Simulating intracellular calcium dynamics

Calcium concentration in the myocytes is crucial in cardiac excitation-contraction coupling (the conversion of an electrical stimulus to a mechanical contraction of the heart cells). This part of the intricate process can be studied by simulating the calcium dynamics. In this project we consider a model that combines stochastic transitions among different states of channels and deterministic calcium dynamics and perform some computer simulations employing MATLAB. Then, we examine whether the model makes correspondence with empirical results taken from the Cellular Physiology Laboratory of the Cardiovascular Research Center (CSIC-ICCC)2014/201

Impurity Knight shift in quantum dot Josephson junctions

Spectroscopy of a Josephson junction device with an embedded quantum dot reveals the presence of a contribution to level splitting in external magnetic field that is proportional to $\cos \phi$, where $\phi$ is the gauge-invariant phase difference across the junction. To elucidate the origin of this unanticipated effect, we systematically study the Zeeman splitting of spinful subgap states in the superconducting Anderson impurity model. The magnitude of the splitting is renormalized by the exchange interaction between the local moment and the continuum of Bogoliubov quasiparticles in a variant of the Knight shift phenomenon. The leading term in the shift is linear in the hybridisation strength $\Gamma$ (quadratic in electron hopping), while the subleading term is quadratic in $\Gamma$ (quartic in electron hopping) and depends on $\phi$ due to spin-polarization-dependent corrections to the Josephson energy of the device. The amplitude of the $\phi$-dependent part is largest for experimentally relevant parameters beyond the perturbative regime where it is investigated using numerical renormalization group calculations. Such magnetic-field-tunable coupling between the quantum dot spin and the Josephson current could find wide use in superconducting spintronics.Comment: 18 pages, 13 figures. Perturbation theory results available as supplemental material, NRG calculation input files available on Zenod

Gate-tunable kinetic inductance parametric amplifier

Superconducting parametric amplifiers play a crucial role in the preparation and readout of quantum states at microwave frequencies, enabling high-fidelity measurements of superconducting qubits. Most existing implementations of these amplifiers rely on the nonlinearity from Josephson junctions, superconducting quantum interference devices or disordered superconductors. Additionally, frequency tunability arises typically from either flux or current biasing. In contrast, semiconductor-based parametric amplifiers are tunable by local electric fields, which impose a smaller thermal load on the cryogenic setup than current and flux biasing and lead to vanishing crosstalk to other on-chip quantum systems. In this work, we present a gate-tunable parametric amplifier that operates without Josephson junctions, utilizing a proximitized semiconducting nanowire. This design achieves near-quantum-limited performance, featuring more than 20 dB gain and a 30 MHz gain-bandwidth product. The absence of Josephson junctions allows for advantages, including substantial saturation powers of -120dBm, magnetic field compatibility up to 500 mT and frequency tunability over a range of 15 MHz. Our realization of a parametric amplifier supplements efforts towards gate-controlled superconducting electronics, further advancing the abilities for high-performing quantum measurements of semiconductor-based and superconducting quantum devices.Comment: 12 pages, 11 figure

Strong tunable coupling between two distant superconducting spin qubits

Superconducting (or Andreev) spin qubits have recently emerged as an alternative qubit platform with realizations in semiconductor-superconductor hybrid nanowires. In these qubits, the spin degree of freedom is intrinsically coupled to the supercurrent across a Josephson junction via the spin-orbit interaction, which facilitates fast, high-fidelity spin readout using circuit quantum electrodynamics techniques. Moreover, this spin-supercurrent coupling has been predicted to facilitate inductive multi-qubit coupling. In this work, we demonstrate a strong supercurrent-mediated coupling between two distant Andreev spin qubits. This qubit-qubit interaction is of the longitudinal type and we show that it is both gate- and flux-tunable up to a coupling strength of 178 MHz. Finally, we find that the coupling can be switched off in-situ using a magnetic flux. Our results demonstrate that integrating microscopic spin states into a superconducting qubit architecture can combine the advantages of both semiconductors and superconducting circuits and pave the way to fast two-qubit gates between remote spins.Comment: 26 pages, 27 figure

A gate-tunable, field-compatible fluxonium

Circuit quantum electrodynamics, where photons are coherently coupled to artificial atoms built with superconducting circuits, has enabled the investigation and control of macroscopic quantum-mechanical phenomena in superconductors. Recently, hybrid circuits incorporating semiconducting nanowires and other electrostatically-gateable elements have provided new insights into mesoscopic superconductivity. Extending the capabilities of hybrid flux-based circuits to work in magnetic fields would be especially useful both as a probe of spin-polarized Andreev bound states and as a possible platform for topological qubits. The fluxonium is particularly suitable as a readout circuit for topological qubits due to its unique persistent-current based eigenstates. In this Letter, we present a magnetic-field compatible hybrid fluxonium with an electrostatically-tuned semiconducting nanowire as its non-linear element. We operate the fluxonium in magnetic fields up to 1T and use it to observe the $\varphi_0$-Josephson effect. This combination of gate-tunability and field-compatibility opens avenues for the exploration and control of spin-polarized phenomena using superconducting circuits and enables the use of the fluxonium as a readout device for topological qubits

Régimen viscoso del transporte electrónico en una constricción en grafeno

Under mesoscopic conditions, strong electron-electron interactions and weak electron-phonon coupling in graphene lead to an hydrodynamic behaviour of electron dynamics, resulting in unusual transport phenomena. During the last years, there have been several theoretical studies of macroscopic signatures of this collective cooperation of electrons. Here, we want to distinguish this hydrodynamic regime studying the conductance through a narrow geometrical constriction in graphene. To do so, we have fabricated high-quality, low-disorder graphene nano-constriction devices encapsulated by hexagonal boron nitride, where electron-electron scattering dominates impurity scattering. We carried out systematic four-probe conductance measurements on devices with different constriction widths as a function of carrier density and temperature. The observation of quantum transport phenomena that are inconsistent with the non-interacting ballistic free-fermion model suggests a macroscopic transport signature of electron viscosity.n condiciones mesoscópicas, las fuertes interacciones electrón-electrón, junto con el débil acoplamiento electrón-fonón en grafeno, dan lugar a un comportamiento hidrodinámico de la dinámica electrónica que resulta en fenómenos de transporte inusuales. Durante los últimos años ha habido varios estudios teóricos acerca de indicios macroscópicos de esta cooperación colectiva de los electrones. Aquí, intentamos distinguir este régimen hidrodinámico estudiando la conductividad a través de una estrecha constricción geométrica en grafeno. Para hacerlo, hemos fabricado dispositivos con nano-constricciones en grafeno, de alta calidad y bajo nivel de desorden, encapsulados en nitruro de boro hexagonal, en los que la dispersión electrón-electrón domina la dispersión por impurezas. Se llevaron a cabo medidas sistemáticas de la conductividad con cuatro sondas, en dispositivos con distintas anchuras de constricción, en función de la densidad de portadores y de la temperatura. La observación de fenómenos cuánticos de transporte que no son consistentes con el modelo balístico de fermiones libres sin interacción sugiere un indicio de transporte macroscópico de la viscosidad electrónica.Les fortes interaccions electró-electró conjuntament amb el dèbil acoblament electró-fonó al grafè en condicions mesoscòpiques provoquen un comportament hidrodinàmic de la dinàmica electrònica que resulta en fenòmens de transport inusuals. Durant els últims anys hi ha hagut diversos estudis teòrics que buscaven indicis macroscòpics d'aquesta cooperació col·lectiva dels electrons. En aquest treball intentem distingir aquest règim hidrodinàmic estudiant la conductivitat a través d'una constricció geomètrica estreta de grafè. Per fer-ho, hem fabricat dispositius amb nano-constriccions de grafè d'alta qualitat i baix nivell de desordre, encapsulats en dispositius de nitrur de bor hexagonal, en els quals la dispersió electró-electró domina sobre la dispersió per impureses. Es van prendre mesures sistemàtiques de la conductivitat emprant quatre sondes, en dispositius amb diferent amplada de constricció, i en funció de la densitat de portadors i de la temperatura. L'observació de fenòmens quàntics de transport que no són consistents amb el model balístic de fermions lliures sense interacció suggereix un indici de transport macroscòpic de la viscositat electrònica

Régimen viscoso del transporte electrónico en una constricción en grafeno

Under mesoscopic conditions, strong electron-electron interactions and weak electron-phonon coupling in graphene lead to an hydrodynamic behaviour of electron dynamics, resulting in unusual transport phenomena. During the last years, there have been several theoretical studies of macroscopic signatures of this collective cooperation of electrons. Here, we want to distinguish this hydrodynamic regime studying the conductance through a narrow geometrical constriction in graphene. To do so, we have fabricated high-quality, low-disorder graphene nano-constriction devices encapsulated by hexagonal boron nitride, where electron-electron scattering dominates impurity scattering. We carried out systematic four-probe conductance measurements on devices with different constriction widths as a function of carrier density and temperature. The observation of quantum transport phenomena that are inconsistent with the non-interacting ballistic free-fermion model suggests a macroscopic transport signature of electron viscosity.n condiciones mesoscópicas, las fuertes interacciones electrón-electrón, junto con el débil acoplamiento electrón-fonón en grafeno, dan lugar a un comportamiento hidrodinámico de la dinámica electrónica que resulta en fenómenos de transporte inusuales. Durante los últimos años ha habido varios estudios teóricos acerca de indicios macroscópicos de esta cooperación colectiva de los electrones. Aquí, intentamos distinguir este régimen hidrodinámico estudiando la conductividad a través de una estrecha constricción geométrica en grafeno. Para hacerlo, hemos fabricado dispositivos con nano-constricciones en grafeno, de alta calidad y bajo nivel de desorden, encapsulados en nitruro de boro hexagonal, en los que la dispersión electrón-electrón domina la dispersión por impurezas. Se llevaron a cabo medidas sistemáticas de la conductividad con cuatro sondas, en dispositivos con distintas anchuras de constricción, en función de la densidad de portadores y de la temperatura. La observación de fenómenos cuánticos de transporte que no son consistentes con el modelo balístico de fermiones libres sin interacción sugiere un indicio de transporte macroscópico de la viscosidad electrónica.Les fortes interaccions electró-electró conjuntament amb el dèbil acoblament electró-fonó al grafè en condicions mesoscòpiques provoquen un comportament hidrodinàmic de la dinàmica electrònica que resulta en fenòmens de transport inusuals. Durant els últims anys hi ha hagut diversos estudis teòrics que buscaven indicis macroscòpics d'aquesta cooperació col·lectiva dels electrons. En aquest treball intentem distingir aquest règim hidrodinàmic estudiant la conductivitat a través d'una constricció geomètrica estreta de grafè. Per fer-ho, hem fabricat dispositius amb nano-constriccions de grafè d'alta qualitat i baix nivell de desordre, encapsulats en dispositius de nitrur de bor hexagonal, en els quals la dispersió electró-electró domina sobre la dispersió per impureses. Es van prendre mesures sistemàtiques de la conductivitat emprant quatre sondes, en dispositius amb diferent amplada de constricció, i en funció de la densitat de portadors i de la temperatura. L'observació de fenòmens quàntics de transport que no són consistents amb el model balístic de fermions lliures sense interacció suggereix un indici de transport macroscòpic de la viscositat electrònica

Realizing superconducting spin qubits

Josephson junctions implemented in semiconducting nanowires proximitized by a superconductor exhibit intricate physics arising from the interplay of electron-electron interactions, superconductivity, spin-orbit coupling, and the Zeeman effect. This thesis explores these phenomena through a series of experiments conducted using circuit quantum electrodynamics techniques.After establishing the fundamental theoretical concepts and experimental methodologies, we introduce a crucial element for probing our devices with microwaves: magnetic field-compatible resonators. We then describe various experiments conducted over the past years in which superconducting resonators and other circuits are used to explore the physics of nanowire Josephson junctions.In an initial experiment, we develop a magnetic-field-resilient fluxoniumcircuit that incorporates an InAs semiconducting nanowire at its core. We show that the device’s spectrum is highly dependent on both the electrostatic gate voltage and the magnetic field strength, allowing us to detect signatures of non-conventional phenomena in semiconducting Josephson junctions.The bulk of this thesis revolves around a second set of experiments, where a quantum dot is electrostatically defined within the nanowire Josephson junction. This time, we use a transmon circuit to investigate singlet-doublet ground state transitions and their dynamics. The two spinful doublet states of the junction define a novel type of qubit with intriguing properties: a superconducting (or Andreev) spin qubit (ASQ). Thus, we then shift our focus to the doublet states and explore their magnetic field dependence with transmon spectroscopy. Subsequently,we turn to directly investigating the spin-flip transition and the coherence properties of the two spin states. We find that the intrinsic coupling between the spin state and the supercurrent through the junction enablesstrong coupling between the ASQ and the transmon qubit in which it is embedded.In a final experiment, we connect two such Andreev spin qubits in parallel and investigate their supercurrent-mediated longitudinal coupling. We find that the qubits are strongly coupled and their coupling strength can be switched on and off by adjusting the magnetic flux. Notably, given that the spins are placed micrometers apart, this mechanism enables interaction between distant spins. Building on these promising characteristics, we end by introducing a proposal that outlines our vision for scaling up ASQs. The proposed architecture, where multiple ASQs are connected in parallel, enables the selective coupling of any pair of qubits in the system, regardless of their spatial separation, through flux control.This thesis concludes by outlining potential future experiments that could be conducted with devices and techniques similar to those investigated here.QRD/Kouwenhoven La

Development of nanowire-based fluxonium devices

This thesis presents the design, development and first spectroscopy measurements of nanowire-based fluxonium devices. We demonstrate the strong external flux and gate voltage tunability of their spectrum, which allows to accurately tune their first transition frequency over a range of more than 10 GHz. We also show the nanowire fluxonium resilience to magnetic fields up to 800 mT, demonstrating its compatibility with the creation of Majorana bound states (MBSs) at the junction ends, what would open the door to the exploration of new physics and new technological applications. First, the emergence of MBSs in a nanowire fluxonium would result in new Majorana signatures, obtained by radio-frequency spectroscopy techniques. This would complement the current experimental evidence for the creation of MBSs in semiconducting nanowires and would allow to characterize their coupling energy scales, that are, up to date, unknown. And second, the nanowire fluxonium devices presented here can be used for addressing a qubit whose state is topologically protected from local perturbations. Integrating topological qubits into a cQED platform would solve the currently existing problems of the lack of a universal set of quantum gates and reliable methods for qubit operation and readout, establishing a path for the development of topological quantum computing.Applied Physic

Data underlying the manuscript: Singlet-doublet transitions of a quantum dot Josephson junction revealed in a transmon circuit

Data processing and plotting repository of the manuscript titled "Singlet-doublet transitions of a quantum dot Josephson junction revealed in a transmon circuit".This repository contains two zip files: "Data processing and plotting" and "Raw timetraces".The first zip file contains all of the processing and plotting of the data for the manuscript in addition to all of the paper figures. It also contains most of the raw experimental data, with the exception of the raw time traces. These are are very large in filesize, and a representative selection is provided separately in the "Raw timetraces" zip file.The data processing and figure plotting all occur in separate Jupyter notebooks and are executed using Python 3. For this we make use of the conda environment management system, and a self-contained environment that contains all the necessary libraries is provided in the sqds_env.yml file located in "Data processing and plotting". It can be installed with conda env create -f sqds_env.yml.</div