Showing 1–29 of 29 results for author: Kanne, T

Probing Synthetic Caroli-de Gennes-Matricon States Through Critical Current in Full-Shell Nanowire Josephson Junctions

Authors: Carlos Payá, Ángel Ibabe, Mario Gómez, Thomas Kanne, Jesper Nygård, Ramón Aguado, Pablo San-Jose, Eduardo J. H. Lee, Elsa Prada

Abstract: Full-shell hybrid nanowires consisting of a semiconductor core fully enveloped by a superconducting shell have emerged as a platform to study Caroli-de Gennes-Matricon (CdGM) analogs. These subgap states can be considered a synthetic version of CdGM states in Abrikosov vortices. Unlike conventional CdGM states, these analogs exhibit a level spacing comparable to the superconducting gap, making the… ▽ More Full-shell hybrid nanowires consisting of a semiconductor core fully enveloped by a superconducting shell have emerged as a platform to study Caroli-de Gennes-Matricon (CdGM) analogs. These subgap states can be considered a synthetic version of CdGM states in Abrikosov vortices. Unlike conventional CdGM states, these analogs exhibit a level spacing comparable to the superconducting gap, making them readily observable via tunneling spectroscopy techniques. The spectral density of CdGM analogs follows a characteristic skewed pattern as a function of applied axial magnetic field, an effect that is superimposed on the Little-Parks oscillations of the shell's gap induced by fluxoid quantization. Here, we provide experimental evidence for CdGM analogs through a distinctive skewness fingerprint in the critical current and zero-bias resistance of overdamped Josephson junctions based on full-shell nanowires. △ Less

Submitted 20 July, 2026; originally announced July 2026.

Comments: 5 pages (main) + 2 pages (End Matter) + 4 pages (Supp. Mat.). 3 + 6 figures

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

Authors: I. Casal Iglesias, F. J. Matute-Cañadas, G. O. Steffensen, A. Ibabe, L. Splitthoff, T. Kanne, J. Nygard, V. Rollano, D. Granados, A. Gomez, R. Aguado, A. Levy Yeyati, E. J. H. Lee

Abstract: 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 Jayne… ▽ More 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. △ Less

Submitted 19 March, 2026; originally announced March 2026.

Comments: 7 pages, 6 figures

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

Authors: G. M. Oliveira, G. O. Steffensen, I. Casal Iglesias, M. Gomez, A. Ibabe, T. Kanne, J. Nygard, R. Aguado, A. Levy Yeyati, E. J. H. Lee

Abstract: 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 t… ▽ More 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. △ Less

Submitted 2 December, 2025; originally announced December 2025.

Nanoscale photonic neuron with biological signal processing

Authors: Joachim E. Sestoft, Thomas K. Jensen, Vidar Flodgren, Abhijit Das, Rasmus D. Schlosser, David Alcer, Mariia Lamers, Thomas Kanne, Magnus T. Borgström, Jesper Nygård, Anders Mikkelsen

Abstract: Computational hardware designed to mimic biological neural networks holds the promise to resolve the drastically growing global energy demand of artificial intelligence. A wide variety of hardware concepts have been proposed, and among these, photonic approaches offer immense strengths in terms of power efficiency, speed and synaptic connectivity. However, existing solutions have large circuit foo… ▽ More Computational hardware designed to mimic biological neural networks holds the promise to resolve the drastically growing global energy demand of artificial intelligence. A wide variety of hardware concepts have been proposed, and among these, photonic approaches offer immense strengths in terms of power efficiency, speed and synaptic connectivity. However, existing solutions have large circuit footprints limiting scaling potential and they miss key biological functions, like inhibition. We demonstrate an artificial nano-optoelectronic neuron with a circuit footprint size reduced by at least a factor of 100 compared to existing technologies and operating powers in the picowatt regime. The neuron can deterministically receive both exciting and inhibiting signals that can be summed and treated with a non-linear function. It demonstrates several biological relevant responses and memory timescales, as well as weighting of input channels. The neuron is compatible with commercial silicon technology, operates at multiple wavelengths and can be used for both computing and optical sensing. This work paves the way for two important research paths: photonic neuromorphic computing with nanosized footprints and low power consumption, and adaptive optical sensing, using the same architecture as a compact, modular front end △ Less

Submitted 12 September, 2025; v1 submitted 8 September, 2025; originally announced September 2025.

Report number: NBI QDEV 2025

Synthetic spin-orbit coupling in superconductor-semiconductor hybrid nanowires with micromagnet arrays

Authors: M. P. Hynes, D. Burke, K. Ganesh, A. Vekris, B. J. Villis, J. C. Gartside, T. Kanne, J. Nygård, K. Moors, W. R. Branford, M. R. Connolly, M. R. Buitelaar

Abstract: Spin-orbit interaction accounts for the coupling of momentum and spin degrees of freedom of electrons and holes in semiconductor materials. In quantum information processing, it allows for electrical control of spin states and for the engineering of topologically protected Majorana zero modes. Although such functionalities were previously considered to be limited to semiconductor materials with st… ▽ More Spin-orbit interaction accounts for the coupling of momentum and spin degrees of freedom of electrons and holes in semiconductor materials. In quantum information processing, it allows for electrical control of spin states and for the engineering of topologically protected Majorana zero modes. Although such functionalities were previously considered to be limited to semiconductor materials with strong intrinsic spin-orbit interactions only, recent theoretical work proposes using external rotating magnetic fields to engineer synthetic spin-orbit coupling. This would relax material constraints and open up new research directions for materials with low intrinsic spin-orbit interaction or augment existing spin-orbit interaction in materials in which this interaction is already strong. Here we demonstrate the feasibility of this approach and introduce rotating magnetic fields along an InAs/Al hybrid nanowire using permalloy micromagnet arrays which yields an estimated synthetic Rashba spin-orbit interaction coefficient of 0.022 eV nm. We use transport spectroscopy and the energy dependence of Andreev bound states in the nanowires as a probe of the magnetic field profiles of the micromagnets which are reconfigurably prepared in parallel or antiparallel magnetization configurations. △ Less

Submitted 9 May, 2025; originally announced May 2025.

Multimode operation of a superconducting nanowire switch in the nanosecond regime

Authors: Zoltán Scherübl, Mátyás Kocsis, Tosson Elalaily, Lőrinc Kupás, Martin Berke, Gergő Fülöp, Thomas Kanne, Karl Berggren, Jesper Nygård, Szabolcs Csonka, Péter Makk

Abstract: Superconducting circuits are promising candidates for future computational architectures, however, practical applications require fast operation. Here, we demonstrate fast, gate-based switching of an Al nanowire-based superconducting switch in time-domain experiments. We apply voltage pulses on the gate while monitoring the microwave transmission of the device. Utilizing the usual leakage-based op… ▽ More Superconducting circuits are promising candidates for future computational architectures, however, practical applications require fast operation. Here, we demonstrate fast, gate-based switching of an Al nanowire-based superconducting switch in time-domain experiments. We apply voltage pulses on the gate while monitoring the microwave transmission of the device. Utilizing the usual leakage-based operation these measurements yield a fast, 1--2~ns switching time to the normal state, possibly limited by the bandwidth of our setup, and a 10--20~ns delay in the normal to superconducting transition. However, having a significant capacitance between the gate and the device allows for a novel operation, where the displacement current, induced by the fast gate pulses, drives the transition. The switching from superconducting to the normal state yields a similar fast timescale, while in the opposite direction the switching is significantly faster (4--6~ns) than the leakage based operation, which may be further improved by better thermal design. The measured short timescales and novel switching operation open the way for future fast and low-power-consumption applications. △ Less

Submitted 25 February, 2025; originally announced February 2025.

Comments: 10 pages, 6 figures

Journal ref: ACS Nano 19, 29207 (2025)

Microwave dynamics of gated Al/InAs superconducting nanowires

Authors: Vittorio Buccheri, François Joint, Kazi Rafsamjani Amin, Tosson Elalaily, Olivér Kürtössy, Zoltán Scherübl, Gergő Fülöp, Thomas Kanne, Jesper Nygård, Péter Makk, Szabolcs Csonka, Simone Gasparinetti

Abstract: Several experiments have recently reported on gate-tunable superconducting properties in metallic devices, holding promise for the realization of cryogenic switches, tunable resonators, and superconducting logic. In particular, the suppression of the critical current as a function of the gate voltage has been widely investigated. However, time-domain studies are discussed only in a few cases. In t… ▽ More Several experiments have recently reported on gate-tunable superconducting properties in metallic devices, holding promise for the realization of cryogenic switches, tunable resonators, and superconducting logic. In particular, the suppression of the critical current as a function of the gate voltage has been widely investigated. However, time-domain studies are discussed only in a few cases. In this paper, we present a microwave characterization of a gate-controlled Al-capped InAs nanowire embedded in a $λ/4$ coplanar waveguide resonator. We observe a shift in the resonator frequency and an increase in its internal losses as a function of the gate voltage, which we relate to a change in the imaginary and real components of the nanowire impedance, respectively. We demonstrate that these changes are described by the Mattis-Bardeen model with an effective temperature. We further study the resonator response to fast gate signals and measure characteristic response times of the order of 40 ns, both in time-domain and parametric modulation experiments. Our study elucidates the impact of the gate on the complex impedance of the nanowire in the superconducting state, as well as its dynamic performance, providing a foundation for the design of gate-controlled superconducting devices. △ Less

Submitted 25 February, 2025; originally announced February 2025.

Journal ref: Appl. Phys. Lett. 126, 232602 (2025)

Approaching the ultrastrong coupling regime between an Andreev level and a microwave resonator

Authors: O. O. Shvetsov, A. Khola, V. Buccheri, I. P. C. Cools, N. Trnjanin, T. Kanne, J. Nygård, A. Geresdi

Abstract: Josephson junctions formed in semiconductor nanowires host Andreev bound states and serve as a physical platform to realize Andreev qubits tuned by electrostatic gating. With the Andreev bound state being confined to the nanoscale weak link, it couples to a circuit-QED architecture via the state-dependent supercurrent flowing through the weak link. Thus, increasing this coupling strength is a cruc… ▽ More Josephson junctions formed in semiconductor nanowires host Andreev bound states and serve as a physical platform to realize Andreev qubits tuned by electrostatic gating. With the Andreev bound state being confined to the nanoscale weak link, it couples to a circuit-QED architecture via the state-dependent supercurrent flowing through the weak link. Thus, increasing this coupling strength is a crucial challenge for this architecture. Here, we demonstrate the fabrication and microwave characterization of a weak link which is defined in an InAs-Al (core-half shell) nanowire and embedded in a superconducting loop with a lumped-element resonator patterned from a thin NbTiN film with high kinetic inductance. We investigated several devices with various weak link lengths and performed spectroscopy that revealed pair transitions and single-quasiparticle transitions arising from spin-orbit split Andreev bound states. Our approach offers a compact geometry and a large resonator impedance above 12 k$Ω$ at a resonator frequency of 8 GHz, which facilitates large coupling in the system. For the pair transitions, the experimentally observed energy level splitting demonstrates the coupling to an Andreev level of 490 MHz. We apply a perturbative model that shows good agreement with the experimental data and extract the maximum coupling of 968~MHz. Moreover, we show that the coupling is even stronger to an Andreev level with a higher transmission. In addition, spectroscopy of single-quasiparticle transitions reveals spin-orbit split Andreev bound states with the extracted spin-photon coupling of 77 MHz. △ Less

Submitted 15 September, 2025; v1 submitted 13 February, 2025; originally announced February 2025.

Andreev spin relaxation time in a shadow-evaporated InAs weak link

Authors: Haoran Lu, David F. Bofill, Zhenhai Sun, Thomas Kanne, Jesper Nygård, Morten Kjaergaard, Valla Fatemi

Abstract: Andreev spin qubits are a new qubit platform that merges superconductivity with semiconductor physics. The mechanisms dominating observed energy relaxation remain unidentified. We report here on three steps taken to address these questions in an InAs nanowire weak link. First, we designed a microwave readout circuit tuned to be directly sensitive to the spin-dependent inductance of the weak link s… ▽ More Andreev spin qubits are a new qubit platform that merges superconductivity with semiconductor physics. The mechanisms dominating observed energy relaxation remain unidentified. We report here on three steps taken to address these questions in an InAs nanowire weak link. First, we designed a microwave readout circuit tuned to be directly sensitive to the spin-dependent inductance of the weak link so that higher orbital states are not necessary for readout -- this resulted in larger windows in parameter space in which the spin state properties can be probed. Second, we implemented a successful gap-engineering strategy to mitigate quasiparticle poisoning. Third, the weak link was fabricated by \textit{in situ} shadow evaporation, which has been shown to improve atomic-scale disorder. We show how our design allows characterization of the spin stability and coherence over the full range of magnetic flux and gate voltage of an odd parity bias point. The spin relaxation and dephasing rates are comparable with the best devices previously reported, suggestive that surface atomic-scale disorder and QP poisoning are not linked to spin relaxation in InAs nanowires. Our design strategies are transferrable to novel materials platforms for Andreev qubits such as germanium and carbon. △ Less

Submitted 20 January, 2025; originally announced January 2025.

Journal ref: Phys. Rev. Applied 24, 024046 (2025)

Heteroatomic Andreev molecule in a superconducting island-double quantum dot hybrid

Authors: Olivér Kürtössy, Mihály Bodócs, Cătălin Paşcu Moca, Zoltán Scherübl, Ella Nikodem, Thomas Kanne, Jesper Nygård, Gergely Zaránd, Péter Makk, Szabolcs Csonka

Abstract: Topological superconductors (SCs) hold great promise for fault-tolerant quantum hardware, however, their experimental realization is very challenging. Recently, superconducting artificial molecules (Andreev molecules) have opened new avenues to engineer topological superconducting materials. In this work, we demonstrate a heteroatomic Andreev molecule, where two normal artificial atoms realized by… ▽ More Topological superconductors (SCs) hold great promise for fault-tolerant quantum hardware, however, their experimental realization is very challenging. Recently, superconducting artificial molecules (Andreev molecules) have opened new avenues to engineer topological superconducting materials. In this work, we demonstrate a heteroatomic Andreev molecule, where two normal artificial atoms realized by quantum dots (QDs) are coupled by a superconducting island (SCI). We show that the two normal atoms strongly hybridize and form a 3-electron-based molecular state. Our density matrix renormalization group (DMRG) calculations explain quantitatively the robust binding of electrons. The tunability of the structure allows us to drive a quantum phase transition from an antiferromagnetic Andreev molecular state to a heteroatomic Andreev molecule with ferromagnetically coupled QDs using simple electrical gating. △ Less

Submitted 30 June, 2024; originally announced July 2024.

Switching dynamics in Al/InAs nanowire-based gate-controlled superconducting switch

Authors: Tosson Elalaily, Martin Berke, Ilari Lilja, Alexander Savin, Gergő Fülöp, Lőrinc Kupás, Thomas Kanne, Jesper Nygård, Péter Makk, Pertti Hakonen, Szabolcs Csonka

Abstract: The observation of the gate-controlled supercurrent (GCS) effect in superconducting nanostructures increased the hopes for realizing a superconducting equivalent of semiconductor field-effect transistors. However, recent works attribute this effect to various leakage-based scenarios, giving rise to a debate on its origin. A proper understanding of the microscopic process underlying the GCS effect… ▽ More The observation of the gate-controlled supercurrent (GCS) effect in superconducting nanostructures increased the hopes for realizing a superconducting equivalent of semiconductor field-effect transistors. However, recent works attribute this effect to various leakage-based scenarios, giving rise to a debate on its origin. A proper understanding of the microscopic process underlying the GCS effect and the relevant time scales would be beneficial to evaluate the possible applications. In this work, we observed gate-induced two-level fluctuations between the superconducting state and normal state in Al/InAs nanowires (NWs). Noise correlation measurements show a strong correlation with leakage current fluctuations. The time-domain measurements show that these fluctuations have Poissonian statistics. Our detailed analysis of the leakage current measurements reveals that it is consistent with the stress-induced leakage current (SILC), in which inelastic tunneling with phonon generation is the predominant transport mechanism. Our findings shed light on the microscopic origin of the GCS effect and give deeper insight into the switching dynamics of the superconducting NW under the influence of the strong gate voltage. △ Less

Submitted 28 May, 2024; v1 submitted 24 December, 2023; originally announced December 2023.

Heat dissipation mechanisms in hybrid superconductor-semiconductor devices revealed by Joule spectroscopy

Authors: Angel Ibabe, Gorm O. Steffensen, Ignacio Casal, Mario Gomez, Thomas Kanne, Jesper Nygard, Alfredo Levy Yeyati, Eduardo J. H. Lee

Abstract: Understanding heating and cooling mechanisms in mesoscopic superconductor-semiconductor hybrid devices is crucial for their application in quantum technologies. Owing to the poor thermal conductivity of typical devices, heating effects can drive superconducting-to-normal phase transitions even at low applied bias, observed as sharp conductance dips through the loss of Andreev excess currents. Trac… ▽ More Understanding heating and cooling mechanisms in mesoscopic superconductor-semiconductor hybrid devices is crucial for their application in quantum technologies. Owing to the poor thermal conductivity of typical devices, heating effects can drive superconducting-to-normal phase transitions even at low applied bias, observed as sharp conductance dips through the loss of Andreev excess currents. Tracking such dips across magnetic field, cryostat temperature, and applied microwave power, which constitutes Joule spectroscopy, allows to uncover the underlying cooling bottlenecks in different parts of a device. By applying this technique, we analyze heat dissipation in devices based on full-shell InAs-Al nanowires and reveal that superconducting islands are strongly susceptible to heating as their cooling is limited by the rather inefficient electron-phonon coupling, as opposed to grounded superconductors that primarily cool by quasiparticle diffusion. Our measurements indicate that powers as low as 50-150 pW are able to fully suprpress the superconductivity of an island. Finally, we show that applied microwaves lead to similar heating effects as DC signals, and explore the interplay of the microwave frequency and the effective electron-phonon relaxation time. △ Less

Submitted 22 November, 2023; originally announced November 2023.

Comments: 9 pages, 4 figures

Journal ref: Nano Lett. 24, 6488, 2024

Photon-mediated long range coupling of two Andreev level qubits

Authors: L. Y. Cheung, R. Haller, A. Kononov, C. Ciaccia, J. H. Ungerer, T. Kanne, J. Nygård, P. Winkel, T. Reisinger, I. M. Pop, A. Baumgartner, C. Schönenberger

Abstract: In a superconducting weak link, the supercurrent is carried by Andreev bound states (ABSs) formed by the phase-coherent reflection of electrons and their time-reversed partners. A single, highly transmissive ABS can serve as an ideal, compact two-level system, due to a potentially large energy difference to the next ABS. While the coherent manipulation of such Andreev levels qubits (ALQs) has been… ▽ More In a superconducting weak link, the supercurrent is carried by Andreev bound states (ABSs) formed by the phase-coherent reflection of electrons and their time-reversed partners. A single, highly transmissive ABS can serve as an ideal, compact two-level system, due to a potentially large energy difference to the next ABS. While the coherent manipulation of such Andreev levels qubits (ALQs) has been demonstrated, a long-range coupling between two ALQs, necessary for advanced qubit architectures, has not been achieved, yet. Here, we demonstrate a coherent remote coupling between two ALQs, mediated by a microwave photon in a novel superconducting microwave cavity coupler. The latter hosts two modes with different coupling rates to an external port. This allows us to perform fast readout of each qubit using the strongly coupled mode, while the weakly coupled mode is utilized to mediate the coupling between the qubits. When both qubits are tuned into resonance with the latter mode, we find excitation spectra with avoided-crossings, in very good agreement with the Tavis-Cummings model. Based on this model, we identify highly entangled two-qubit states for which the entanglement is mediated over a distance of six millimeters. This work establishes ALQs as compact and scalable solid-state qubits. △ Less

Submitted 24 October, 2023; originally announced October 2023.

Comments: 13 pages, 7 figures

Signatures of gate-driven out of equilibrium superconductivity in Ta/InAs nanowires

Authors: Tosson Elalaily, Martin Berke, Máté Kedves, Gergő Fülöp, Zoltán Scherübl, Thomas Kanne, Jesper Nygård, Péter Makk, Szabolcs Csonka

Abstract: Understanding the microscopic origin of the gate-controlled supercurrent (GCS) in superconducting nanobridges is crucial for engineering superconducting switches suitable for a variety of electronic applications. The origin of GCS is controversial, and various mechanisms have been proposed to explain it. In this work, we have investigated the GCS in a Ta layer deposited on the surface of InAs nano… ▽ More Understanding the microscopic origin of the gate-controlled supercurrent (GCS) in superconducting nanobridges is crucial for engineering superconducting switches suitable for a variety of electronic applications. The origin of GCS is controversial, and various mechanisms have been proposed to explain it. In this work, we have investigated the GCS in a Ta layer deposited on the surface of InAs nanowires. Comparison between switching current distributions at opposite gate polarities and between the gate dependence of two opposite side gates with different nanowire$-$gate spacings shows that the GCS is determined by the power dissipated by the gate leakage. We also found a substantial difference between the influence of the gate and elevated bath temperature on the magnetic field dependence of the supercurrent. Detailed analysis of the switching dynamics at high gate voltages shows that the device is driven into the multiple phase slips regime by high-energy fluctuations arising from the leakage current. △ Less

Submitted 30 October, 2022; originally announced October 2022.

Joule spectroscopy of hybrid superconductor-semiconductor nanodevices

Authors: Angel Ibabe, Mario Gomez, Gorm O. Steffensen, Thomas Kanne, Jesper Nygard, Alfredo Levy Yeyati, Eduardo J. H. Lee

Abstract: Hybrid superconductor-semiconductor devices offer highly tunable platforms, potentially suitable for quantum technology applications, that have been intensively studied in the past decade. Here we establish that measurements of the superconductor-to-normal transition originating from Joule heating provide a powerful spectroscopical tool to characterize such hybrid devices. Concretely, we apply thi… ▽ More Hybrid superconductor-semiconductor devices offer highly tunable platforms, potentially suitable for quantum technology applications, that have been intensively studied in the past decade. Here we establish that measurements of the superconductor-to-normal transition originating from Joule heating provide a powerful spectroscopical tool to characterize such hybrid devices. Concretely, we apply this technique to junctions in full-shell Al-InAs nanowires in the Little-Parks regime and obtain detailed information of each lead independently and in a single measurement, including differences in the superconducting coherence lengths of the leads, inhomogeneous covering of the epitaxial shell, and the inverse superconducting proximity effect; all-in-all constituting a unique fingerprint of each device and highlighting the large variability present in these systems. Besides the practical uses, our work also underscores the importance of heating in hybrid devices, an effect that is often overlooked. △ Less

Submitted 2 October, 2022; originally announced October 2022.

Journal ref: Nat Commun 14, 2873 (2023)

Parallel InAs nanowires for Cooper pair splitters with Coulomb repulsion

Authors: Olivér Kürtössy, Zoltán Scherübl, Gergő Fülöp, István Endre Lukács, Thomas Kanne, Jesper Nygård, Péter Makk, Szabolcs Csonka

Abstract: Hybrid nanostructures consisting of two parallel InAs nanowires connected by an epitaxially grown superconductor (SC) shell recently became available. Due to the defect-free SC-semiconductor interface and the two quasi-one-dimensional channels being close by, these novel platforms can be utilized to spatially separate entangled pairs of electrons by using quantum dots (QD) in the so-called Cooper… ▽ More Hybrid nanostructures consisting of two parallel InAs nanowires connected by an epitaxially grown superconductor (SC) shell recently became available. Due to the defect-free SC-semiconductor interface and the two quasi-one-dimensional channels being close by, these novel platforms can be utilized to spatially separate entangled pairs of electrons by using quantum dots (QD) in the so-called Cooper pair splitting (CPS) process. The minimized distance between the QDs overcomes the limitations of single-wire-based geometries and can boost the splitting efficiency. Here we investigate CPS in such a device, for the first time, where strong inter-dot Coulomb repulsion is also present and studied thoroughly. We analyze theoretically the slight reduction of the CPS efficiency imposed by the Coulomb interaction and compare it to the experiments. Despite the competition between crossed Andreev reflection (CAR) and inter-wire capacitance, a significant CPS signal is observed indicating the dominance of the superconducting coupling. Our results demonstrate that the application of parallel InAs nanowires with epitaxial SC is a promising route for the realization of parafermionic states relying on enhanced CAR between the wires. △ Less

Submitted 27 March, 2022; originally announced March 2022.

Electronic transport in double-nanowire superconducting islands with multiple terminals

Authors: Alexandros Vekris, Juan Carlos Estrada Saldaña, Thomas Kanne, Thor Hvid-Olsen, Mikelis Marnauza, Dags Olsteins, Matteo M. Wauters, Michele Burrello, Jesper Nygård, Kasper Grove-Rasmussen

Abstract: We characterize in-situ grown parallel nanowires bridged by a superconducting island. The magnetic-field and temperature dependence of Coulomb blockade peaks measured across different pairs of nanowire ends are consistent with a sub-gap state extended over the hybrid parallel-nanowire island. Being gate-tunable, accessible by multiple terminals and free of quasiparticle poisoning, these nanowires… ▽ More We characterize in-situ grown parallel nanowires bridged by a superconducting island. The magnetic-field and temperature dependence of Coulomb blockade peaks measured across different pairs of nanowire ends are consistent with a sub-gap state extended over the hybrid parallel-nanowire island. Being gate-tunable, accessible by multiple terminals and free of quasiparticle poisoning, these nanowires show promise for the implementation of several proposals that rely on parallel nanowire platforms. △ Less

Submitted 17 March, 2022; originally announced March 2022.

Comments: 9 pages, 5 figures

Report number: NBI QDEV 2022

Superconductivity and parity preservation in as-grown In islands on InAs nanowires

Authors: Martin Saurbrey Bjergfelt, Damon J. Carrad, Thomas Kanne, Erik Johnson, Elisabetta M. Fiordaliso, Thomas Sand Jespersen, Jesper Nygård

Abstract: We report in-situ synthesis of crystalline indium islands on InAs nanowires grown by molecular beam epitaxy. Structural analysis by transmission electron microscopy showed that In crystals grew in a tetragonal body-centred crystal structure within two families of orientations relative to wurtzite InAs. The crystalline islands had lengths < 500 nm and low-energy surfaces, suggesting that growth was… ▽ More We report in-situ synthesis of crystalline indium islands on InAs nanowires grown by molecular beam epitaxy. Structural analysis by transmission electron microscopy showed that In crystals grew in a tetragonal body-centred crystal structure within two families of orientations relative to wurtzite InAs. The crystalline islands had lengths < 500 nm and low-energy surfaces, suggesting that growth was driven mainly by surface energy minimization. Electrical transport through In/InAs devices exhibited Cooper pair charging, evidencing charge parity preservation and a pristine In/InAs interface, with an induced superconducting gap ~ 0.45 meV. Cooper pair charging persisted to temperatures > 1.2 K and magnetic fields ~ 0.7 T, demonstrating that In/InAs hybrids belong to an expanding class of semiconductor/superconductor hybrids operating over a wider parameter space than state-of-the-art Al-based hybrids. Engineering crystal morphology while isolating single islands using shadow epitaxy provides an interesting alternative to previous semiconductor/superconductor hybrid morphologies and device geometries. △ Less

Submitted 23 November, 2021; originally announced November 2021.

Comments: Published in Nano Letters: https://pubs.acs.org/doi/10.1021/acs.nanolett.1c02487

Scalable platform for nanocrystal-based quantum electronics

Authors: Joachim E. Sestoft, Aske N. Gejl, Thomas Kanne, Rasmus D. Schlosser, Daniel Ross, Daniel Kjær, Kasper Grove-Rasmussen, Jesper Nygård

Abstract: Unlocking the full potential of nanocrystals in electronic devices requires scalable and deterministic manufacturing techniques. A platform offering promising alternative paths to scalable production is microtomy, the technique of cutting thin lamellae with large areas containing embedded nanostructures. This platform has so far not been used for fabrication of electronic quantum devices. Here, we… ▽ More Unlocking the full potential of nanocrystals in electronic devices requires scalable and deterministic manufacturing techniques. A platform offering promising alternative paths to scalable production is microtomy, the technique of cutting thin lamellae with large areas containing embedded nanostructures. This platform has so far not been used for fabrication of electronic quantum devices. Here, we combine microtomy with vapor-liquid-solid growth of III/V nanowires to create a scalable platform that can deterministically transfer large arrays of single and fused nanocrystals - offering single unit control and free choice of target substrate. We fabricate electronic devices on cross-sectioned InAs nanowires with good yield and demonstrate their ability to exhibit quantum phenomena such as conductance quantization, single electron charging, and wave interference. Finally, we devise how the platform can host rationally designed semiconductor/superconductor networks relevant for emerging quantum technologies. △ Less

Submitted 9 November, 2021; originally announced November 2021.

Report number: NBI QDEV 2021

Gate-controlled supercurrent in epitaxial Al/InAs nanowires

Authors: Tosson Elalaily, Olivér Kürtössy, Zoltán Scherübl, Martin Berke, Gergő Fülöp, István Endre Lukács, Thomas Kanne, Jesper Nygård, Kenji Watanabe, Takashi Taniguchi, Péter Makk, Szabolcs Csonka

Abstract: Gate-controlled supercurrent (GCS) in superconductor nanobridges has recently attracted attention as a means to create superconducting field effect transistors. Despite the clear advantage for applications with low power consumption and high switching speeds, the microscopic mechanism of the field effect is still under debate. In this work, we realize GCS for the first time in an epitaxial superco… ▽ More Gate-controlled supercurrent (GCS) in superconductor nanobridges has recently attracted attention as a means to create superconducting field effect transistors. Despite the clear advantage for applications with low power consumption and high switching speeds, the microscopic mechanism of the field effect is still under debate. In this work, we realize GCS for the first time in an epitaxial superconductor, which is created as a shell on an InAs nanowire. We show that the supercurrent in the epitaxial Al layer can be switched to the normal state by applying $\simeq\pm$ 23$\,$V on a bottom gate insulated from the nanowire by a crystalline hBN layer. Our extensive study on the temperature and magnetic field dependencies of GCS suggests that hot electron injection alone cannot explain our experimental findings. △ Less

Submitted 21 June, 2021; v1 submitted 11 June, 2021; originally announced June 2021.

Asymmetric Little-Parks Oscillations in Full Shell Double Nanowires

Authors: Alexandros Vekris, Juan Carlos Estrada Saldaña, Joeri de Bruijckere, Sara Lorić, Thomas Kanne, Mikelis Marnauza, Dags Olsteins, Jesper Nygård, Kasper Grove-Rasmussen

Abstract: Little-Parks oscillations of a hollow superconducting cylinder are of interest for flux-driven topological superconductivity in single Rashba nanowires. The oscillations are typically symmetric in the orientation of the applied magnetic flux. Using double InAs nanowires coated by an epitaxial superconducting Al shell which, despite the non-centro-symmetric geometry, behaves effectively as one holl… ▽ More Little-Parks oscillations of a hollow superconducting cylinder are of interest for flux-driven topological superconductivity in single Rashba nanowires. The oscillations are typically symmetric in the orientation of the applied magnetic flux. Using double InAs nanowires coated by an epitaxial superconducting Al shell which, despite the non-centro-symmetric geometry, behaves effectively as one hollow cylinder, we demonstrate that a small misalignment of the applied parallel field with respect to the axis of the nanowires can produce field-asymmetric Little-Parks oscillations. These are revealed by the simultaneous application of a magnetic field perpendicular to the misaligned parallel field direction. The asymmetry occurs in both the destructive regime, in which superconductivity is destroyed for half-integer quanta of flux through the shell, and in the non-destructive regime, where superconductivity is depressed but not fully destroyed at these flux values. △ Less

Submitted 2 June, 2021; originally announced June 2021.

Comments: 9 pages, 3 figures

Report number: NBI QDEV 2021

Journal ref: Scientific Reports 11, 19034 (2021)

Josephson junctions in double nanowires bridged by in-situ deposited superconductors

Authors: Alexandros Vekris, Juan Carlos Estrada Saldaña, Thomas Kanne, Mikelis Marnauza, Dags Olsteins, Furong Fan, Xiaobo Li, Thor Hvid-Olsen, Xiaohui Qiu, Hongqi Xu, Jesper Nygård, Kasper Grove-Rasmussen

Abstract: We characterize parallel double quantum dot Josephson junctions based on closely-grown double nanowires bridged by in-situ deposited superconductors. The parallel double dot behavior occurs despite the closeness of the nanowires and the potential risk of nanowire clamping during growth. By tuning the charge filling and lead couplings, we map out the simplest parallel double quantum dot Yu-Shiba-Ru… ▽ More We characterize parallel double quantum dot Josephson junctions based on closely-grown double nanowires bridged by in-situ deposited superconductors. The parallel double dot behavior occurs despite the closeness of the nanowires and the potential risk of nanowire clamping during growth. By tuning the charge filling and lead couplings, we map out the simplest parallel double quantum dot Yu-Shiba-Rusinov phase diagram. Our quasi-independent two-wire hybrids show promise for the realization of exotic topological phases. △ Less

Submitted 4 April, 2021; originally announced April 2021.

Comments: 8 pages, 4 figures

Report number: NBI QDEV 2021

Journal ref: Phys. Rev. Research 3, 033240 (2021)

Andreev molecule in parallel InAs nanowires

Authors: Olivér Kürtössy, Zoltán Scherübl, Gergő Fülöp, István Endre Lukács, Thomas Kanne, Jesper Nygård, Péter Makk, Szabolcs Csonka

Abstract: Coupling individual atoms via tunneling fundamentally changes the state of matter: electrons bound to atomic cores become delocalized resulting in a change from an insulating to a metallic state, as it is well known from the canonical example of solids. A chain of atoms could lead to more exotic states if the tunneling takes place via the superconducting vacuum and can induce topologically protect… ▽ More Coupling individual atoms via tunneling fundamentally changes the state of matter: electrons bound to atomic cores become delocalized resulting in a change from an insulating to a metallic state, as it is well known from the canonical example of solids. A chain of atoms could lead to more exotic states if the tunneling takes place via the superconducting vacuum and can induce topologically protected excitations like Majorana or parafermions. Toward the realization of such artificial chains, coupling a single atom to the superconducting vacuum is well studied, but the hybridization of two sites via the superconductor was not yet reported. The peculiar vacuum of the BCS condensate opens the way to annihilate or generate two electrons from the bulk resulting in a so-called Andreev molecular state. By employing parallel nanowires with an Al superconductor shell, two artificial atoms were created at a minimal distance with an epitaxial superconducting link between. Hybridization via the BCS vacuum was observed between the two artificial atoms for the first time, as a demonstration of an Andreev molecular state. △ Less

Submitted 29 March, 2021; v1 submitted 25 March, 2021; originally announced March 2021.

Comments: References were updated

Double nanowires for hybrid quantum devices

Authors: Thomas Kanne, Dags Olsteins, Mikelis Marnauza, Alexandros Vekris, Juan Carlos Estrada Saldana, Sara Loric, Rasmus D. Schlosser, Daniel Ross, Szabolcs Csonka, Kasper Grove-Rasmussen, Jesper Nygård

Abstract: Parallel one-dimensional semiconductor channels connected by a superconducting strip constitute the core platform in several recent quantum device proposals that rely e.g. on Andreev processes or topological effects. In order to realize these proposals, the actual material systems must have high crystalline purity and the coupling between the different elements should be controllable in terms of t… ▽ More Parallel one-dimensional semiconductor channels connected by a superconducting strip constitute the core platform in several recent quantum device proposals that rely e.g. on Andreev processes or topological effects. In order to realize these proposals, the actual material systems must have high crystalline purity and the coupling between the different elements should be controllable in terms of their interfaces and geometry. We present a strategy for synthesizing double InAs nanowires by the vapor-liquid-solid mechanism using III-V molecular beam epitaxy. A superconducting layer is deposited onto nanowires without breaking vacuum, ensuring pristine interfaces between the superconductor and the two semiconductor nanowires. The method allows for a high yield of merged as well as separate parallel nanowires, with full or half-shell superconductor coatings. We demonstrate their utility in complex quantum devices by electron transport measurements. △ Less

Submitted 25 March, 2021; originally announced March 2021.

Report number: NBI QDEV 2021

Temperature Induced Shifts of Yu-Shiba-Rusinov Resonances in Nanowire-Based Hybrid Quantum Dots

Authors: Juan Carlos Estrada Saldaña, Alexandros Vekris, Victoria Sosnovtseva, Thomas Kanne, Peter Krogstrup, Kasper Grove-Rasmussen, Jesper Nygård

Abstract: The strong coupling of a superconductor to a spinful quantum dot results in Yu-Shiba-Rusinov (YSR) discrete subgap excitations. In isolation and at zero temperature, the excitations are $δ$ resonances. In transport experiments, however, they show as broad differential conductance peaks. We obtain the lineshape of the peaks and their temperature dependence in superconductor-quantum-dot-metal (S-QD-… ▽ More The strong coupling of a superconductor to a spinful quantum dot results in Yu-Shiba-Rusinov (YSR) discrete subgap excitations. In isolation and at zero temperature, the excitations are $δ$ resonances. In transport experiments, however, they show as broad differential conductance peaks. We obtain the lineshape of the peaks and their temperature dependence in superconductor-quantum-dot-metal (S-QD-N) nanowire-based devices. Unexpectedly, we find that the peaks shift in energy with temperature, with the shift magnitude and sign depending on ground state parity and bias voltage. Additionally, we empirically find a power-law scaling of the peak area versus temperature. These observations are not explained by current models. △ Less

Submitted 28 February, 2020; originally announced February 2020.

Report number: NBI QDEV 2020

Journal ref: Commun Phys 3, 125 (2020) https://rdcu.be/b5ymM

Epitaxial Pb on InAs nanowires

Authors: Thomas Kanne, Mikelis Marnauza, Dags Olsteins, Damon J. Carrad, Joachim E. Sestoft, Joeri de Bruijckere, Lunjie Zeng, Erik Johnson, Eva Olsson, Kasper Grove-Rasmussen, Jesper Nygård

Abstract: Semiconductor-superconductor hybrids are widely used for realising complex quantum phenomena such as topological superconductivity and spins coupled to Cooper pairs. Accessing exotic regimes at high magnetic fields and increasing operating temperatures beyond the state-of-the-art requires new, epitaxially matched semiconductor-superconductor materials. The challenge is to generate favourable condi… ▽ More Semiconductor-superconductor hybrids are widely used for realising complex quantum phenomena such as topological superconductivity and spins coupled to Cooper pairs. Accessing exotic regimes at high magnetic fields and increasing operating temperatures beyond the state-of-the-art requires new, epitaxially matched semiconductor-superconductor materials. The challenge is to generate favourable conditions for heterostructure formation between materials with the desired inherent properties. Here, we harness increased knowledge of metal-on-semiconductor growth to develop InAs nanowires with epitaxially matched, single crystal, atomically flat Pb films along the entire nanowire. These highly ordered heterostructures have a critical temperature of 7 K and a superconducting gap of 1.25 meV, which remains hard at 8.5 T, thereby more than doubling the available parameter space. Additionally, InAs/Pb island devics exhibit magnetic field-driven transitions from Cooper pair to single electron charging; a pre-requisite for use in topological quantum computation. Introducing semiconductor-Pb hybrids potentially enables access to entirely new regimes for an array of quantum systems. △ Less

Submitted 26 February, 2020; originally announced February 2020.

Report number: NBI QDEV 2020

Journal ref: Nature Nanotechnology (2021)

Shadow epitaxy for in-situ growth of generic semiconductor/superconductor devices

Authors: Damon J. Carrad, Martin Bjergfelt, Thomas Kanne, Martin Aagesen, Filip Krizek, Elisabetta M. Fiordaliso, Erik Johnson, Jesper Nygård, Thomas Sand Jespersen

Abstract: Uniform, defect-free crystal interfaces and surfaces are crucial ingredients for realizing high-performance nanoscale devices. A pertinent example is that advances in gate-tunable and topological superconductivity using semiconductor/superconductor electronic devices are currently built on the hard proximity-induced superconducting gap obtained from epitaxial indium arsenide/aluminium heterostruct… ▽ More Uniform, defect-free crystal interfaces and surfaces are crucial ingredients for realizing high-performance nanoscale devices. A pertinent example is that advances in gate-tunable and topological superconductivity using semiconductor/superconductor electronic devices are currently built on the hard proximity-induced superconducting gap obtained from epitaxial indium arsenide/aluminium heterostructures. Fabrication of devices requires selective etch processes; these exist only for InAs/Al hybrids, precluding the use of other, potentially superior material combinations. We present a crystal growth platform -- based on three-dimensional structuring of growth substrates -- which enables synthesis of semiconductor nanowire hybrids with in-situ patterned superconductor shells. This platform eliminates the need for etching, thereby enabling full freedom in choice of hybrid constituents. We realise and characterise all the most frequently used architectures in superconducting hybrid devices, finding increased yield and electrostatic stability compared to etched devices, along with evidence of ballistic superconductivity. In addition to aluminium, we present hybrid devices based on tantalum, niobium and vanadium. This is the submitted version of the manuscript. The accepted, peer reviewed version is available from Advanced Materials: http://doi.org/10.1002/adma.201908411 Previous title: Shadow lithography for in-situ growth of generic semiconductor/superconductor devices △ Less

Submitted 8 May, 2020; v1 submitted 1 November, 2019; originally announced November 2019.

Report number: NBI QDEV 2019

Journal ref: Advanced Materials (2020) 1908411

Semiconductor - Ferromagnetic Insulator - Superconductor Nanowires: Stray Field and Exchange Field

Authors: Yu Liu, Saulius Vaitiekenas, Sara Marti-Sanchez, Christian Koch, Sean Hart, Zheng Cui, Thomas Kanne, Sabbir A. Khan, Rawa Tanta, Shivendra Upadhyay, Martin Espineira Cachaza, Charles M. Marcus, Jordi Arbiol, Kathryn A. Moler, Peter Krogstrup

Abstract: Nanowires can serve as flexible substrates for hybrid epitaxial growth on selected facets, allowing for design of heterostructures with complex material combinations and geometries. In this work we report on hybrid epitaxy of semiconductor - ferromagnetic insulator - superconductor (InAs/EuS/Al) nanowire heterostructures. We study the crystal growth and complex epitaxial matching of wurtzite InAs… ▽ More Nanowires can serve as flexible substrates for hybrid epitaxial growth on selected facets, allowing for design of heterostructures with complex material combinations and geometries. In this work we report on hybrid epitaxy of semiconductor - ferromagnetic insulator - superconductor (InAs/EuS/Al) nanowire heterostructures. We study the crystal growth and complex epitaxial matching of wurtzite InAs / rock-salt EuS interfaces as well as rock-salt EuS / face-centered cubic Al interfaces. Because of the magnetic anisotropy originating from the nanowire shape, the magnetic structure of the EuS phase are easily tuned into single magnetic domains. This effect efficiently ejects the stray field lines along the nanowires. With tunnel spectroscopy measurements of the density of states, we show the material has a hard induced superconducting gap, and magnetic hysteretic evolution which indicates that the magnetic exchange fields are not negligible. These hybrid nanowires fulfil key material requirements for serving as a platform for spin-based quantum applications, such as scalable topological quantum computing. △ Less

Submitted 8 October, 2019; originally announced October 2019.

Comments: 15 pages, 5 figures

Report number: NBI QDEV 2019

Journal ref: Nano Lett. 20, 456 (2019)

Engineering Hybrid Epitaxial InAsSb/Al Nanowire Materials for Stronger Topological Protection

Authors: Joachim E. Sestoft, Thomas Kanne, Aske Nørskov Gejl, Merlin von Soosten, Jeremy S. Yodh, Daniel Sherman, Brian Tarasinski, Michael Wimmer, Erik Johnson, Mingtang Deng, Jesper Nygård, Thomas Sand Jespersen, Charles M. Marcus, Peter Krogstrup

Abstract: The combination of strong spin-orbit coupling, large $g$-factors, and the coupling to a superconductor can be used to create a topologically protected state in a semiconductor nanowire. Here we report on growth and characterization of hybrid epitaxial InAsSb/Al nanowires, with varying composition and crystal structure. We find the strongest spin-orbit interaction at intermediate compositions in zi… ▽ More The combination of strong spin-orbit coupling, large $g$-factors, and the coupling to a superconductor can be used to create a topologically protected state in a semiconductor nanowire. Here we report on growth and characterization of hybrid epitaxial InAsSb/Al nanowires, with varying composition and crystal structure. We find the strongest spin-orbit interaction at intermediate compositions in zincblende InAs$_{1-x}$Sb$_{x}$ nanowires, exceeding that of both InAs and InSb materials, confirming recent theoretical studies \cite{winkler2016topological}. We show that the epitaxial InAsSb/Al interfaces allows for a hard induced superconducting gap and 2$e$ transport in Coulomb charging experiments, similar to experiments on InAs/Al and InSb/Al materials, and find measurements consistent with topological phase transitions at low magnetic fields due to large effective $g$-factors. Finally we present a method to grow pure wurtzite InAsSb nanowires which are predicted to exhibit even stronger spin-orbit coupling than the zincblende structure. △ Less

Submitted 29 December, 2017; v1 submitted 18 November, 2017; originally announced November 2017.

Comments: 10 pages and 5 figures

Report number: NBI QDEV 2017

Journal ref: Phys. Rev. Materials 2, 044202 (2018)