9 research outputs found
Parity transitions in the superconducting ground state of hybrid InSb-Al Coulomb islands
No full textThe number of electrons in small metallic or semiconducting islands is quantised. When tunnelling is enabled via opaque barriers this number can change by an integer. In superconductors the addition is in units of two electron charges (2e), reflecting that the Cooper pair condensate must have an even parity. This ground state (GS) is foundational for all superconducting qubit devices. Here, we study a hybrid superconducting-semiconducting island and find three typical GS evolutions in a parallel magnetic field: a robust 2e-periodic even-parity GS, a transition to a 2e-periodic odd-parity GS, and a transition from a 2e- to a 1e-periodic GS. The 2e-periodic odd-parity GS persistent in gate-voltage occurs when a spin-resolved subgap state crosses zero energy. For our 1e-periodic GSs we explicitly show the origin being a single zero-energy state gapped from the continuum, i.e., compatible with an Andreev bound states stabilized at zero energy or the presence of Majorana zero modes.QRD/Kouwenhoven LabApplied SciencesQRD/Geresdi LabQN/Bakkers La
Observation of Conductance Quantization in InSb Nanowire Networks
No full textMajorana zero modes (MZMs) are prime candidates for robust topological quantum bits, holding a great promise for quantum computing. Semiconducting nanowires with strong spin orbit coupling offer a promising platform to harness one-dimensional electron transport for Majorana physics. Demonstrating the topological nature of MZMs relies on braiding, accomplished by moving MZMs around each other in a certain sequence. Most of the proposed Majorana braiding circuits require nanowire networks with minimal disorder. Here, the electronic transport across a junction between two merged InSb nanowires is studied to investigate how disordered these nanowire networks are. Conductance quantization plateaus are observed in most of the contact pairs of the epitaxial InSb nanowire networks: the hallmark of ballistic transport behavior.QRD/Kouwenhoven LabQN/Conesa-Boj LabQN/Bakkers La
Crossed InSb nanowire junctions for Majorana operations
No full textIn this work we report on recent advances in the fabrication and characterization of crossed InSb nanowires. The yield of crystalline nanowire crosses has been increased by growing the wires on 111 facets created in 100-oriented InP substrates by wet chemical etching. Ebeam lithography on the tilted facets has been developed to precisely control the position of the catalysts particles, crucial for an optimized crossing process. With transmission electron microscopy we investigate the crystalline quality of the wire-wire interface. Low-temperature transport studies show quantized conductance across the junction indicating the high quality of the merged nanowires
Quantized Majorana conductance
No full textMajorana zero-modes - a type of localized quasiparticle - hold great promise for topological quantum computing. Tunnelling spectroscopy in electrical transport is the primary tool for identifying the presence of Majorana zero-modes, for instance as a zero-bias peak in differential conductance. The height of the Majorana zero-bias peak is predicted to be quantized at the universal conductance value of 2e 2 /h at zero temperature (where e is the charge of an electron and h is the Planck constant), as a direct consequence of the famous Majorana symmetry in which a particle is its own antiparticle. The Majorana symmetry protects the quantization against disorder, interactions and variations in the tunnel coupling. Previous experiments, however, have mostly shown zero-bias peaks much smaller than 2e 2 /h, with a recent observation of a peak height close to 2e 2 /h. Here we report a quantized conductance plateau at 2e 2 /h in the zero-bias conductance measured in indium antimonide semiconductor nanowires covered with an aluminium superconducting shell. The height of our zero-bias peak remains constant despite changing parameters such as the magnetic field and tunnel coupling, indicating that it is a quantized conductance plateau. We distinguish this quantized Majorana peak from possible non-Majorana origins by investigating its robustness to electric and magnetic fields as well as its temperature dependence. The observation of a quantized conductance plateau strongly supports the existence of Majorana zero-modes in the system, consequently paving the way for future braiding experiments that could lead to topological quantum computing
Control of CO2 vibrational dynamics via shaped-pulse coherent anti-Stokes Raman spectroscopy
No full textThesis (PhD)--Stellenbosch University, 2017ENGLISH ABSTRACT : In this work we investigate the coherent control of carbon dioxide (CO2) vibrational dynamics
using Coherent anti-Stokes Raman Scattering (CARS). During CARS, vibrational modes are
excited via stimulated Raman scattering (SRS). Subsequently a narrowband probe field interacts
with the molecular ensemble providing not only information about the modes populated, but
also on the evolution of the wave-packet created during excitation. By spectrally shaping one of
the SRS pump fields the vibrational dynamics can be controlled. In this work it was assumed
that the pump pulse structure which will lead to a desired dynamics is unknown. To find that
structure, a learning algorithm was developed which utilizes a spatial light modulator (SLM) in
a 4f-optical con guration to spectrally shape the pump. Both a time-frequency representation of
the shaped pulse (called the von Neumann basis) and a standard Fourier domain representation
were bench-marked during optimization of a second harmonic generation (SHG) signal in a BBO
crystal to ascertain which will suit the optimization problem best in terms of convergence rate
and parameter space size. It was found that the von Neumann basis converged faster than the
standard Fourier domain representation while still operating on a larger parameter space and
therefore it was used in all subsequent work. In addition, we developed a quantum mechanical
theoretical model of the CARS process to ensure proper understanding of our measurements. We
demonstrated experimentally that mode excitation selectivity can be achieved using the pump fields extracted by the learning algorithm, and we explore the underlying selectivity mechanisms.
Control of the relative phase of oscillation of different vibrational modes is also observed. Our
work demonstrates coherent quantum control of all relevant aspects of the molecular vibrational
dynamics of CO2.AFRIKAANSE OPSOMMING : In hierdie werk ondersoek ons die koherente beheer van koolstofdioksied (CO2) vibrasionele
dinamika met behulp van koherente anti-Stokes Raman verstrooiing(KARV). Gedurende KARV
word vibrasionele modusse opgewek deur middel van gestimuleerde Raman verstrooiing (GRV).
Vervolgens meet n nou-bandwydte meet puls die molekulêre toestand asook die tydsontwikkeling
van die golf-pakkie wat geskep is tydens opwekking. Deur een van die GRV velde spektraal te
vervorm kan die vibrasionele dinamika beheer word. In hierdie werk is aanvaar dat die pomp
puls struktuur wat sal lei tot 'n gewenste dinamika onbekend is. Om daardie struktuur te vind,
word n leer algoritme ontwikkel wat n ruimtelike lig modulator (RLM) in 'n 4f-optiese opstelling
gebruik om die pomp te vervorm. Beide 'n tyd-frekwensie voorstelling van die gevormde veld
(bekend as die von Neumann basis) en 'n standaard Fourier voorstelling was getoets gedurende
optimering van 'n tweede harmoniese opwekking (THO) in 'n BBO kristal om vas te stel wat
die optimering probleem die beste sal pas in terme van konvergensie koers en parameter ruimte
grootte. Daar is bevind dat die von Neumann basis vinniger konvergeer as die standaard Fourier
verteenwoordiging terwyl dit op 'n groter parameter ruimte werk en is dus gebruik in alle werk
wat daarop volg. Daarbenewens het ons 'n kwantummeganiese teoretiese model van die proses
ontwikkel om behoorlike begrip van ons metings te verseker. Ons demonstreer eksperimenteel
dat modus opwekking selektief gedoen kan word met behulp van die pomp velde verkry vanaf
die leer algoritme, en ons ondersoek die onderliggende selektiwiteit meganismes. Beheer van die
relatiewe fase van ossillasie van verskillende vibrasionele modusse is ook waargeneem. Ons werk
toon kwantum beheer van alle relevante aspekte van die molekulêre vibrasionele dinamika van
CO2.Doctora
InSb nanowires with built-in GaxIn1-xSb tunnel barriers for Majorana devices
No full textMajorana zero modes (MZMs), prime candidates for topological quantum bits, are detected as zero bias conductance peaks (ZBPs) in tunneling spectroscopy measurements. Implementation of a narrow and high tunnel barrier in the next generation of Majorana devices can help to achieve the theoretically predicted quantized height of the ZBP. We propose a material-oriented approach to engineer a sharp and narrow tunnel barrier by synthesizing a thin axial segment of GaxIn1–xSb within an InSb nanowire. By varying the precursor molar fraction and the growth time, we accurately control the composition and the length of the barriers. The height and the width of the GaxIn1–xSb tunnel barrier are extracted from the Wentzel–Kramers-Brillouin (WKB) fits to the experimental I–V traces
Electric field tunable superconductor-semiconductor coupling in Majorana nanowires
No full textWe study the effect of external electric fields on superconductor-semiconductor coupling by measuring the electron transport in InSb semiconductor nanowires coupled to an epitaxially grown Al superconductor. We find that the gate voltage induced electric fields can greatly modify the coupling strength, which has consequences for the proximity induced superconducting gap, effective g-factor, and spin-orbit coupling, which all play a key role in understanding Majorana physics. We further show that level repulsion due to spin-orbit coupling in a finite size system can lead to seemingly stable zero bias conductance peaks, which mimic the behavior of Majorana zero modes. Our results improve the understanding of realistic Majorana nanowire systems
Mirage Andreev Spectra Generated by Mesoscopic Leads in Nanowire Quantum Dots
No full textWe 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 structuresS. M. F. is supported by NSF DMR-1743972, ONR, and ARO. C. J. P. and S.M. F. acknowledge NSF PIRE-1743717. R. A., P.S-J., and E. P. acknowledge support from
the Spanish Ministry of Economy and Competitiveness through Grants No. FIS2015-64654-P (MINECO/FEDER), No. FIS2015-65706-P (MINECO/FEDER), and No. FIS2016-80434-P (AEI/FEDER, EU), and the Ramón y Cajal programme Grant No. RYC-2011-09345. E. J. H. L. acknowledges ERC Grant No. 716559, the Maria de Maeztu programme for Units of Excellence in R&D (MDM-2014–0377), and the Ramón y Cajal programme
(RYC-2015-17973
Epitaxy of advanced nanowire quantum devices
No full textSemiconductor nanowires are ideal for realizing various low-dimensional quantum devices. In particular, topological phases of matter hosting non-Abelian quasiparticles (such as anyons) can emerge when a semiconductor nanowire with strong spin-orbit coupling is brought into contact with a superconductor. To exploit the potential of non-Abelian anyons - which are key elements of topological quantum computing - fully, they need to be exchanged in a well-controlled braiding operation. Essential hardware for braiding is a network of crystalline nanowires coupled to superconducting islands. Here we demonstrate a technique for generic bottom-up synthesis of complex quantum devices with a special focus on nanowire networks with a predefined number of superconducting islands. Structural analysis confirms the high crystalline quality of the nanowire junctions, as well as an epitaxial superconductor-semiconductor interface. Quantum transport measurements of nanowire 'hashtags' reveal Aharonov-Bohm and weak-antilocalization effects, indicating a phase-coherent system with strong spin-orbit coupling. In addition, a proximity-induced hard superconducting gap (with vanishing sub-gap conductance) is demonstrated in these hybrid superconductor-semiconductor nanowires, highlighting the successful materials development necessary for a first braiding experiment. Our approach opens up new avenues for the realization of epitaxial three-dimensional quantum architectures which have the potential to become key components of various quantum devices
