R. Brunner

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.

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.

Open quantum dots—probing the quantum to classical transition

Quantum dots provide a natural system in which to study both quantum and classical features of transport. As a closed testbed, they provide a natural system with a very rich set of eigenstates. When coupled to the environment through a pair of quantum point contacts, each of which passes several modes, the original quantum environment evolves into a set of decoherent and coherent states, which classically would compose a mixed phase space. The manner of this breakup is governed strongly by Zureks decoherence theory, and the remaining coherent states possess all the properties of his pointer states. These states are naturally studied via traditional magnetotransport at low temperatures. More recently, we have used scanning gate (conductance) microscopy to probe the nature of the coherent states, and have shown that families of states exist through the spectrum in a manner consistent with quantum Darwinism. In this review, we discuss the nature of the various states, how they are formed, and the signatures that appear in magnetotransport and general conductance studies.

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.

Magneto-transport in open quantum dot arrays at the transition from low to high magnetic fields: Regularity and chaos

The mixed phase space in an open quantum dot array with a soft confining potential is studied in low and high magnetic fields. Chaos occurs due to the perturbations by the constrictions connecting the dots of the array. Regular orbits and the areas of Kalmogorov-Arnold-Moser islands in phase space become increasingly dominating with increasing magnetic field. The correspondence to the high-field quantum-mechanical picture in a 2D electron system with boundaries is discussed.

Aluminum oxide for an effective gate in Si/SiGe two-dimensional electron gas systems

We investigate the gating properties of Si/SiGe two-dimensional electron gas systems with various gate materials and fabricate a lateral Si/SiGe quantum dot by gating through an Al2O3 film. In comparison to the conventional surface Schottky gates, gating through a thin Al2O3 layer provides a strong suppression of leakage current. The fabricated quantum dot shows a periodic current oscillation or Coulomb oscillation with negligible gate leakage, indicating that the gating employed may be good for implementing Si/SiGe-based spin qubit devices with quantum dots.

Quantitative Evaluation of Charge Sensing Sensitivity in a Laterally Defined Triple Quantum Dot

We have established a quantitative method to evaluate the charge sensing sensitivity using a quantum point contact. From the experimental data of a lateral triple quantum dot, we have obtained potential modulations at the saddle point imposed by an electron in each quantum dot of ∼1 μeV. The estimated screening lengths for each quantum dot showed a clear position dependence probably due to the gate structures.

Coherent control of two individual electron spins and influence of hyperfine coupling in a double quantum dot

Electric dipole spin resonance of two individual electrons and the influence of hyperfine coupling on the spin resonance are studied for a double quantum dot equipped with a micro-magnet. The spin resonance occurs by oscillating the electron in each dot at microwave (MW) frequencies in the presence of a micro-magnet induced stray field. The observed continuous wave (CW) and time-resolved spin resonances are consistent with calculations in which the MW induced AC electric field and micro-magnet induced stray field are taken into account. The influence of hyperfine coupling causes an increase and broadening of the respective CW spin resonance peaks through dynamical nuclear polarization when sweeping up the magnetic field. This behaviour appears stronger for the larger of the two spin resonance peaks and in general becomes more pronounced as the MW power increases, both reflecting that the electron-nuclei interaction is more efficient for the stronger spin resonance. In addition the hyperfine coupling effect only becomes pronounced when the MW induced AC magnetic field exceeds the fluctuating nuclear field.

Dynamical polarization effect of nuclear spin bath dragged by electron spin resonance in double quantum dot integrated with micro-magnet

We studied on Overhauser shift of electron dipole spin resonance (EDSR) peaks by using a double quantum dot integrated with a micro-magnet. Two EDSR peaks are well resolved, reflecting electron spin flip events at different resonance conditions between two dots, which depend on the in-plane field at the two dots produced by a micro magnet. One of the two peaks is significantly higher than the other and shows a larger Overhauser shift, indicating that an electron spin flip process local to the dot causes dynamical polarization of local nuclear spins to the same dot. After the nuclear spin polarization is saturated, we observed the decay of the Overhauser shift by repeatedly measuring the EDSR peak with a minimum microwave power. The decay time constant is much longer than by other groups. We discuss the possible reason for this difference.

Magnetoresistance experiments and quasi‐classical calculations regarding backscattering in open quantum dots

Closed and open quantum dots are low‐dimensional electron systems with different strength of coupling to the two‐dimensional environment. In the case of a the closed dot, only transport by tunneling is possible. Our work, on the the other hand, investigates the transport of open quantum dots strongly coupled to the surrounding two‐dimensional electron system. The open dot show peculiar peaks in their magnetoresistance. We compare experimental results with those of a semi‐classical model using trajectories in a parabolic potential. Using a velocity distribution of the incoming electrons based on Fermi‐Dirac statistics we can explain the temperature dependence of the peak heights.