arXiv subjects

E. P. A. M. Bakkers

Publications and source records attributed to E. P. A. M. Bakkers.

At least 37 records · Page 2Linked to original sources

arXiv2018

Spin-orbit interaction and induced superconductivity in an one-dimensional hole gas

Low dimensional semiconducting structures with strong spin-orbit interaction (SOI) and induced superconductivity attracted much interest in the search for topological superconductors. Both the strong SOI and hard superconducting gap are directly related to the topological protection of the predicted Majorana bound states. Here we explore the one-dimensional hole gas in germanium silicon (Ge-Si) core-shell nanowires (NWs) as a new material candidate for creating a topological superconductor. Fitting multiple Andreev reflection measurements shows that the NW has two transport channels only, underlining its one-dimensionality. Furthermore, we find anisotropy of the Lande g-factor, that, combined with band structure calculations, provides us qualitative evidence for direct Rashba SOI and a strong orbital effect of the magnetic field. Finally, a hard superconducting gap is found in the tunneling regime, and the open regime, where we use the Kondo peak as a new tool to gauge the quality of the superconducting gap.

cond-mat.mes-hall↗

arXiv2018

Mirage Andreev spectra generated by mesoscopic leads in nanowire quantum dots

We study transport mediated by Andreev bound states formed in InSb nanowire quantum dots. Two kinds of superconducting source and drain contacts are used: epitaxial Al/InSb devices exhibit a doubling of tunneling resonances, while in NbTiN/InSb devices Andreev spectra of the dot appear to be replicated multiple times at increasing source-drain bias voltages. In both devices, a mirage of a crowded spectrum is created. To describe the observations a model is developed that combines the effects of a soft induced gap and of additional Andreev bound states both in the quantum dot and in the finite regions of the nanowire adjacent to the quantum dot. Understanding of Andreev spectroscopy is important for the correct interpretation of Majorana experiments done on the same structures.

cond-mat.mes-hall↗

arXiv2018

Single, double, and triple quantum dots in Ge/Si nanowires

We report highly tunable control of holes in Ge/Si core/shell nanowires (NWs). We demonstrate the ability to create single quantum dots (QDs) of various sizes, with low hole occupation numbers and clearly observable excited states. For the smallest dot size we observe indications of single-hole occupation. Moreover, we create double and triple tunnel-coupled quantum dot arrays. In the double quantum dot configuration we observe Pauli spin blockade (PSB). These results open the way to perform hole spin qubit experiments in these devices.

cond-mat.mes-hall↗

arXiv2018

Split-Channel Ballistic Transport in an InSb Nanowire

We report an experimental study of one-dimensional (1D) electronic transport in an InSb semiconducting nanowire. Three bottom gates are used to locally deplete the nanowire creating a ballistic quantum point contact with only a few conducting channels. In a magnetic field, the Zeeman splitting of the corresponding 1D subbands is revealed by the emergence of conductance plateaus at multiples of $e^2$/h, yet we find a quantized conductance pattern largely dependent on the configuration of voltages applied to the bottom gates. In particular, we can make the first plateau disappear leaving a first conductance step of 2$e^2/h$, which is indicative of a remarkable two-fold subband degeneracy that can persist up to several Tesla. For certain gate voltage settings, we also observe the presence of discrete resonant states producing conductance features that can resemble those expected from the opening of a helical gap in the subband structure. We explain our experimental findings through the formation of two spatially separated 1D conduction channels.

cond-mat.mes-hall↗

arXiv2018

Realizing Majorana zero modes in superconductor-semiconductor heterostructures

Realizing topological superconductivity and Majorana zero modes in the laboratory is one of the major goals in condensed matter physics. We review the current status of this rapidly-developing field, focusing on semiconductor-superconductor proposals for topological superconductivity. Material science progress and robust signatures of Majorana zero modes in recent experiments are discussed. After a brief introduction to the subject, we outline several next-generation experiments probing exotic properties of Majorana zero modes, including fusion rules and non-Abelian exchange statistics. Finally, we discuss prospects for implementing Majorana-based topological quantum computation in these systems.

cond-mat.supr-con↗

arXiv2016

Hybrid Superconductor-Quantum Point Contact Devices using InSb Nanowires

Proposals for studying topological superconductivity and Majorana bound states in nanowires proximity coupled to superconductors require that transport in the nanowire is ballistic. Previous work on hybrid nanowire-superconductor systems has shown evidence for Majorana bound states, but these experiments were also marked by disorder, which disrupts ballistic transport. In this letter, we demonstrate ballistic transport in InSb nanowires interfaced directly with superconducting Al by observing quantized conductance at zero-magnetic field. Additionally, we demonstrate that the nanowire is proximity coupled to the superconducting contacts by observing Andreev reflection. These results are important steps for robustly establishing topological superconductivity in InSb nanowires.

cond-mat.mes-hall↗

arXiv2016

Josephson $φ_{0}$-junction in nanowire quantum dots

The Josephson effect describes supercurrent flowing through a junction connecting two superconducting leads by a thin barrier [1]. This current is driven by a superconducting phase difference $ϕ$ between the leads. In the presence of chiral and time reversal symmetry of the Cooper pair tunneling process [2] the current is strictly zero when $ϕ$ vanishes. Only if these underlying symmetries are broken the supercurrent for $ϕ=0$ may be finite [3-5]. This corresponds to a ground state of the junction being offset by a phase $ϕ_{0}$, different from 0 or $π$. Here, we report such a Josephson $ϕ_{0}$-junction based on a nanowire quantum dot. We use a quantum interferometer device in order to investigate phase offsets and demonstrate that $ϕ_{0}$ can be controlled by electrostatic gating. Our results have possible far reaching implications for superconducting flux and phase defined quantum bits as well as for exploring topological superconductivity in quantum dot systems.

cond-mat.mes-hall↗

arXiv2015

Spin-orbit interaction in InSb nanowires

We use magnetoconductance measurements in dual-gated InSb nanowire devices together with a theoretical analysis of weak antilocalization to accurately extract spin-orbit strength. In particular, we show that magnetoconductance in our three-dimensional wires is very different compared to wires in two-dimensional electron gases. We obtain a large Rashba spin-orbit strength of $0.5 -1\,\text{eVÅ}$ corresponding to a spin-orbit energy of $0.25-1\,\text{meV}$. These values underline the potential of InSb nanowires in the study of Majorana fermions in hybrid semiconductor-superconductor devices.

cond-mat.mes-hall↗

arXiv2015

Realization of microwave quantum circuits using hybrid superconducting-semiconducting nanowire Josephson elements

We report the realization of quantum microwave circuits using hybrid superconductor-semiconductor Josephson elements comprised of InAs nanowires contacted by NbTiN. Capacitively-shunted single elements behave as transmon qubits with electrically tunable transition frequencies. Two-element circuits also exhibit transmon-like behavior near zero applied flux, but behave as flux qubits at half the flux quantum, where non-sinusoidal current-phase relations in the elements produce a double-well Josephson potential. These hybrid Josephson elements are promising for applications requiring microwave superconducting circuits operating in magnetic field.

cond-mat.mes-hall↗

arXiv2013

Harnessing nuclear spin polarization fluctuations in a semiconductor nanowire

Soon after the first measurements of nuclear magnetic resonance (NMR) in a condensed matter system, Bloch predicted the presence of statistical fluctuations proportional to $1/\sqrt{N}$ in the polarization of an ensemble of $N$ spins. First observed by Sleator et al., so-called "spin noise" has recently emerged as a critical ingredient in nanometer-scale magnetic resonance imaging (nanoMRI). This prominence is a direct result of MRI resolution improving to better than 100 nm^3, a size-scale in which statistical spin fluctuations begin to dominate the polarization dynamics. We demonstrate a technique that creates spin order in nanometer-scale ensembles of nuclear spins by harnessing these fluctuations to produce polarizations both larger and narrower than the natural thermal distribution. We focus on ensembles containing ~10^6 phosphorus and hydrogen spins associated with single InP and GaP nanowires (NWs) and their hydrogen-containing adsorbate layers. We monitor, control, and capture fluctuations in the ensemble's spin polarization in real-time and store them for extended periods. This selective capture of large polarization fluctuations may provide a route for enhancing the weak magnetic signals produced by nanometer-scale volumes of nuclear spins. The scheme may also prove useful for initializing the nuclear hyperfine field of electron spin qubits in the solid-state.

cond-mat.mes-hall↗

arXiv2013

Electrical control over single hole spins in nanowire quantum dots

Single electron spins in semiconductor quantum dots (QDs) are a versatile platform for quantum information processing, however controlling decoherence remains a considerable challenge. Recently, hole spins have emerged as a promising alternative. Holes in III-V semiconductors have unique properties, such as strong spin-orbit interaction and weak coupling to nuclear spins, and therefore have potential for enhanced spin control and longer coherence times. Weaker hyperfine interaction has already been reported in self-assembled quantum dots using quantum optics techniques. However, challenging fabrication has so far kept the promise of hole-spin-based electronic devices out of reach in conventional III-V heterostructures. Here, we report gate-tuneable hole quantum dots formed in InSb nanowires. Using these devices we demonstrate Pauli spin blockade and electrical control of single hole spins. The devices are fully tuneable between hole and electron QDs, enabling direct comparison between the hyperfine interaction strengths, g-factors and spin blockade anisotropies in the two regimes.

cond-mat.mes-hall↗

arXiv2012

Fast spin-orbit qubit in an indium antimonide nanowire

Due to the strong spin-orbit interaction in indium antimonide, orbital motion and spin are no longer separated. This enables fast manipulation of qubit states by means of microwave electric fields. We report Rabi oscillation frequencies exceeding 100 MHz for spin-orbit qubits in InSb nanowires. Individual qubits can be selectively addressed due to intrinsic dierences in their g-factors. Based on Ramsey fringe measurements, we extract a coherence time T_2* = 8 +/- 1 ns at a driving frequency of 18.65 GHz. Applying a Hahn echo sequence extends this coherence time to 35 ns.

cond-mat.mes-hall↗

arXiv2012

Suppression of Zeeman gradients by nuclear polarization in double quantum dots

We use electric dipole spin resonance to measure dynamic nuclear polarization in InAs nanowire quantum dots. The resonance shifts in frequency when the system transitions between metastable high and low current states, indicating the presence of nuclear polarization. We propose that the low and the high current states correspond to different total Zeeman energy gradients between the two quantum dots. In the low current state, dynamic nuclear polarization efficiently compensates the Zeeman gradient due to the $g$-factor mismatch, resulting in a suppressed total Zeeman gradient. We present a theoretical model of electron-nuclear feedback that demonstrates a fixed point in nuclear polarization for nearly equal Zeeman splittings in the two dots and predicts a narrowed hyperfine gradient distribution.

cond-mat.mes-hall↗

arXiv2012

Spectroscopy of spin-orbit quantum bits in indium antimonide nanowires

Double quantum dot in the few-electron regime is achieved using local gating in an InSb nanowire. The spectrum of two-electron eigenstates is investigated using electric dipole spin resonance. Singlet-triplet level repulsion caused by spin-orbit interaction is observed. The size and the anisotropy of singlet-triplet repulsion are used to determine the magnitude and the orientation of the spin-orbit effective field in an InSb nanowire double dot. The obtained results are confirmed using spin blockade leakage current anisotropy and transport spectroscopy of individual quantum dots.

cond-mat.mes-hall↗

arXiv2012

Signatures of Majorana fermions in hybrid superconductor-semiconductor nanowire devices

Majorana fermions are particles identical to their own antiparticles. They have been theoretically predicted to exist in topological superconductors. We report electrical measurements on InSb nanowires contacted with one normal (Au) and one superconducting electrode (NbTiN). Gate voltages vary electron density and define a tunnel barrier between normal and superconducting contacts. In the presence of magnetic fields of order 100 mT we observe bound, mid-gap states at zero bias voltage. These bound states remain fixed to zero bias even when magnetic fields and gate voltages are changed over considerable ranges. Our observations support the hypothesis of Majorana fermions in nanowires coupled to superconductors.

cond-mat.mes-hall↗

arXiv2011

Measurement of g-factor tensor in a quantum dot and disentanglement of exciton spins

We perform polarization-resolved magneto-optical measurements on single InAsP quantum dots embedded in an InP nanowire. In order to determine all elements of the electron and hole $g$-factor tensors, we measure in magnetic field with different orientations. The results of these measurements are in good agreement with a model based on exchange terms and Zeeman interaction. In our experiment, polarization analysis delivers a powerful tool that not only significantly increases the precision of the measurements, but also enables us to probe the exciton spin state evolution in magnetic fields. We propose a disentangling scheme of heavy-hole exciton spins enabling a measurement of the electron spin $T_2$ time.

cond-mat.mtrl-sci↗

arXiv2010

Spin-orbit qubit in a semiconductor nanowire

Motion of electrons can influence their spins through a fundamental effect called spin-orbit interaction. This interaction provides a way to electrically control spins and as such lies at the foundation of spintronics. Even at the level of single electrons, spin-orbit interaction has proven promising for coherent spin rotations. Here we report a spin-orbit quantum bit implemented in an InAs nanowire, where spin-orbit interaction is so strong that spin and motion can no longer be separated. In this regime we realize fast qubit rotations and universal single qubit control using only electric fields. We enhance coherence by dynamically decoupling the qubit from the environment. Our qubits are individually addressable: they are hosted in single-electron quantum dots, each of which has a different Landé g-factor. The demonstration of a nanowire qubit opens ways to harness the advantages of nanowires for use in quantum computing. Nanowires can serve as one-dimensional templates for scalable qubit registers. Unique to nanowires is the possibility to easily vary the material even during wire growth. Such flexibility can be used to design wires with suppressed decoherence and push semiconductor qubit fidelities towards error-correction levels. Furthermore, electrical dots can be integrated with optical dots in p-n junction nanowires. The coherence times achieved here are sufficient for the conversion of an electronic qubit into a photon, the flying qubit, for long-distance quantum communication.

cond-mat.mes-hall↗

arXiv2010

Disentangling the effects of spin-orbit and hyperfine interactions on spin blockade

We have achieved the few-electron regime in InAs nanowire double quantum dots. Spin blockade is observed for the first two half-filled orbitals, where the transport cycle is interrupted by forbidden transitions between triplet and singlet states. Partial lifting of spin blockade is explained by spin-orbit and hyperfine mechanisms that enable triplet to singlet transitions. The measurements over a wide range of interdot coupling and tunneling rates to the leads are well reproduced by a simple transport model. This allows us to separate and quantify the contributions of the spin-orbit and hyperfine interactions.

cond-mat.mes-hall↗