arXiv subjects

Andrea Maiani

Publications and source records attributed to Andrea Maiani.

12 recordsLinked to original sources

arXiv2026

All-electrical dephasing-protected spin qubits in altermagnets

We propose altermagnetic semiconductors as a platform for field-free, all-electrically controlled spin qubits in gate-defined quantum dots. The momentum-dependent spin splitting of an altermagnet produces a Zeeman-like qubit splitting whose magnitude and sign are set by the dot ellipticity, enabling local frequency tunability without external magnetic fields or micromagnets. Because the splitting is tied to a fixed altermagnetic quantization axis, electric-field noise is longitudinally suppressed at leading order, while quantization-axis fluctuations couple transversely and therefore cause relaxation rather than pure dephasing. The compensated magnetic order also avoids stray fields, making the platform naturally compatible with superconducting resonators and dispersive circuit-QED readout through the qubit's spin-dependent electric dipole. Starting from an effective quantum-dot model, supported by a microscopic lattice model, we derive the single- and two-dot Hamiltonian models. We show that electric-dipole spin resonance enables single-qubit control, while tunable exchange and electrically addressable qubit frequencies realize fSim two-qubit gates. The same double-dot architecture also supports singlet-triplet qubits with electrical control of both exchange and splitting gradients, removing the need for micromagnets or nuclear-polarization gradients. These results establish altermagnetic quantum dots as a route to field-free spin qubits with intrinsic electrical tunability and enhanced dephasing protection.

cond-mat.mes-hall↗

arXiv2026

Conductivity anisotropy and linear dichroism in spin-textured altermagnets

Spin textures are ubiquitous in antiferromagnets, yet their consequences for altermagnets remain largely unexplored. We show that smooth spatial variations of the N\'eel order act on itinerant electrons as emergent gauge fields, producing strong, tunable in-plane anisotropies in dc transport and interband optical absorption, even without intrinsic spin-orbit coupling. As a concrete example, we analyze a coplanar spin helix and predict that the principal axes of the conductivity and linear dichroism are set by the helix wave vector. Moreover, the optical anisotropy exhibits two distinct frequency regimes separated by a crossover: at low frequencies the absorption axis is locked to crystal axes, while at high frequencies it tracks the helix. Our results identify polarization-resolved optics and anisotropic transport as direct probes of textured altermagnetic states and suggest a simple route to direction-selective electronic and optical functionality in altermagnets.

cond-mat.mes-hall↗

arXiv2025

Directional conductance of Andreev crystals in hybrid Josephson junction arrays

Andreev bound states are coherent electron-hole superpositions that form in a normal metal through repeated Andreev reflection at a superconducting interface. When the length of a superconducting segment is comparable to the coherence length, the bound states on opposite sides of the segment hybridize through quasiparticle tunneling. In a periodic array, these hybridized Andreev bound states form energy bands below the superconducting gap. We develop a theoretical framework for transport in such Andreev crystals. We demonstrate that, at high interface transparency, a constant phase bias between neighboring superconductors renders the bands directional: one band contains only right-moving and the other only left-moving electronic states. This property leads to a directional conductance that enables the device to operate as a flux- and bias-voltage-tunable filter that allows signal transmission in only one direction.

cond-mat.mes-hall↗

arXiv2024

Impurity states in altermagnetic superconductors

Altermagnetic superconductors offer the possibility of exploring unconventional superconductivity, including topological states and finite-momentum superconductivity, with promising applications in spintronics and quantum information. However, a direct experimental confirmation of their existence remains elusive. In this work, we propose non-magnetic impurities as probes of the interplay between altermagnetism and superconductivity. These impurities induce spin-polarized subgap states whose spatial extension reflects the magnetic order of the substrate material. Depending on whether or not the impurity respects the bulk symmetries, these states form spin-degenerate or spin-split doublets. An external magnetic field aligned with the N\'eel vector can further control the doublet splitting. We further demonstrate that coupling between impurity states leads to a position-dependent, spin-sensitive hybridization, enabling another approach for \textit{in situ} control of atomic-size quantum devices. These findings provide unambiguous experimental signatures of altermagnetic superconductivity accessible via local measurements such as scanning tunneling microscopy and open unexplored pathways for designing tunable quantum devices.

cond-mat.supr-con↗

arXiv2024

Gatemonium: A Voltage-Tunable Fluxonium

We present a new style of fluxonium qubit, gatemonium, based on an all superconductorsemiconductor hybrid platform. The linear inductance is achieved using six hundred planar Al-InAs Josephson junctions (JJs) in series. By tuning the single junction with a gate voltage, we demonstrate electrostatic control of the effective Josephson energy, tuning the weight of the fictitious phase particle. One and two-tone spectroscopy of the gatemonium transitions further reveal details of the hybrid plasmon-fluxon spectrum. Accounting for the nonsinusoidal current-phase relation of the single junction, we fit the measured spectra to extract charging and inductive energies. We conduct time domain characterization of the plasmon modes in a second gatemonium device with different charging energy and JJ array inductance. We discuss future directions for this platform in gate voltage-tunable, high plasma frequency, enhanced impedance junction arrays, and enhanced coherence times for voltage tunable architectures.

cond-mat.mes-hall↗

arXiv2023

Nonsinusoidal current-phase relations in semiconductor-superconductor-ferromagnetic insulator devices

Coherent tunneling processes of multiple Cooper pairs across a Josephson junction give rise to higher harmonics in the current phase relation. In this work, we propose and study Josephson junctions based on semiconductor-superconductor-ferromagnetic insulator heterostructures to engineer nonsinusoidal current-phase relations. The gate-tunability of charge carriers density in the semiconductor, together with the adjustable magnetization of the ferromagnetic insulator, provides control over the content of the supercurrent harmonics. At finite exchange field, hybrid junctions can undergo a 0\,--\,$π$ phase transition, resulting in the supercurrent reversal. Close to the transition, single-pair tunneling is suppressed and the current-phase relation is dominated by the second-harmonic, indicating transport primarily by pairs of Cooper pairs. Finally, we demonstrate that non-collinear magnetization or spin-orbit coupling in the leads and the junction can lead to a gate-tunable Josephson diode effect with efficiencies of up to $\sim30\%$.

cond-mat.mes-hall↗

arXiv2023

Orbital-free approach for large-scale electrostatic simulations of quantum nanoelectronics devices

The route to reliable quantum nanoelectronic devices hinges on precise control of the electrostatic environment. For this reason, accurate methods for electrostatic simulations are essential in the design process. The most widespread methods for this purpose are the Thomas-Fermi approximation, which provides quick approximate results, and the Schrödinger-Poisson method, which better takes into account quantum mechanical effects. The mentioned methods suffer from relevant shortcomings: the Thomas-Fermi method fails to take into account quantum confinement effects that are crucial in heterostructures, while the Schrödinger-Poisson method suffers severe scalability problems. This paper outlines the application of an orbital-free approach inspired by density functional theory. By introducing gradient terms in the kinetic energy functional, our proposed method incorporates corrections to the electronic density due to quantum confinement while it preserves the scalability of a theory that can be expressed as a functional minimization problem. This method offers a new approach to addressing large-scale electrostatic simulations of quantum nanoelectronic devices.

cond-mat.mes-hall↗

arXiv2022

Conductance matrix symmetries of multiterminal semiconductor-superconductor devices

Nonlocal tunneling spectroscopy of multiterminal semiconductor-superconductor hybrid devices is a powerful tool to investigate the Andreev bound states below the parent superconducting gap. We examine how to exploit both microscopic and geometrical symmetries of the system to extract information on the normal and Andreev transmission probabilities from the multiterminal electric or thermoelectric differential conductance matrix under the assumption of an electrostatic potential landscape independent of the bias voltages, as well as the absence of leakage currents. These assumptions lead to several symmetry relations on the conductance matrix. Next, by considering a numerical model of a proximitized semiconductor wire with spin-orbit coupling and two normal contacts at its ends, we show how such symmetries can be used to identify the direction and relative strength of Rashba versus Dresselhaus spin-orbit coupling. Finally, we study how a voltage-bias-dependent electrostatic potential as well as quasiparticle leakage break the derived symmetry relations and investigate characteristic signatures of these two contributions.

cond-mat.mes-hall↗

arXiv2022

Semiconductor-ferromagnet-superconductor planar heterostructures for 1D topological superconductivity

Hybrid structures of semiconducting (SM) nanowires, epitaxially grown superconductors (SC), and ferromagnetic-insulator (FI) layers have been explored experimentally and theoretically as alternative platforms for topological superconductivity at zero magnetic field. Here, we analyze a tripartite SM/FI/SC heterostructure but realized in a planar stacking geometry, where the thin FI layer acts as a spin-polarized tunneling barrier between the SM and the SC. We optimize the system's geometrical parameters using microscopic simulations, finding the range of FI thicknesses for which the hybrid system can be tuned into the topological regime. Within this range, and thanks to the vertical confinement provided by the stacking geometry, trivial and topological phases alternate regularly as the external gate is varied, displaying a hard topological gap that can reach half of the SC one. This is a significant improvement compared to setups using hexagonal nanowires, which show erratic topological regions with typically smaller and softer gaps. Our proposal provides a magnetic field-free planar design for quasi-one-dimensional topological superconductivity with attractive properties for experimental control and scalability.

cond-mat.mes-hall↗

arXiv2022

Entangling transmons with low-frequency protected superconducting qubits

Novel qubits with intrinsic noise protection constitute a promising route for improving the coherence of quantum information in superconducting circuits. However, many protected superconducting qubits exhibit relatively low transition frequencies, which could make their integration with conventional transmon circuits challenging. In this work, we propose and study a scheme for entangling a tunable transmon with a Cooper-pair parity-protected qubit, a paradigmatic example of a low-frequency protected qubit that stores quantum information in opposite Cooper-pair parity states on a superconducting island. By tuning the external flux on the transmon, we show that non-computational states can mediate a two-qubit entangling gate that preserves the Cooper-pair parity independent of the detailed pulse sequence. Interestingly, the entangling gate bears similarities to a controlled-phase gate in conventional transmon devices. Hence, our results suggest that standard high-precision gate calibration protocols could be repurposed for operating hybrid qubit devices.

quant-ph↗

arXiv2022

Vortex nucleation barrier in superconductors beyond the Bean-Livingston approximation: A numerical approach for the sphaleron problem in a gauge theory

The knowledge of vortex nucleation barriers is crucial for applications of superconductors, such as single-photon detectors and superconductor-based qubits. Contrarily to the problem of finding energy minima and critical fields, there are no controllable methods to explore the energy landscape, identify saddle points, and compute associated barriers. Similar problems exist in high-energy physics where the saddle-point configurations are called sphalerons. Here, we present a generalization of the string method to gauge field theories, which allows the calculation of energy barriers in superconductors. We solve the problem of vortex nucleation, assessing the effects of the nonlinearity of the model, complicated geometry, surface roughness, and pinning.

cond-mat.supr-con↗

arXiv2022

Vortex nucleation barriers and stable fractional vortices near boundaries in multicomponent superconductors

The magnetization process of a superconductor is determined by the potential barrier for vortex nucleation and escape. In multicomponent superconductors, fractional vortices with a winding in the phase of only one of the components can be stable topological solitons that carry a fraction of the flux quantum. While the formation of such objects in the bulk costs logarithmically or linearly divergent energy, these objects were shown to be stable near samples' boundaries in the two-component London model. Therefore, the conventional Bean-Livingston picture of magnetic flux entry does not apply to these superconductors, since the entry process can involve fractionalization of a vortex. In this paper, we address the nonlinear problem of determining the potential barrier for fluxoid penetration in a multicomponent superconductor, including the effects of various intercomponent couplings, by using the recently developed gauged string method. The method allows numerically exact (i.e., convergent) calculation of a sphaleron configuration in a gauge theory and thus the height of the nucleation barrier. We show how the fractionalized nucleation processes result in multiple sphalerons and intermediate states due to the complex shape of the energy landscape of multicomponent superconductors.

cond-mat.supr-con↗