The superconducting proximity effect has been the focus of significant research efforts over many years and has recently attracted renewed interest as the basis of topologically nontrivial states in materials with a large spin orbit interaction, with protected boundary states useful for quantum information technologies. However, spectroscopy of these states is challenging because of the limited spatial and energetic control of conventional tunnel barriers.Here, we report electronic spectroscopy measurements of the proximity gap in a semiconducting indium arsenide (InAs) nanowire (NW) segment coupled to a superconductor (SC), using a spatially separated quantum dot (QD) formed deterministically during the crystal growth. We extract the characteristic parameters describing the proximity gap which is suppressed for lower electron densities and fully developed for larger ones. This gate-tunable transition of the proximity effect can be understood as a transition from the long to the short junction regime of subgap bound states in the NW segment. Our device architecture opens up the way to systematic, unambiguous spectroscopy studies of subgap bound states, such as 1 arXiv:1812.06850v1 [cond-mat.mes-hall]

We report on the fabrication and electrical characterization of an InAs double-nanowire (NW) device consisting of two closely placed parallel NWs coupled to a common superconducting electrode on one side and individual normal metal leads on the other. In this new type of device we detect Cooper-pair splitting (CPS) with a sizeable efficiency of correlated currents in both NWs. In contrast to earlier experiments, where CPS was realized in a single NW, demonstrating an intrawire electron pairing mediated by the superconductor (SC), our experiment demonstrates an interwire interaction mediated by the common SC. The latter is the key for the realization of zero-magnetic field Majorana bound states, or Parafermions; in NWs and therefore constitutes a milestone towards topological superconductivity. In addition, we observe transport resonances that occur only in the superconducting state, which we tentatively attribute to Andreev bound states and/or Yu-Shiba resonances that form in the proximitized section of one NW.

Using angle-resolved photoelectron spectroscopy we investigate the surface electronic structure of the threedimensional topological insulator (TI) Sb2Te3(0001). Our data show the presence of a topological surface state in the bulk energy gap with the Dirac-point located above the Fermi level. The adsorption of Cs-atoms on Sb2Te3(0001) gives rise to a downward energy shift of the electronic valence band states which saturates at a value of ∼200 meV. For the saturation coverage the Dirac-point of the linearly dispersive surface state resides in close proximity to the Fermi level. The electronic structure of the Cs/Sb2Te3 interface therefore considerably deviates from previously studied metal-TI interfaces based on the isostructural compound Bi2Se3 which points to the importance of atomic composition in these hetero systems.Three-dimensional topological insulators (TIs) are currently generating widespread scientific interest in the condensed matter physics community as the distinct topology of their bulk band structure provokes the existence of robust metallic surface states with unique physical properties.1,2 The surface states locally span the global energy gap in the electronic excitation spectrum of the bulk material and their existence is protected by time-reversal symmetry.3,4 A salient feature of topological surface states (TSSs) lies in their characteristic spin structure introduced by spin-orbit coupling which locks the spin to the direction perpendicular to the wave vector.5 As a consequence of this spin structure the backscattering of the surface state electrons from non-magnetic impurities is strongly suppressed.6 Currently, most research on TIs is devoted to the chalcogenide semiconductors Bi 2 Se 3 7,8 and Bi 2 Te 3 8,9 , related ternary compounds 10,11 as well as to HgTe quantum wells. 12 The TSSs of these materials show a particularly simple dispersion consisting of a single spin-polarized Dirac-cone.The experimental realization of the most appealing properties of TSSs that have been predicted so far will require interface structures between TIs and metal films. This holds for example for the topological magnetoelectric effect at the interface of a TI and a ferromagnet 13,14 as well as for Majorana fermions at the interface of a TI and a superconductor. 15 It is therefore important to investigate the influence of metallic adlayers on the electronic structure of TI surfaces. Surfacesensitive spectroscopic techniques and in particular angleresolved photoelectron spectroscopy (ARPES), which also played a key role in the discovery of TIs 1 , are suitable methods to study modifications in the electronic structure during the formation of interfaces 16 . Indeed, great experimental effort on the basis of ARPES is currently directed towards an improved understanding of the influence of adsorbates on the electronic structure of TI surfaces. [17][18][19][20][21][22][23] However, most of these works have focused on the TI surface Bi 2 Se 3 (0001). It therefore appears essential to expand the present investigati...

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Sub-gap states in semiconducting-superconducting nanowire hybrid devices are controversially discussed as potential topologically non-trivial quantum states. One source of ambiguity is the lack of an energetically and spatially well defined tunnel spectrometer. Here, we use quantum dots directly integrated into the nanowire during the growth process to perform tunnel spectroscopy of discrete sub-gap states in a long nanowire segment. In addition to sub-gap states with a standard magnetic field dependence, we find topologically trivial sub-gap states that are independent of the external magnetic field, i.e. that are pinned to a constant energy as a function of field. We explain this effect qualitatively and quantitatively by taking into account the strong spin-orbit interaction in the nanowire, which can lead to a decoupling of Andreev bound states from the field due to a spatial spin texture of the confined eigenstates.

We present a comprehensive electrical characterization of an InAs/InP nanowire (NW) heterostructure, comprising of two InP barriers forming a quantum dot (QD), two adjacent lead segments and two metallic contacts. We demonstrate how to extract valuable quantitative information of the QD. The QD shows very regular Coulomb blockade resonances over a large gate voltage range. By analyzing the resonance line shapes, we map the evolution of the tunnel couplings from the few to the many electron regime, with electrically tunable tunnel couplings from <1 μeV to >600 μeV, and a transition from the temperature to the lifetime broadened regime. The InP segments form tunnel barriers with almost fully symmetric tunnel couplings and a barrier height of ∼350 meV. All of these findings can be understood in great detail based on the deterministic material composition and geometry. Our results demonstrate that integrated InAs/InP QDs provide a promising platform for electron tunneling spectroscopy in InAs NWs, which can readily be contacted by a variety of superconducting materials to investigate subgap states in proximitized NW regions, or be used to characterize thermoelectric nanoscale devices in the quantum regime.

Ternary quantum information processing in superconducting devices poses a promising alternative to its more popular binary counterpart through larger, more connected computational spaces and proposed advantages in quantum simulation and error correction. Although generally operated as qubits, transmons have readily addressable higher levels, making them natural candidates for operation as quantum three-level systems (qutrits). Recent works in transmon devices have realized high fidelity single qutrit operation. Nonetheless, effectively engineering a high-fidelity two-qutrit entanglement remains a central challenge for realizing qutrit processing in a transmon device. In this work, we apply the differential AC Stark shift to implement a flexible, microwave-activated, and dynamic cross-Kerr entanglement between two fixed-frequency transmon qutrits, expanding on work performed for the ZZ interaction with transmon qubits. We then use this interaction to engineer efficient, high-fidelity qutrit CZ† and CZ gates, with estimated process fidelities of 97.3(1)% and 95.2(3)% respectively, a significant step forward for operating qutrits on a multi-transmon device.

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