We demonstrate fast universal electrical spin manipulation with inhomogeneous magnetic fields. With fast Rabi frequency up to 127 MHz, we leave the conventional regime of strong nuclear-spin influence and observe a spin-flip fidelity >96%, a distinct chevron Rabi pattern in the spectral-time domain, and a spin resonance linewidth limited by the Rabi frequency, not by the dephasing rate. In addition, we establish fast z rotations up to 54 MHz by directly controlling the spin phase. Our findings will significantly facilitate tomography and error correction with electron spins in quantum dots.

Tailoring spin coupling to electric fields is central to spintronics and spin-based quantum information processing. We present an optimal micromagnet design that produces appropriate stray magnetic fields to mediate fast electrical spin manipulations in nanodevices. We quantify the practical requirements for spatial field inhomogeneity and tolerance for misalignment with spins, and propose a design scheme to improve the spin-rotation frequency (to exceed 50MHz in GaAs nanostructures). We then validate our design by experiments in separate devices. Our results will open a route to rapidly control solid-state electron spins with limited lifetimes and to study coherent spin dynamics in solids.

We prepare a gate-defined quadruple quantum dot to study the gate-tunability of single to quadruple quantum dots with finite inter-dot tunnel couplings. The measured charging energies of various double dots suggest that the dot size is governed by gate geometry. For the triple and quadruple dots we study gate-tunable inter-dot tunnel couplings. Particularly for the triple dot we find that the effective tunnel coupling between side dots significantly depends on the alignment of the center dot potential. These results imply that the present quadruple dot device has gate performance relevant for implementing spin-based four-qubit systems with controllable exchange couplings.Quantum dots (QDs) are artificial structures fabricated in semiconductors in which electrons are confined within the size of their de-Broglie wave length, typically tens of nanometers. Since QDs can trap single electrons isolated from the environment and these electron states are precisely controlled, they are attractive systems for both basic research of electron interaction and applications to quantum information processing. Recently several challenging experiments have demonstrated coherent manipulation of electron spins 1-4 following the proposal of electron-spin-based quantum computation 5 . We previously demonstrated two spin-1/2 qubits and exchange control with a double quantum dot (DQD) with a micro-magnet (MM) 4 and proposed a triple QD (TQD) with a MM suitable for implementing three spin-1/2 qubits 6 . Extending the number of qubits is an important step toward realization of quantum computation. Several types of few-electron TQDs have been demonstrated in recent years [7][8][9] . As for quadruple QDs (QQDs), some systems consisting simply of two capacitively coupled DQDs have been studied [10][11][12][13] . In these devices each DQD is used as a charge qubit 10,11 or a singlet-triplet spin qubit 12, 13 and the capacitive coupling between the two DQDs has been used to perform conditional operations between the qubits. However, no QQDs having finite tunnel couplings between all the neighboring dots have ever been fabricated. Furthermore, integration with a MM favors multiple QDs in a linear array. In this Letter we fabricated collinear QQDs with inter-dot tunnel coupling, which are designed to be fitted with a MM, and demonstrated gate-tunable formation of single, double, triple and quadruple QDs by adjusting gate voltages and observed the effect of inter-dot tunneling in the stability diagram.Figure 1(a) shows a scanning electron micrograph of our device. The geometry of the surface gate electrodes are designed by using the numerical simulation 14 of electrostatic potential to create four dots in a row. The gate-defined QQD is formed in a 100-nm deep two-dimensional electron gas (2DEG) at a GaAs/AlGaAs hetero-interface with a capping gate on top to effectively reduce the 2DEG density. All experiments to identify the charge states of the fabricated devices were performed at a bath temperature of 50 mK.Initially we applied appropriate gat...

Electron spin confined in quantum dots is a promising candidate for experimental qubits. Aiming at realizing a three spin-qubit system, we designed split micromagnets suitable for the lateral triple quantum dots. From numerical simulations of the stray magnetic field distribution, field gradients ∼0.8 T/μm and differences of in-plane components ∼10 mT can be attained, which enable the electrical and addressable manipulation of three qubits. Furthermore, this technique can be applied for up to 25 qubits in realistic multiple quantum dots. For the first step of implementing such three-qubit systems, a relevant triple quantum dot device has been fabricated and characteristic charge states were observed.

We developed a resistive-pulse sensor with a solid-state pore and measured the latex agglutination of submicron particles induced by antigen-antibody interaction for single-molecule detection of proteins. We fabricated the pore based on numerical simulation to clearly distinguish between monomer and dimer latex particles. By measuring single dimers agglutinated in the single-molecule regime, we detected single human alpha-fetoprotein molecules. Adjusting the initial particle concentration improves the limit of detection (LOD) to 95 fmol/l. We established a theoretical model of the LOD by combining the reaction kinetics and the counting statistics to explain the effect of initial particle concentration on the LOD. The theoretical model shows how to improve the LOD quantitatively. The single-molecule detection studied here indicates the feasibility of implementing a highly sensitive immunoassay by a simple measurement method using resistive-pulse sensing.