Toshiaki Obata

Electrically driven single-electron spin resonance in a slanting Zeeman field

The integration of a micrometre-sized magnet with a semiconductor device has enabled the individual manipulation of two single electron spins. This approach may provide a scalable route for quantum computing with electron spins confined in quantum dots.

Coherent Single Electron Spin Control in a Slanting Zeeman Field

We consider a single electron in a 1D quantum dot with a static slanting Zeeman field. By combining the spin and orbital degrees of freedom of the electron, an effective quantum two-level (qubit) system is defined. This pseudospin can be coherently manipulated by the voltage applied to the gate electrodes, without the need for an external time-dependent magnetic field or spin-orbit coupling. Single-qubit rotations and the controlled-NOT operation can be realized. We estimated the relaxation (T1) and coherence (T2) times and the (tunable) quality factor. This scheme implies important experimental advantages for single electron spin control.

Two-qubit gate of combined single-spin rotation and interdot spin exchange in a double quantum dot.

A crucial requirement for quantum-information processing is the realization of multiple-qubit quantum gates. Here, we demonstrate an electron spin-based all-electrical two-qubit gate consisting of single-spin rotations and interdot spin exchange in a double quantum dot. A partially entangled output state is obtained by the application of the two-qubit gate to an initial, uncorrelated state. We find that the degree of entanglement is controllable by the exchange operation time. The approach represents a key step towards the realization of universal multiple-qubit gates.

Single to quadruple quantum dots with tunable tunnel couplings

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 the gate geometry. For the triple and quadruple dots, we study the gate-tunable inter-dot tunnel couplings. 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 has a gate performance relevant for implementing spin-based four-qubits with controllable exchange couplings.

Triple quantum dot device designed for three spin qubits

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.

Micromagnets for coherent control of spin-charge qubit in lateral quantum dots

A lateral quantum dot design for coherent electrical manipulation of a two-level spin-charge system is presented. Two micron-size permanent magnets integrated to high-frequency electrodes produce a static slanting magnetic field suitable for voltage controlled single qubit gate operations. Stray field deviation from the slanting form is taken into account in the Hamiltonian describing the two-level system, which involves hybridization of a single electron spin to the quantum dot’s orbitals. Operation speed and gate fidelity are related to device parameters. Sub-100-ns π pulse duration can be achieved with lattice fluctuation coherence time of 4ms for GaAs.

Superfluidity and quantized vortex studies under rotation up to 4 Hz at mK and 1 Hz at sub-mK temperatures

Abstract Two new high-speed rotating cryostats at the ISSP, University of Tokyo, capable of operating at mK temperatures, are currently employed for studies of superfluidity and quantized vortex states in 4 He and 3 He . A stable high-Q torsional oscillator is used for the study of superfluidity in low-density 4 He films adsorbed on 3-D connected porous substrates. At Tc, the critical phenomena of 3-D Bose superfluids is seen. Under rotation a new type of vortex state is observed. High-resolution NMR studies of superfluid 3 He , contained in capillary arrays of two different diameters, are made as a function of rotational speed. In these experiments, single vortex intrusion events as well as texural modification under rotation are clearly resolved.

Coherent manipulation of individual electron spin in a double quantum dot integrated with a micromagnet

We report the coherent manipulation of electron spins in a double quantum dot integrated with a micro-magnet. We performed electric dipole spin resonance experiments in the continuous wave (CW) and pump-and-probe modes. We observed two resonant CW peaks and two Rabi oscillations of the quantum dot current by sweeping an external magnetic field at a fixed frequency. Two peaks and oscillations are measured at different resonant magnetic field, which reflects the fact that the local magnetic fields at each quantum dot are modulated by the stray field of a micro-magnet. As predicted with a density matrix approach, the CW current is quadratic with respect to microwave (MW) voltage while the Rabi frequency (\nu_Rabi) is linear. The difference between the \nu_Rabi values of two Rabi oscillations directly reflects the MW electric field across the two dots. These results show that the spins on each dot can be manipulated coherently at will by tuning the micro-magnet alignment and MW electric field.

Robust micromagnet design for fast electrical manipulations of single 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 50 MHz 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.

Characterization and suppression of low-frequency noise in Si/SiGe quantum point contacts and quantum dots

We report on the effects of a global top gate on low-frequency noise in Schottky gate-defined quantum point contacts (QPCs) and quantum dots (QDs) in a modulation-doped Si/SiGe heterostructure. For a relatively large top gate voltage, the QPC current shows frequent switching with 1/f2 Lorentzian type charge noise. As the top gate voltage is decreased, the QPC pinch-off voltage becomes less negative, and the 1/f2 noise becomes rapidly suppressed in a homogeneous background 1/f noise. We apply this top-gating technique to double QDs to stabilize the charge state for the electron number down to zero.