arXiv subjects

J. Nygard

Publications and source records attributed to J. Nygard.

18 recordsLinked to original sources

arXiv2026

Ultrastrong Coupling and Coherent Dynamics in a Gate-Tunable Transmon Qubit

Ultrastrong light-matter coupling (USC) gives access to exotic quantum phenomena and promises faster quantum gates, yet coherent time-domain control in this regime remains largely unexplored. Here, we realize USC in a hybrid system consisting of an InAs nanowire-based gatemon qubit coupled to a superconducting resonator. Spectroscopy reveals an avoided crossing that cannot be captured by the Jaynes-Cummings (JC) model, as well as photon-number-dependent transitions whose energies deviate markedly from the JC ladder expected in the strong coupling regime. Beyond demonstrating USC, we achieve time-resolved coherent control of the qubit and measure coherence times comparable to gatemons operating outside the USC regime. These results establish that hybrid semiconductor-superconductor qubits can retain coherent control in USC and provide a platform for exploring quantum dynamics and device concepts in this regime.

cond-mat.mes-hall↗

arXiv2025

Anomalous metallic phase and reduced critical current in superconducting nanowires due to inverse proximity effect

Superconductor-to-metal transitions (SMTs) are key probes of mesoscopic superconductivity, but their interpretation can be complicated by device geometry and measurement conditions. Here, we study epitaxial InAs-Al nanowires and show that metallic contacts induce an inverse proximity effect (IPE), creating weak spots in the superconductor that strongly suppress the critical current and give rise to an anomalous metallic phase. Using transport measurements supported by Usadel theory, we demonstrate that this phase originates from the contact-induced weakening of superconductivity together with Joule heating, rather than intrinsic material properties. Our findings reveal an overlooked observer effect in mesoscopic superconductors and provide essential guidance for interpreting SMTs and for designing devices based on these systems.

cond-mat.mes-hall↗

arXiv2023

Integrated bioelectronic proton-gated logic elements utilizing nanoscale patterned Nafion

A central endeavour in bioelectronics is the development of logic elements to transduce and process ionic to electronic signals. Motivated by this challenge, we report fully monolithic, nanoscale logic elements featuring n- and p-type nanowires as electronic channels that are proton-gated by electron-beam patterned Nafion. We demonstrate inverter circuits with state-of-the-art ion-to-electron transduction performance giving DC gain exceeding 5 and frequency response up to 2 kHz. A key innovation facilitating the logic integration is a new electron-beam process for patterning Nafion with linewidths down to 125 nm. This process delivers feature sizes compatible with low voltage, fast switching elements. This expands the scope for Nafion as a versatile patternable high-proton-conductivity element for bioelectronics and other applications requiring nanoengineered protonic membranes and electrodes.

physics.app-ph↗

arXiv2021

Microwave susceptibility observation of interacting many-body Andreev states

Electrostatic charging affects the many-body spectrum of Andreev states, yet its influence on their microwave properties has not been elucidated. We developed a circuit quantum electrodynamics probe that, in addition to transition spectroscopy, measures the microwave susceptibility of different states of a semiconductor nanowire weak link with a single dominant (spin-degenerate) Andreev level. We found that the microwave susceptibility does not exhibit a particle-hole symmetry, which we qualitatively explain as an influence of Coulomb interaction. Moreover, our state-selective measurement reveals a large, $π$-phase shifted contribution to the response common to all many-body states which can be interpreted as arising from a phase-dependent continuum in the superconducting density of states.

cond-mat.mes-hall↗

arXiv2018

p-GaAs nanowire MESFETs with near-thermal limit gating

Difficulties in obtaining high-performance p-type transistors and gate insulator charge-trapping effects present two major challenges for III-V complementary metal-oxide semiconductor (CMOS) electronics. We report a p-GaAs nanowire metal-semiconductor field-effect transistor (MESFET) that eliminates the need for a gate insulator by exploiting the Schottky barrier at the metal-GaAs interface. Our device beats the best-performing p-GaSb nanowire metal-oxide-semiconductor field effect transistor (MOSFET), giving a typical sub-threshold swing of 62 mV/dec, within 4% of the thermal limit, on-off ratio $\sim 10^{5}$, on-resistance ~700 k$Ω$, contact resistance ~30 k$Ω$, peak transconductance 1.2 $μ$S/$μ$m and high-fidelity ac operation at frequencies up to 10 kHz. The device consists of a GaAs nanowire with an undoped core and heavily Be-doped shell. We carefully etch back the nanowire at the gate locations to obtain Schottky-barrier insulated gates whilst leaving the doped shell intact at the contacts to obtain low contact resistance. Our device opens a path to all-GaAs nanowire MESFET complementary circuits with simplified fabrication and improved performance.

cond-mat.mes-hall↗

arXiv2017

Evolution of Nanowire Transmons and Their Quantum Coherence in Magnetic Field

We present an experimental study of nanowire transmons at zero and applied in-plane magnetic field. With Josephson non-linearities provided by the nanowires, our qubits operate at higher magnetic fields than standard transmons. Nanowire transmons exhibit coherence up to 70 mT, where the induced superconducting gap in the nanowire closes. We demonstrate that on-chip charge noise coupling to the Josephson energy plays a dominant role in the qubit dephasing. This takes the form of strongly-coupled two-level systems switching on 100 ms timescales and a more weakly coupled background producing $1/f$ noise. Several observations, including the field dependence of qubit energy relaxation and dephasing, are not fully understood, inviting further experimental investigation and theory. Using nanowires with a thinner superconducting shell will enable operation of these circuits up to 0.5 T, a regime relevant for topological quantum computation.

quant-ph↗

arXiv2016

InAs nanowire with epitaxial aluminium as a single-electron transistor with fixed tunnel barriers

We report on fabrication of single-electron transistors using InAs nanowires with epitaxial aluminium with fixed tunnel barriers made of aluminium oxide. The devices exhibit a hard superconducting gap induced by the proximized aluminium cover shell and they behave as metallic single-electron transistors. In contrast to the typical few channel contacts in semiconducting devices, our approach forms opaque multichannel contacts to a semiconducting wire and thus provides a complementary way to study them. In addition, we confirm that unwanted extra quantum dots can appear at the surface of the nanowire. Their presence is prevented in our devices, and also by inserting a protective layer of GaAs between the InAs and Al, the latter being suitable for standard measurement methods.

cond-mat.mes-hall↗

arXiv2016

Raman spectroscopy and electrical properties of InAs nanowires with local oxidation enabled by substrate micro-trenches and laser irradiation

The thermal gradient along indium-arsenide nanowires was engineered by a combination of fabricated micro- trenches in the supporting substrate and focused laser irradiation. This allowed local control of thermally activated oxidation reactions of the nanowire on the scale of the diffraction limit. The locality of the oxidation was detected by micro-Raman mapping, and the results were found consistent with numerical simulations of the temperature profile. Applying the technique to nanowires in electrical devices the locally oxidized nanowires remained conducting with a lower conductance as expected for an effectively thinner conducting core.

physics.optics↗

arXiv2015

A Semiconductor Nanowire-Based Superconducting Qubit

We introduce a hybrid qubit based on a semiconductor nanowire with an epitaxially grown superconductor layer. Josephson energy of the transmon-like device ("gatemon") is controlled by an electrostatic gate that depletes carriers in a semiconducting weak link region. Strong coupling to an on-chip microwave cavity and coherent qubit control via gate voltage pulses is demonstrated, yielding reasonably long relaxation times (0.8 μs) and dephasing times (1 μs), exceeding gate operation times by two orders of magnitude, in these first-generation devices. Because qubit control relies on voltages rather than fluxes, dissipation in resistive control lines is reduced, screening reduces crosstalk, and the absence of flux control allows operation in a magnetic field, relevant for topological quantum information.

cond-mat.mes-hall↗

arXiv2015

Parity lifetime of bound states in a proximitized semiconductor nanowire

Quasiparticle excitations can compromise the performance of superconducting devices, causing high frequency dissipation, decoherence in Josephson qubits, and braiding errors in proposed Majorana-based topological quantum computers. Quasiparticle dynamics have been studied in detail in metallic superconductors but remain relatively unexplored in semiconductor-superconductor structures, which are now being intensely pursued in the context of topological superconductivity. To this end, we introduce a new physical system comprised of a gate-confined semiconductor nanowire with an epitaxially grown superconductor layer, yielding an isolated, proximitized nanowire segment. We identify Andreev-like bound states in the semiconductor via bias spectroscopy, determine the characteristic temperatures and magnetic fields for quasiparticle excitations, and extract a parity lifetime (poisoning time) of the bound state in the semiconductor exceeding 10 ms.

cond-mat.mes-hall↗

arXiv2012

Tunneling Spectroscopy of Quasiparticle Bound States in a Spinful Josephson Junction

The spectrum of a segment of InAs nanowire, confined between two superconducting leads, was measured as function of gate voltage and superconducting phase difference using a third normal-metal tunnel probe. Sub-gap resonances for odd electron occupancy---interpreted as bound states involving a confined electron and a quasiparticle from the superconducting leads, reminiscent of Yu-Shiba-Rusinov states---evolve into Kondo-related resonances at higher magnetic fields. An additional zero bias peak of unknown origin is observed to coexist with the quasiparticle bound states.

cond-mat.mes-hall↗

arXiv2010

Ferromagnetic proximity effect in F-QDot-S device

Ferromagnetic proximity effect is studied in InAs nanowire (NW) based quantum dots (QD) strongly coupled to a ferromagnetic (F) and a superconducting (S) lead. The influence of the F lead is detected through the splitting of the spin-1/2 Kondo resonance. We show that the F lead induces a local exchange field on the QD, which has varying amplitude and a sign depending on the charge states. The interplay of the F and S correlations generates an exchange field related supgap feature. This novel mini-gap allows now the visualization of the exchange field also in even charge states

cond-mat.mes-hall↗

arXiv2008

Giant fluctuations and gate control of the g-factor in InAs Nanowire Quantum Dots

We study the g-factor of discrete electron states in InAs nanowire based quantum dots. The g values are determined from the magnetic field splitting of the zero bias anomaly due to the spin 1/2-Kondo effect. Unlike to previous studies based on 2DEG quantum dots, the g-factors of neighboring electron states show a surprisingly large fluctuation: g can scatter between 2 and 18. Furthermore electric gate tunability of the g-factor is demonstrated.

cond-mat.mes-hall↗

arXiv2006

Nonequilibrium Singlet-Triplet Kondo Effect in Carbon Nanotubes

The Kondo-effect is a many-body phenomenon arising due to conduction electrons scattering off a localized spin. Coherent spin-flip scattering off such a quantum impurity correlates the conduction electrons and at low temperature this leads to a zero-bias conductance anomaly. This has become a common signature in bias-spectroscopy of single-electron transistors, observed in GaAs quantum dots as well as in various single-molecule transistors. While the zero-bias Kondo effect is well established it remains uncertain to what extent Kondo correlations persist in non-equilibrium situations where inelastic processes induce decoherence. Here we report on a pronounced conductance peak observed at finite bias-voltage in a carbon nanotube quantum dot in the spin singlet ground state. We explain this finite-bias conductance anomaly by a nonequilibrium Kondo-effect involving excitations into a spin triplet state. Excellent agreement between calculated and measured nonlinear conductance is obtained, thus strongly supporting the correlated nature of this nonequilibrium resonance.

cond-mat.str-el↗

arXiv2004

Zero-field splitting of Kondo resonances in a carbon nanotube quantum dot

We present low-temperature electron transport measurements on a single-wall carbon nanotube quantum dot exhibiting Kondo resonances at low temperature. Contrary to the usual behavior for the spin-1/2 Kondo effect we find that the temperature dependence of the zero bias conductance is nonmonotonic. In nonlinear transport measurements low-energy splittings of the Kondo resonances are observed at zero magnetic field. We suggest that these anomalies reflect interactions between the nanotube and a magnetic (catalyst) particle. The nanotube device may effectively act as a ferromagnetically contacted Kondo dot.

cond-mat.mes-hall↗

arXiv2000

Bias and temperature dependence of the 0.7 conductance anomaly in Quantum Point Contacts

The 0.7 (2e^2/h) conductance anomaly is studied in strongly confined, etched GaAs/GaAlAs quantum point contacts, by measuring the differential conductance as a function of source-drain and gate bias as well as a function of temperature. We investigate in detail how, for a given gate voltage, the differential conductance depends on the finite bias voltage and find a so-called self-gating effect, which we correct for. The 0.7 anomaly at zero bias is found to evolve smoothly into a conductance plateau at 0.85 (2e^2/h) at finite bias. Varying the gate voltage the transition between the 1.0 and the 0.85 (2e^2/h) plateaus occurs for definite bias voltages, which defines a gate voltage dependent energy difference $Δ$. This energy difference is compared with the activation temperature T_a extracted from the experimentally observed activated behavior of the 0.7 anomaly at low bias. We find Δ= k_B T_a which lends support to the idea that the conductance anomaly is due to transmission through two conduction channels, of which the one with its subband edge Δbelow the chemical potential becomes thermally depopulated as the temperature is increased.

cond-mat.mes-hall↗

arXiv1998

Activated Behavior of the 0.7 2(e^2)/h Conductance Anomaly in Quantum Point Contacts

The 0.7 conductance anomaly in the quantized conductance of trench etched GaAs quantum point contacts is studied experimentally. The temperature dependence of the anomaly measured with vanishing source-drain bias reveals the same activated behavior as reported earlier for top-gated structures. Our main result is that the zero bias, high temperature 0.7 anomaly found in activation measurements and the finite bias, low temperature 0.9 anomaly found in transport spectroscopy have the same origin: a density dependent excitation gap.

cond-mat.mes-hall↗

arXiv1998

Temperature dependence of the ``0.7'' 2(e^2)/h quasi plateau in strongly confined quantum point contacts

We present new results of the ``0.7'' 2(e^2)/h structure or quasi plateau in some of the most strongly confined point contacts so far reported. This strong confinement is obtained by a combination of shallow etching and metal gate deposition on modulation doped GaAs/GaAlAs heterostructures. The resulting subband separations are up to 20 meV, and as a consequence the quantized conductance can be followed at temperatures up to 30 K, an order of magnitude higher than in conventional split gate devices. We observe pronounced quasi plateaus at several of the lowest conductance steps all the way from their formation around 1 K to 30 K, where the entire conductance quantization is smeared out thermally. We study the deviation of the conductance from ideal integer quantization as a function of temperature, and we find an activated behavior, exp(-T_a/T), with a density dependent activation temperature T_a of the order of 2 K. We analyze our results in terms of a simple theoretical model involving scattering against plasmons in the constriction.

cond-mat.mes-hall↗