We introduce a scheme for preparation, manipulation, and read out of Majorana zero modes in semiconducting wires with mesoscopic superconducting islands. Our approach synthesizes recent advances in materials growth with tools commonly used in quantum-dot experiments, including gate control of tunnel barriers and Coulomb effects, charge sensing, and charge pumping. We outline a sequence of milestones interpolating between zero-mode detection and quantum computing that includes (1) detection of fusion rules for non-Abelian anyons using either proximal charge sensors or pumped current, (2) validation of a prototype topological qubit, and (3) demonstration of non-Abelian statistics by braiding in a branched geometry. The first two milestones require only a single wire with two islands, and additionally enable sensitive measurements of the system's excitation gap, quasiparticle poisoning rates, residual Majorana zero-mode splittings, and topological-qubit coherence times. These pre-braiding experiments can be adapted to other manipulation and read out schemes as well.

We observed mixing between two-electron singlet and triplet states in a double quantum dot, caused by interactions with nuclear spins in the host semiconductor. This mixing was suppressed when we applied a small magnetic field or increased the interdot tunnel coupling and thereby the singlet-triplet splitting. Electron transport involving transitions between triplets and singlets in turn polarized the nuclei, resulting in marked bistabilities. We extract from the fluctuating nuclear field a limitation on the time-averaged spin coherence time T2* of 25 nanoseconds. Control of the electron-nuclear interaction will therefore be crucial for the coherent manipulation of individual electron spins.

We report transport measurements through a single-molecule magnet, the Mn 12 derivative Mn 12 O 12 O 2 C-C 6 H 4 -SAc 16 H 2 O 4 , in a single-molecule transistor geometry. Thiol groups connect the molecule to gold electrodes that are fabricated by electromigration. Striking observations are regions of complete current suppression and excitations of negative differential conductance on the energy scale of the anisotropy barrier of the molecule. Transport calculations, taking into account the high-spin ground state and magnetic excitations of the molecule, reveal a blocking mechanism of the current involving nondegenerate spin multiplets.

The layered semimetal WTe2 has recently been found to be a two-dimensional topological insulator (2D TI) when thinned down to a single monolayer, with conducting helical edge channels. We report here that intrinsic superconductivity can be induced in this monolayer 2D TI by mild electrostatic doping, at temperatures below 1 K. The 2D TI-superconductor transition can be easily driven by applying a just a small gate voltage. This discovery offers new possibilities for gatecontrolled devices combining superconductivity and topology, and could provide a basis for quantum information schemes based on topological protection. Main text:Many of the most important, and fascinating, phenomena in condensed matter emerge from the quantum mechanics of electrons in a lattice. The periodic potential of the lattice gives rise to Bloch energy bands, and so to the physics of semiconductors that underlies all modern-day electronics. On the more exotic side, electrons in a lattice can pair up to act as bosons and condense into a macroscopic quantum state conducting electricity with zero resistance. More recently, it was realized that Bloch wavefunctions can have a non-trivial topology, incorporating twists analogous to a Möbius strip. This led to the discovery of topological insulators-materials that are electrically insulating in their interior but have conducting boundary modes that result from the topological discontinuity between inside and outside(1). The first of these to be studied was the so-called 2D topological insulator (2D TI), in which the one-dimensional helical edge modes (spin locked to momentum) give rise to the quantum spin Hall effect(2-4).Materials that combine non-trivial topology with superconductivity have been the subject of active investigation in recent years. For example, hybrid structures that couple an s-wave superconductor to a 2D TI have also been proposed as platform for Majorana modes(5), whose non-abelian exchange properties might be harnessed for qubits(6) with coherence times far longer than those built on conventional platforms. There are also topological superconductors, in which vortices or boundaries can host Majorana modes(7).Here we report the remarkable finding that monolayer WTe2, recently shown(8-13) to be an intrinsic 2D TI, itself turns superconducting under moderate electrostatic gating. Several other non-topological layered materials superconduct in the monolayer limit, either intrinsically or under heavy doping using ionic liquid gates(14-22). However, the present case constitutes the first instance of a phase transition from a 2D topological insulator to a superconductor, which moreover is readily controlled by a gate voltage. The discovery creates new opportunities for gateable superconducting circuitry, and offers the potential to develop topological superconducting devices in a single material, as opposed to the hybrid constructions currently required.

We report measurements of mesoscopic fluctuations of Coulomb blockade peaks in a shapedeformable GaAs quantum dot. Distributions of peak heights agree with predicted universal functions for both zero and nonzero magnetic fields. Parametric fluctuations of peak height and position, measured using a two-dimensional sweep over gate voltage and magnetic field, yield autocorrelations of height fluctuations consistent with a predicted Lorentzian-squared form for the unitary ensemble. We discuss the dependence of the correlation field on temperature and coupling to the leads as the dot is opened.

We measure transport through gold grain quantum dots fabricated using electromigration, with magnetic impurities in the leads. A Kondo interaction is observed between dot and leads, but the presence of magnetic impurities results in a gate-dependent zero-bias conductance peak that is split due to a RKKY interaction between the spin of the dot and the static spins of the impurities. A magnetic field restores the single Kondo peak in the case of an antiferromagnetic RKKY interaction. This system provides a new platform to study Kondo and RKKY interactions in metals at the level of a single spin. DOI: 10.1103/PhysRevLett.96.017205 PACS numbers: 75.30.Hx, 72.15.Qm, 73.23.ÿb, 73.63.Kv The observation of the Kondo effect in quantum dot systems has generated renewed experimental and theoretical interest in this many-body effect. The Kondo effect is the screening of a localized spin by surrounding conduction electrons. The localized spin can take the form of a magnetic atom, or the net spin in a quantum dot (QD). The Kondo effect has been studied extensively in quantum dot systems such as semiconductor quantum dots [1,2], carbon nanotubes [3], and single molecules contacted by metal leads [4 -7].The Kondo effect in a quantum dot can be used to probe interactions of a local spin with other magnetic moments. Whereas the Kondo effect enhances the zero-bias conductance through spin-flip processes, exchange interactions tend to freeze the spin of the QD. This competition results in a suppression and splitting of the Kondo resonance. The Kondo effect has been used to study the direct interaction between spins on a double dot [8,9], the exchange interaction with ferromagnetic leads [10], and the indirect Ruderman-Kittel-Kasuya-Yoshida (RKKY) interaction of two QDs separated by a larger dot [11]. In bulk metals with embedded magnetic impurities, the competition between the Kondo effect and RKKY coupling between impurities gives rise to complex magnetic states such as spin glasses [12].In this Letter, we use the Kondo effect to study the RKKY interaction between the net spin of a quantum dot and magnetic impurities in the leads of an all-metal device. The system consists of a small gold grain in the vicinity of magnetic cobalt impurities [ Fig. 2(a)]. By itself, the Kondo interaction with the net spin on such a grain induces a zerobias peak in conductance. This feature is regularly observed in samples without impurities [13]. In the present experiment, cobalt impurities deposited intentionally cause the zero-bias peak to split. The splitting is explained by the RKKY interaction between the impurities and the spin of the grain. Temperature and magnetic-field dependence of the split zero-bias peak (SZBP) confirm this interpretation.Measurements are performed on gold wires that have been broken by a controlled electromigration process, which is tailored to produce narrow gaps. Two substantially different procedures were followed, in two laboratories, but yielded similar results. Both procedures begin with a 12 nm gold bridge on...

Coupling a two-dimensional (2D) semiconductor heterostructure to a superconductor opens new research and technology opportunities, including fundamental problems in mesoscopic superconductivity, scalable superconducting electronics, and new topological states of matter. One route towards topological matter is by coupling a 2D electron gas with strong spin–orbit interaction to an s-wave superconductor. Previous efforts along these lines have been adversely affected by interface disorder and unstable gating. Here we show measurements on a gateable InGaAs/InAs 2DEG with patterned epitaxial Al, yielding devices with atomically pristine interfaces between semiconductor and superconductor. Using surface gates to form a quantum point contact (QPC), we find a hard superconducting gap in the tunnelling regime. When the QPC is in the open regime, we observe a first conductance plateau at 4e2/h, consistent with theory. The hard-gap semiconductor–superconductor system demonstrated here is amenable to top-down processing and provides a new avenue towards low-dissipation electronics and topological quantum systems.

We present gate-dependent transport measurements of Kondo impurities in bare gold break junctions, generated with high yield using an electromigration process that is actively controlled. Thirty percent of measured devices show zero-bias conductance peaks. Temperature dependence suggests Kondo temperatures approximately 7 K. The peak splitting in magnetic field is consistent with theoretical predictions for g = 2, though in many devices the splitting is offset from 2g mu(B)B by a fixed energy. The Kondo resonances observed here may be due to atomic-scale metallic grains formed during electromigration.