Quantum computer has a great attention all over the world, several attempts to construct it by supposing a theoretical or experimental model for qubit which is the basic unit of quantum computer. Qubit is formed from two level physical system, So, semiconductor quantum dot is suitable for qubit where qubit state formed from superposition between two spin states , silicon (Si) quantum dot is one of suitable system for qubit and quantum logic gate because of its long coherence time. In this work, we study theoretical model of quantum dots from silicon (Si) which coupling through superposition and entanglement mechanism to form quantum logic gate. we have investigated magnetic field effect on exchange interaction by using two approximate methods Hund – Mullikan (HM) and Hitler – London (LH) for calculation and evaluation of the exchange interaction coupling , also study the influence of inter – dots distance on the exchange coupling. From investigation of exchange interaction, silicon quantum dots is the best suitable for quantum logic gate.
Quantum computer has a great attention all over the world, several attempts to construct it by supposing a theoretical or experimental model for qubit which is the basic unit of quantum computer. Qubit is formed from two level physical system, So, semiconductor quantum dot is suitable for qubit where qubit state formed from superposition between two spin states , silicon (Si) quantum dot is one of suitable system for qubit and quantum logic gate because of its long coherence time. In this work, we study theoretical model of quantum dots from silicon (Si) which coupling through superposition and entanglement mechanism to form quantum logic gate. we have investigated magnetic field effect on exchange interaction by using two approximate methods Hund – Mullikan (HM) and Hitler – London (LH) for calculation and evaluation of the exchange interaction coupling , also study the influence of inter – dots distance on the exchange coupling. From investigation of exchange interaction, silicon quantum dots is the best suitable for quantum logic gate.
We consider a quantum-gate mechanism based on electron spins in coupled semiconductor quantum dots. Such gates provide a general source of spin entanglement and can be used for quantum computers. We determine the exchange coupling J in the effective Heisenberg model as a function of magnetic (B) and electric fields, and of the interdot distance a within the Heitler-London approximation of molecular physics. This result is refined by using sp hybridization, and by the Hund-Mulliken molecular-orbit approach, which leads to an extended Hubbard description for the two-dot system that shows a remarkable dependence on B and a due to the long-range Coulomb interaction. We find that the exchange J changes sign at a finite field (leading to a pronounced jump in the magnetization) and then decays exponentially. The magnetization and the spin susceptibilities of the coupled dots are calculated. We show that the dephasing due to nuclear spins in GaAs can be strongly suppressed by dynamical nuclear-spin polarization and/or by magnetic fields.
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.
We report on our recent progress in applying semiconductor quantum dots for spin-based quantum computation, as proposed by Loss and DiVincenzo (1998 Phys. Rev. A 57 120). For the purpose of single-electron spin resonance, we study different types of single quantum dot devices that are designed for the generation of a local ac magnetic field in the vicinity of the dot. We observe photon-assisted tunnelling as well as pumping due to the ac voltage induced by the ac current driven through a wire in the vicinity of the dot, but no evidence for ESR so far. Analogue concepts for a double quantum dot and the hydrogen molecule are discussed in detail. Our experimental results in laterally coupled vertical double quantum dot device show that the Heitler–London model forms a good approximation of the two-electron wavefunction. The exchange coupling constant J is estimated. The relevance of this system for two-qubit gates, in particular the SWAP operation, is discussed. Density functional calculations reveal the importance of the gate electrode geometry in lateral quantum dots for the tunability of J in realistic two-qubit gates.
The universal quantum computation is obtained when there exists asymmetric anisotropic exchange between electron spins in coupled semiconductor quantum dots. The asymmetric Heisenberg model can be transformed into the isotropic model through the control of two local unitary rotations for the realization of essential quantum gates. The rotations on each qubit are symmetrical and depend on the strength and orientation of asymmetric exchange. The implementation of the axially symmetric local magnetic fields can assist the construction of quantum logic gates in anisotropic coupled quantum dots. This proposal can efficiently use each physical electron spin as a logical qubit in the universal quantum computation.
Several logical qubits and quantum gates have been proposed for semiconductor quantum dots controlled by voltages applied to top gates. The different schemes can be difficult to compare meaningfully. Here we develop a theoretical framework to evaluate disparate qubit-gating schemes on an equal footing. We apply the procedure to two types of double-dot qubits: the singlet-triplet and the semiconducting quantum dot hybrid qubit. We investigate three quantum gates that flip the qubit state: a DC pulsed gate, an AC gate based on logical qubit resonance, and a gate-like process known as stimulated Raman adiabatic passage. These gates are all mediated by an exchange interaction that is controlled experimentally using the interdot tunnel coupling g and the detuning [Symbol: see text], which sets the energy difference between the dots. Our procedure has two steps. First, we optimize the gate fidelity (f) for fixed g as a function of the other control parameters; this yields an f(opt)(g) that is universal for different types of gates. Next, we identify physical constraints on the control parameters; this yields an upper bound f(max) that is specific to the qubit-gate combination. We show that similar gate fidelities (~99:5%) should be attainable for singlet-triplet qubits in isotopically purified Si, and for hybrid qubits in natural Si. Considerably lower fidelities are obtained for GaAs devices, due to the fluctuating magnetic fields ΔB produced by nuclear spins.
Quantum physics governs the world at the microscopic scale and determines the properties of matter. While classical computers exploit quantum phenomena passively by relying on transistor technology, the concept of a quantum computer with active control over quantum states promises unprecedented possibilities for information encoding and processing. Already developed quantum algorithms are known to perform certain tasks faster than any existing classical algorithms, and quantum computers also promise the possibility for precise simulations of complex quantum systems, a task infeasible for classical computers. These points emphasize the need for the development of quantum computers. Considering quantum computers, the fundamental unit of information is a qubit, a two-level quantum system. Qubits, in contrast to classical bits which are either in state 0 or 1, can exist in quantum superposition of the states 0 and 1, allowing for more complex and parallel computations. While there are many options to encode a logical qubit in a physical system, spin 1/2 particles are considered a natural choice because their spin degree of freedom defines a qubit intrinsically. Semiconductors are a promising platform for hosting spin qubits, as they enable the confinement of electrons with spin 1/2 either through gate-defined quantum dots (QDs) or donor atoms. Among the possible choices of donor atoms, phosphorous 31P is particularly noteworthy because its nucleus has nuclear spin 1/2. Nuclear spins are prime candidates for qubit implementation because they demonstrated exceptionally long coherence times, but the underlying good isolation from their environment is a challenge when it comes to readout and control. In this thesis, we face this challenge by proposing a hybrid quantum dot-donor (QDD) system consisting of a gate defined silicon (Si) QD and a thereto laterally displaced 31P phosphorous donor atom implanted in the Si host material. A single electron is presumed to be confined in the system, which can be moved between the QD and the donor on demand. If its wave function overlaps with the donor atom, electron spin and the donor nuclear spin interact via the hyperfine interaction and the electron can act as a mediator between the nuclear spin qubit and the environment for readout and control, whereas the nuclear spin qubit can be set in an idling mode by shifting the electron to the QD. Importantly, the proposed system can be embedded in circuit quantum electrodynamics (cQED) architectures enabling strong coupling between the system’s electron spin and microwave resonator photons. This coupling is fundamental for the research presented in this thesis. Analyzing the envisioned system, we first propose a novel method for nuclear-spin readout by probing the resonator transmission. This method is based on the observation that the signature of strong electron spin-photon coupling in the resonator transmission is altered due to the hyperfine interaction, in such a manner that well separated signatures for the electron spin-photon coupling conditioned on the state of the nuclear spin arise. As the next step, we turn our attention to the control of nuclear spin qubits and especially to the challenge of realizing a two-qubit gate between distant nuclear spins. In this course, we derive an effective nuclear spin-photon coupling. While expected to be weak, this interaction can be harnessed to couple the nuclear spins of two distant QDD systems dispersively to a microwave resonator resulting in an entangling nuclear spin two-qubit √iSWAP gate with a gate fidelity approaching 90%. These two key findings represent a significant and valuable contribution to the field of quantum information processing using nuclear spins.