Quantum Information Processing Based On Quantum Dot System And Spin Chain

The rapid rise and development of Quantum information science(QIS) has been made in the mid-90th of last century. QIS is a newly developed cross-field which draws upon the disciplines of physical science, mathematics, computer science, and engineer-ing. With the help of rapid progress in science and technology, the miniaturiuzation of electronics is ever-increasing. It is predicted that in2020the size of electronics will reach a scale of the order of10nm, where the quantum effects of electrons and atoms will become dominant. Then the Moore’s law will eventually be limited in the next decades due to the quantum effect. The classical mechanics, which rules the world of information for years, will become invalid to address those problems. We need to exploit some new principle, such as quantum mechanics. Due to the postulates of quan-tum mechanics, there are some new properties of quantum information, such as linear superposition of quantum mechanics and entanglement, which improve the quality of information processing, such as computing, storage, communication and so on. At-tribute to the linear superposition of quantum state, quantum paralleism can greatly improve the computing rate of quantum computing. quantum paralleism also can for-bid the probability of arbitrary quantum state cloning, which guarantees the security of quantum cryptographic protocols. With the existence of entanglement property, many quantum information protocols, such as quantum teleportation, dense coding, and re-mote quantum state preparation become true.Those fantastic advantages of quantum information science surge broad interest of physical realization of powerful quantum computer. However, There are many difficul-ties in the process towards the practical application. For example, significant progress has been made in quantum information processing based on well-known quantum sys-tems, such as optics, ion traps, quantum dot, superconduction circuit, and so on. The manipulation of one or a few quantum qubits becomes maturity in experimental re-search. However, at present, people are not sure about how to construct quantum com-puter large enough to manipulate hundreds and thousands of quantum qubits. The main obstacle is that the quantum coherence and entanglement will be damaged by the un-wanted coupling between quantum system and environment. This is called quantum decoherence. As the number of quantum qubit increases, quantum decoherence be-comes so serious that no quantum operation can be done. In this thesis, we just start our topic from quantum information processing based on quantum dot system and spin chain. We propose to build multi-qubit quantum phase gate and prepare two-body steady entanglement in quantum dot system. We also inves-tigate quantum state transfer in spin chain. We give a careful analysis of the effect of different decoherence mechanism on quantum computing and quantum state transfer. In terms of quantum decoherence, we also consider the dissipation engineering ways to design decoherence assisted protocols, or use the dynamical decoupling to suppress the decoherence in quantum systems. The detail goes as follows:1.We propose a scheme to construct three-qubit phase gate in quantum dot system.In principle, the single qubit rotations and two qubit controlled NOT(CNOT) gates can be used to implement an arbitrary unitary operation on n qubits, and therefore are universal for quantum computing. But in order to simplify the structure of quantum computer, it is of practical significance to develop multi-qubit non-local quantum gates in quantum system. With the help of inborn Coulomb interaction between nearest quan-tum dots, an all-optical one-step three-qubit controlled phase gate can be implemented by driving a laser. This scheme requires three n-doped quantum dots of the same energy structure. Each quantum dot is doped with a single excess electron, and information is encoded on the spin of electron. The advantages of our scheme are shown as follows:(1) all optical scheme;(2) do not require single-site addressing. We investigate the effect of configuration, spontaneous emission and the acoustic phonon on the scheme. We find that the gate performance is primarily related to quantum-dot configuration and temperature, and can be significantly improved by selecting lower single-exciton resonance in line-type configuration. Our work gives a careful analysis on the effect of arrangement of QD arrays, which has some practical reference value for fabrication of multi-qubit quantum computing devices based on quantum dot system and implement-ing complicated large scale quantum computing in the future.2. We design a protocol for decoherence-assisted preparation of steady state entanglement.Generally speaking, the noise induced by environment is the No.1Enemy of quantum computing and quantum communication. It can destroy the coherence of quantum entanglement and quantum operation. By careful designing quantum co-herence control and quantum decoherence operators, the dissipation may become the source of quantum information processing. Here driven by optical pumping, the sys- tem will be forced to its dark state with the help of decoherence. If the dark state is an entangled state, the steady state entanglement is implemented. The advantages of this scheme are:(1) no requirement of initialization, This is because our protocol implements initialization and entanglement preparation at the same time.(2) The en-tanglement is deterministically generated without measurement.(3) There is no need to precisely control the time of interaction between laser and quantum dot. Furthermore, we consider the effects of phonon and electron tunneling on our protocol, and we find that the dominant source for decoherence is deformation phonon in InAs/GaAs quan-tum dot.3. We analyze the effect of disorder in quantum state transfer based on quan-tum dot arrays.We investigate the quantum state transfer in quantum dot arrays. Hyperfine inter-action, which induced by the coupling between electron spins and nuclear spins in sur-rounding bulk materials, plays deleterious role in quantum state transfer in spin chains constructed by quantum dot arrays. Hyperfine interaction can be simulated by a three dimensional effective magnetic filed, and it can be taken as a static disorder during a trial when we plan to average hundreds of trails to calculate the average fidelity of quantum state transfer. Hyperfine interaction breaks the conservation of the excitation number, thus the density matrix in dynamic evolution should be expanded in the whole Hilbert space. Moreover, we consider the exchange coupling fluctuations due to the imperfect control of quantum barrier. It depends on time, and particularly, it behaves as1/f noise. We find that the hyperfine interaction is the main decoherence source in quantum state transfer in quantum dot system.4. We propose a new state transfer protocol in strongly correlated system.Due to the inherent interaction between nearest spins, spin chain has been consid-ered as a nature atomic-scale quantum channel for quantum state transfer. The typical quantum state transfer protocol is to attach a quantum qubit, which has been encoded with an unknown or known quantum state, to quantum channel. Then let the system evolves under the system Hamiltonian, the information will be received at the end of the quantum channel. In this protocol, the inherent entanglement plays a little role and only the symmetries of the state and the Hamiltonian seem to be important. The entangle-ment in strongly correlated system is typically short ranged. It cannot be used directly to teleport an unknown state, accomplish remote state preparation of a known state, or double the rate of classical communication by dense coding. However, assisted by quantum dynamical evolution, information can be encoded on the first spin of quantum channel by applying single-qubit rotation. After a certain period of time, the informa-tion can be decoded by measuring the last two spins of the channel. This protocol can propagate both classical information and quantum information, and it can realize the dense-coding-like or remote-state-preparation-like despite the absence of long-range entanglement.5. We exploit dynamic decoupling in dissipation assisted non-classical state preparation to greatly improve the quality of those non-classical state.The dissipation induced by the undesired coupling between system and environ-ment will lead to decoherence in quantum system. The careful designed dissipation process can be used as the source of quantum information processing. For example, the non-classical states can be generated due to the effective non-linear interaction in-duced by dissipation. But the fidelity of those non-classical state is still limited by the unwanted dispassion. As is well known, dynamic decoupling is an effective method to persist coherence of quantum qubit. Here we try to combine the advantages of dynamic decoupling technique and dissipation assisted non-classical state preparation protocol, and we find that the fidelity of non-classical state can be greatly improved.