum information processing.     

Quantum computing by optical control of electron spins

We review the progress and main challenges in implementing large-scale quantum computing by optical control of electron spins in quantum dots (QDs). Relevant systems include self-assembled QDs of III–V or II–VI compound semiconductors (such as InGaAs and CdSe), monolayer fluctuation QDs in compound semiconductor quantum wells, and impurity centres in solids, such as P-donors in silicon and nitrogen-vacancy centres in diamond. The decoherence of the electron spin qubits is discussed and various schemes for countering the decoherence problem are reviewed. We put forward designs of local nodes consisting of a few qubits which can be individually addressed and controlled. Remotely separated local nodes are connected by photonic structures (microcavities and waveguides) to form a large-scale distributed quantum system or a quantum network. The operation of the quantum network consists of optical control of a single electron spin, coupling of two spins in a local nodes, optically controlled quantum interfacing between stationary spin qubits in QDs and flying photon qubits in waveguides, rapid initialization of spin qubits and qubit-specific single-shot non-demolition quantum measurement. The rapid qubit initialization may be realized by selectively enhancing certain entropy dumping channels via phonon or photon baths. The single-shot quantum measurement may be in situ implemented through the integrated photonic network. The relevance of quantum non-demolition measurement to large-scale quantum computation is discussed. To illustrate the feasibility and demand, the resources are estimated for the benchmark problem of factorizing 15 with Shor's algorithm.     

Magnetism in closed-shell quantum dots: emergence of magnetic bipolarons

Similar to atoms and nuclei, semiconductor quantum dots exhibit the formation of shells. Predictions of magnetic behavior of the dots are often based on the shell occupancies. Thus, closed-shell quantum dots are assumed to be inherently nonmagnetic. Here, we propose a possibility of magnetism in such dots doped with magnetic impurities. On the example of the system of two interacting fermions, the simplest embodiment of the closed-shell structure, we demonstrate the emergence of a novel broken-symmetry ground state that is neither spin singlet nor spin triplet. We propose experimental tests of our predictions and the magnetic-dot structures to perform them.     

半导体中的自旋弛豫——从体材料到量子阱、量子线、量子点

本文对半导体中的自旋弛豫过程给出一个简要的回顾,介绍了半导体材料从体材料到量子阱、量子线、量子点不同维数的结构中各种自旋弛豫过程,主要关注了自旋去相位和相干控制。对于不同材料中的各种弛豫机制,关注的重点在于如何能够在实验上以一种可以控制的方式来改变可调参数从而达到控制自旋弛豫过程。这些参数主要有电场、磁场、温度、应变、有效g因子等等。本文的组织上,首先介绍研究前景,第1部分简要介绍了自旋弛豫的四种机制。第2部分按照维数的不同将半导体中自旋弛豫分为3个部分:体材料、量子阱、量子线、量子点,在每一部分中又基本上按照电子、空穴、激子的顺序进行了简要的总结:对于不同的载流子,考虑了自旋弛豫对可调参数的依赖关系。这些结果要么试图解释了已有的实验结果,要么从理论上给出预言从而给实验指明了方向,为室温下可以使用的自旋电子学器件设计提供了依据,为固态量子计算和量子信息处理铺平了道路。最后简单地给出展望。     

半导体中的自旋弛豫--从体材料到量子阱、量子线、量子点

本文对半导体中的自旋弛豫过程给出一个简要的回顾,介绍了半导体材料从体材料到量子阱、量子线、量子点不同维数的结构中各种自旋弛豫过程,主要关注了自旋去相位和相干控制。对于不同材料中的各种弛豫机制,关注的重点在于如何能够在实验上以一种可以控制的方式来改变可调参数从而达到控制自旋弛豫过程。这些参数主要有电场、磁场、温度、应变、有效g因子等等。本文的组织上,首先介绍研究前景,第1部分简要介绍了自旋弛豫的四种机制。第2部分按照维数的不同将半导体中自旋弛豫分为3个部分:体材料、量子阱、量子线、量子点,在每一部分中又基本上按照电子、空穴、激子的顺序进行了简要的总结:对于不同的载流子,考虑了自旋弛豫对可调参数的依赖关系。这些结果要么试图解释了已有的实验结果,要么从理论上给出预言从而给实验指明了方向,为室温下可以使用的自旋电子学器件设计提供了依据,为固态量子计算和量子信息处理铺平了道路。最后简单地给出展望。     

Induced quantum dots and wires: electron storage and delivery

We show that quantum dots and quantum wires are formed underneath metal electrodes deposited on a planar semiconductor heterostructure containing a quantum well. The confinement is due to the self-focusing mechanism of an electron wave packet interacting with the charge induced on the metal surface. Induced quantum wires guide the transfer of electrons along metal paths and induced quantum dots store the electrons in specific locations of the nanostructure. Induced dots and wires can be useful for devices operating on the electron spin. An application for a spin readout device is proposed.