arXiv subjects

Dominik M. Zumbühl

Publications and source records attributed to Dominik M. Zumbühl.

At least 19 recordsLinked to original sources

arXiv2026

Discrete states and ballistic interference in quantum wires approaching macroscopic lengths

Increasing the size of a system showing quantum effects is a difficult task limited by decoherence, a diminishing quantum level spacing, and the effects of disorder spoiling the quantum behavior when growing in size. Systems in 1D offer very strong confinement in the transverse directions, thus generally enhancing quantum effects, but are notoriously sensitive to disorder. In this work, we present a system of 1D electrons exhibiting discrete quantum levels and fully ballistic coherent quantum interference with lengths of up to 18\,$μ$m. Tunneling spectroscopy between two parallel quantum wires with a central gated segment shows intricate interference patterns exhibiting several different periods in magnetic field and density. An analysis over three different wire lengths and a comparison with single particle numerical simulations without any free parameters remarkably explains the full pattern including the observed periods. Therefore, these wires are essentially ideal 1D systems with aspect ratios approaching 1'000. In addition, at low bias, we also observe not only the Coulomb charging energies but can clearly resolve the discrete orbital and spin states in up to 10\,$μ$m long wires when filling 100 electrons with the center gate. This is made visible by a state-of-the-art low temperature and low noise measurement system. The spin filling sequence is completely regular, strictly alternating spin up and down, avoiding high spin states, while the peak conductance is modulated in accordance with the previously discussed interference patterns. These striking results show that single particle Schrödinger quantum mechanics such as ballistic quantum interference and discrete quantum states may be observed, under the right conditions, in systems of up to 18\,$μ$m length, thus approaching macroscopic sizes.

cond-mat.mes-hall↗

arXiv2026

High-Q cryogenic surface acoustic wave resonators in the GHz range

Surface acoustic wave (SAW) resonators provide a compact platform for confining microwave-frequency phonons and are widely used in radio-frequency technologies, but their operation at gigahertz frequencies and cryogenic temperatures remains challenging. In this regime, conventional design rules do not directly apply, and achieving high-quality acoustic confinement requires careful consideration about geometry and loss mechanisms. Here, we present a systematic experimental study of SAW resonators on gallium arsenide, a platform of particular interest for hybrid quantum devices but comparatively unexplored for high-Q SAW cavities. By varying key design parameters such as cavity length, wavelength, and crystal orientation, we study resonator performance and achieve quality factors up to 28000 in the gigahertz range. In addition, we introduce mesa steps within the acoustic cavity, mimicking realistic device architectures and providing insight into scattering processes and additional dissipation channels. Our results establish practical design guidelines for GaAs-based SAW resonators and support their development as a scalable platform for quantum acoustics and phonon-mediated hybrid systems.

cond-mat.mes-hall↗

arXiv2026

Compact self-matched gyrators using edge magnetoplasmons

Edge magnetoplasmons provide a natural platform for chiral electrodynamics, where broken time-reversal symmetry enforces unidirectional propagation. When probed at microwave frequencies, they offer a route to compact non-reciprocal devices. So far, implementations have suffered from large losses or required complicated matching networks. Here we show that the circulating modes coupled to capacitive gates give rise to a gyrator response, characterized by directional π phase difference between forward and reverse transmission. By engineering a three-terminal capacitive geometry, we realize a self-impedance matched gyrator in which the gyration points coincide with transmission maxima, enabling nearly lossless gyration without external matching networks. Our devices are implemented on a GaAs 2D gas, operate from 0.2 to 2 GHz, tuned by magnetic field, with sub-millimeter footprints and insertion loss as low as 2 dB. This is a factor of 100 smaller and less lossy than commercial and plasmon units, respectively. A dissipative model, in agreement with experiment, provides the fundamental physics and delivers the key materials parameters, leading the way to even less lossy devices approaching ideal operation by materials improvement. The self-impedance matched concept is broadly applicable to a variety of devices, thus providing a foundation for a new generation of high-quality microwave plasmon technology.

cond-mat.mes-hall↗

arXiv2026

Quantifying Strain and its Effect on Charge Transport in Ge/Si Core/Shell Nanowires

Strain engineering in semiconductor nanostructures offers a promising route to optimize electronic and optical properties for advanced quantum technologies. This study explores the relationship between core and shell thicknesses and strain distribution in Ge/Si core/shell nanowires, targeting their application as hosts for spin qubits. Nanowires were synthesized using an Au-catalyzed chemical vapor deposition technique, achieving control over core and shell dimensions. High-resolution transmission electron microscopy and elemental mapping confirmed structural integrity, while Geometric Phase Analysis and Raman spectroscopy provided quantitative insights into strain variations driven by core and shell dimensions. Furthermore, polarization resolved $μ$-Raman measurements allowed us to quantify the longitudinal and transverse phonon mode splitting as a function of strain in the Ge core. The strain dependent electronic properties were investigated by hole mobility measurements. Finally, we observe a record high hole mobility of 25,500 cm$^2$V$^{-1}$s$^{-1}$, underscoring the potential of these core/shell nanowire structures for the realization of high-fidelity spin qubits. Our findings highlight the critical role of geometry in strain tuning and provide valuable design guidelines for optimizing Ge/Si nanowires in scalable quantum device architectures.

cond-mat.mes-hall↗

arXiv2026

A scalable non-superconducting tunnel junction technology

Tunnel junctions are one of the key elements of chip-scale microsystems serving various technologies from classical microelectronics to quantum information. Aluminium and its oxide (AlOx) have dominated cryogenic tunnel junction technology for decades due to the high quality of AlOx barriers and Al superconducting properties below 1.2 K. However, many applications require non-superconducting junctions, either standalone or in combination with superconducting technology, motivating efforts to suppress Al superconductivity through magnetic fields, doping, or proximity effects -- approaches that so far suffered from integration compatibility and scalability issues. Here, we present a CMOS-compatible normal-metal tunnel junction technology based on TiW alloy and AlOx barriers. We demonstrate wafer-scale fabrication of TiW/Al-AlOx/TiW junctions and validate their performance in Coulomb blockade thermometers operating down to 20 mK, confirming robust normal-state behavior. This TiW-based architecture offers a scalable solution for non-superconducting tunnel junctions across a broad temperature range, enabling integration into advanced cryogenic, quantum and nanoelectronic chip-level systems.

cond-mat.mes-hall↗

arXiv2026

Magnetic Field-Enhanced Graphene Superconductivity with Record Pauli-Limit Violation

Spin-polarized superconductors offer a rare platform for studying electronic correlations, but few candidate systems have been experimentally confirmed to date. Here, we report the observation of a spin-polarized superconducting state, denoted SC5, in WSe2-proximitized rhombohedral trilayer graphene. At in-plane magnetic field B|| = 0 T, SC5 has a critical temperature of 68 mK and an out-of-plane critical magnetic field of only 12 mT. Surprisingly, these values are significantly enhanced as B|| increases, and the superconductivity persists to B|| = 8.8 T. This value corresponds to a record-high Pauli-limit violation ratio of at least 80 among all superconductors, while the true critical field is beyond the limit of our instrument. We conclude that SC5 experiences a canting crossover from Ising-type to spin-polarized superconductor with increased B||.

cond-mat.supr-con↗

arXiv2025

In-situ control of hole-spin driving mechanisms

Hole-spin qubits enable fast, all-electrical spin manipulation through electric-dipole spin resonance (EDSR), arising from two microscopic mechanisms rooted in their intrinsically strong spin-orbit interaction. Depending on how the electric field acts on the quantum dot, the spin can be driven either by a modulation of its g-factor or by a displacement of the wavefunction. Here, we demonstrate in-situ control over the dominant EDSR driving mechanism of a hole-spin qubit in a silicon fin field-effect transistor by applying microwave signals to two different gate electrodes, thereby tuning the orientation of the local electric field. We measure the effective g-factor, its electrical tunability, and the Rabi frequency as functions of magnetic-field orientation. Their distinct angular dependencies, analyzed using a g-matrix formalism, allow us to identify the underlying driving processes and track their relative contributions for different drive configurations. By selecting the drive electrode, we can switch from a regime dominated by g-factor modulation to one with a strong contribution from wavefunction displacement. This in-situ tunability provides direct experimental access to both spin-driving mechanisms and offers a route toward optimized spin-qubit performance.

cond-mat.mes-hall↗

arXiv2025

Family of Unconventional Superconductivities in Crystalline Graphene

Unconventional superconductors exhibit multiple broken symmetries and exceed the range of the Bardeen-Cooper-Schrieffer (BCS) theory. For instance, time-reversal symmetry can be broken in addition to the gauge symmetry, resulting in superconductors that can be enhanced or induced by a magnetic field. However, such unconventional superconductivities are more vulnerable to impurities than their BCS counterparts, requiring highly ordered and clean material systems to observe them. Crystalline rhombohedral multilayer graphene is a promising platform to explore unconventional superconductivity due to its superior material quality and gate-tunable strong correlation effects. Here we report transport measurements of rhombohedral tetralayer and pentalayer graphene, where a spectrum of superconductivities in a clean limit are observed. Three of them (SC2-4) show highly unusual enhancements by magnetic fields: 1. SC2 is strengthened by an in-plane field; 2. SC3 is boosted by a small out-of-plane field; 3. SC4 is induced by an in-plane field. All these superconductors are robust against an in-plane field up to 8.5 Tesla, exceeding the Pauli limit of conventional superconductors by tens of times and suggesting their unconventional nature. Moreover, we observed that proximitized spin-orbit coupling generates a plethora of new superconductors in the phase diagram, while maintaining the high quality of bare rhombohedral graphene. Our work establishes a family of new superconductors in rhombohedral multilayer graphene, which also provides an ideal platform to engineer non-Abelian quasiparticles by proximitizing with quantum anomalous Hall states existing in the same material system.

cond-mat.mes-hall↗

arXiv2025

Compromise-Free Scaling of Qubit Speed and Coherence

Across leading qubit platforms, a common trade-off persists: increasing coherence comes at the cost of operational speed, reflecting the notion that protecting a qubit from its noisy surroundings also limits control over it. This speed-coherence dilemma limits qubit performance across various technologies. Here, we demonstrate a hole spin qubit in a Ge/Si core/shell nanowire that triples its Rabi frequency while simultaneously quadrupling its Hahn-echo coherence time, boosting the Q-factor by over an order of magnitude. This is enabled by the direct Rashba spin-orbit interaction, emerging from heavy-hole-light-hole mixing through strong confinement in two dimensions. Tuning a gate voltage causes this interaction to peak, providing maximum drive speed and a point where the qubit is optimally protected from charge noise, allowing speed and coherence to scale together. Our proof-of-concept shows that careful dot design can overcome a long-standing limitation, offering a new approach towards building high-performance, fault-tolerant qubits.

cond-mat.mes-hall↗

arXiv2025

Dominant end-tunneling effect in two distinct Luttinger liquids coexisting in one quantum wire

Luttinger liquids occupy a special place in physics as the most understood case of essentially quantum many-body systems. The experimental mission of measuring its main prediction, power laws in observable quantities, has already produced a body of exponents in different semiconductor and metallic structures. Here, we combine tunneling spectroscopy with density-dependent transport measurements in the same quantum wires over more than two orders of magnitude in temperature to very low electron temperatures down to $\sim$40 mK. This reveals that, when the second 1D subband becomes populated, the temperature dependence splits into two ranges with different exponents in the power-law dependence of the conductance, both dominated by the finite-size effect of the end-tunneling process. This result demonstrates the importance of measuring the Luttinger parameters as well as the number of modes independently through spectroscopy in addition to the transport exponent in the characterization of Luttinger liquids. This opens a new pathway to unambiguous interpretation of the exponents observed in quantum wires.

cond-mat.str-el↗

arXiv2025

Edge State Selective Measurement of Quantum Hall Dispersions

Edge states reflect the key physical properties yet are difficult to probe individually, particularly when several states are present at an edge. We present momentum resolved tunneling spectroscopy between a quantum well and a quantum wire to extract the dispersions of the quantum Hall edge states. Momentum and energy selective tunneling allows to separately address the different states even if they are spatially overlapping. This delivers the edge state velocities over broad ranges of magnetic field and density, in excellent agreement with a hard-wall model. This technique provides a basis for future edge state selective spectroscopy on quantum materials.

cond-mat.mes-hall↗

arXiv2025

A direct dispersive signature of Pauli spin blockade

Pauli Spin Blockade (PSB) is a key paradigm in semiconductor nanostructures and gives access to the spin physics. We report the direct observation of PSB with gate-dispersive reflectometry on double quantum dots with source-drain bias. The reservoir charge transitions are strongly modulated, turning on and off when entering and leaving the blockaded region, consistent with a simple model. Seen with holes in Ge and Si, the effects are enhanced with larger bias voltage and suppressed by magnetic field. This work lays the foundation for fast probing of spin physics and minimally invasive spin readout.

cond-mat.mes-hall↗

arXiv2025

Finite-bias Coulomb blockade thermometry

Coulomb blockade thermometers (CBTs) are versatile and, in principle, primary thermometers operating down to the micro-Kelvin range but bias heating spoils the thermometry and the primary mode. Here, we introduce a method to extract the CBT electron temperature in the presence of heat created by an arbitrary bias voltage, and without assumptions on the heat flow mechanisms. The charging energy is extracted with high precision and without any other knowledge, thus making true primary thermometry possible. The experiment also reveals a subtle dependence of the charging energy on phonon temperature below 100 mK likely due to the amorphous AlO$_x$ tunnel junctions.

physics.app-ph↗

arXiv2025

Signatures of Chiral Superconductivity in Rhombohedral Graphene

Chiral superconductors are unconventional superconducting states that break time reversal symmetry spontaneously and typically feature Cooper pairing at non-zero angular momentum. Such states may host Majorana fermions and provide an important platform for topological physics research and fault-tolerant quantum computing. Despite intensive search and prolonged studies of several candidate systems, chiral superconductivity has remained elusive so far. Here we report the discovery of robust unconventional superconductivity in rhombohedral tetra- and penta-layer graphene in the absence of moiré superlattice effects. We observed two superconducting states in the gate-induced flat conduction bands with Tc up to 300 mK and charge density ne as low as 2.4*1011 cm-2 in three tetralayer and two pentalayer devices. Spontaneous time-reversal-symmetry-breaking (TRSB) due to electron's orbital motion is found, and several observations indicate the chiral nature of these superconducting states, including: 1. In the superconducting state, Rxx shows magnetic hysteresis in varying out-of-plane magnetic field B, which is absent from all other superconductors; 2. the superconducting states are immune to in-plane magnetic field and are developed within a spin- and valley-polarized quarter-metal phase; 3. the normal states show anomalous Hall signals at zero magnetic field and magnetic hysteresis. We also observed a critical B of up to 1.4 Tesla, higher than any graphene superconductivity reported so far and indicates a strong-coupling superconductivity close to the BCS-BEC crossover. Our observations establish a pure carbon material for the study of topological superconductivity, and pave the way to explore Majorana modes and topological quantum computing.

cond-mat.mes-hall↗

arXiv2024

All-electrical operation of a spin qubit coupled to a high-Q resonator

Building a practical quantum processor involves integrating millions of physical qubits along with the necessary components for individual qubit manipulation and readout. Arrays of gated silicon spins offer a promising route toward achieving this goal. Optimized radio frequency resonators with high internal quality factor are based on superconducting inductors and enable fast spin readout. All-electrical spin control and gate-dispersive readout remove the need for additional device components and simplify scaling. However, superconducting high-Q tank circuits are susceptible to crosstalk induced ringup from electrical qubit control pulses, which causes fluctuations of the quantum dot potential and is suspected to degrade qubit performance. Here, we report on the coherent and all-electrical control of a hole spin qubit at 1.5K, integrated into a silicon fin field-effect transistor and connected to a niobium nitride nanowire inductor gate-sensor. Our experiments show that qubit control pulses with their broad range of higher harmonics ring up the tank when the control pulse spectrum overlaps with the tank resonance. This can cause a reduction of the readout visibility if the tank ringing amplitude exceeds the excited state splitting of the quantum dot, lifting Pauli spin blockade and thus leading to state preparation and measurement errors. We demonstrate how to circumvent these effects by engineering control pulses around the tank resonances. Importantly, we find that the ringup does not limit the spin coherence time, indicating that efficient high-Q resonators in gate-sensing are compatible with all-electrical spin control.

cond-mat.mes-hall↗

arXiv2023

Cryogenic hyperabrupt strontium titanate varactors for sensitive reflectometry of quantum dots

Radio frequency reflectometry techniques enable high bandwidth readout of semiconductor quantum dots. Careful impedance matching of the resonant circuit is required to achieve high sensitivity, which however proves challenging at cryogenic temperatures. Gallium arsenide-based voltage-tunable capacitors, so-called varactor diodes, can be used for in-situ tuning of the circuit impedance but deteriorate and fail at temperatures below 10 K and in magnetic fields. Here, we investigate a varactor based on strontium titanate with hyperabrupt capacitance-voltage characteristic, that is, a capacitance tunability similar to the best gallium arsenide-based devices. The varactor design introduced here is compact, scalable and easy to wirebond with an accessible capacitance range from 45 pF to 3.2 pF. We tune a resonant inductor-capacitor circuit to perfect impedance matching and observe robust, temperature and field independent matching down to 11 mK and up to 2 T in-plane field. Finally, we perform gate-dispersive charge sensing on a germanium/silicon core/shell nanowire hole double quantum dot, paving the way towards gate-based single-shot spin readout. Our results bring small, magnetic field-resilient, highly tunable varactors to mK temperatures, expanding the toolbox of cryo-radio frequency applications.

cond-mat.mes-hall↗

arXiv2023

Identifying Pauli spin blockade using deep learning

Pauli spin blockade (PSB) can be employed as a great resource for spin qubit initialisation and readout even at elevated temperatures but it can be difficult to identify. We present a machine learning algorithm capable of automatically identifying PSB using charge transport measurements. The scarcity of PSB data is circumvented by training the algorithm with simulated data and by using cross-device validation. We demonstrate our approach on a silicon field-effect transistor device and report an accuracy of 96% on different test devices, giving evidence that the approach is robust to device variability. The approach is expected to be employable across all types of quantum dot devices.

cond-mat.mes-hall↗

arXiv2023

Quantum oscillations in 2D electron gases with spin-orbit and Zeeman interactions

Shubnikov-de Haas (SdH) oscillations have served as a paradigmatic experimental probe and tool for extracting key semiconductor parameters such as carrier density, effective mass, Zeeman splitting with g-factor $g^*$, quantum scattering times and spin-orbit (SO) coupling parameters. Here, we derive for the first time an analytical formulation for the SdH oscillations in 2D electron gases (2DEGs) with simultaneous Rashba, Dresselhaus, and Zeeman interactions. Our analytical and numerical calculations allow us to extract both Rashba and Dresselhaus SO coupling parameters, carrier density, quantum lifetimes, and also to understand the role of higher harmonics in the SdH oscillations. More importantly, we derive a simple condition for the vanishing of SO induced SdH beatings for all harmonics in 2DEGs: $α/β= [(1-\tilde Δ)/(1+\tilde Δ)]^{1/2}$, where $\tilde Δ$ is a material parameter given by the ratio of the Zeeman and Landau level splitting. We also predict beatings in the higher harmonics of the SdH oscillations and elucidate the inequivalence of the SdH response of Rashba-dominated ($α>β$) vs Dresselhaus-dominated ($α<β$) 2DEGs in semiconductors with substantial $g^*$. We find excellent agreement with recent available experimental data of Dettwiler ${\it et\thinspace al.}$ Phys. Rev. X $\textbf{7}$, 031010 (2017), and Beukman ${\it et\thinspace al.}$, Phys. Rev. B $\textbf{96}$, 241401 (2017).

cond-mat.mes-hall↗