arXiv subjects

Giovanni Isella

Publications and source records attributed to Giovanni Isella.

At least 19 recordsLinked to original sources

arXiv2026

Growth and characterization of planar hexagonal Ge on CdS

Hexagonal group-IV semiconductors have attracted increasing interest owing to their unconventional electronic and optical properties compared to the cubic diamond phase. However, the stabilization of these metastable allotropes in planar heterostructures remains a major challenge. In this work, we demonstrate the epitaxial growth of planar hexagonal germanium on non-basal $m$-plane CdS substrates by low-energy plasma-enhanced chemical vapor deposition. The role of growth temperature in the formation and stabilization of the hexagonal phase is investigated. X-ray diffraction, scanning transmission electron microscopy, and polarization-resolved Raman spectroscopy reveal the formation of epitaxial hexagonal germanium with the expected crystal symmetry. In particular, the Raman response exhibits the characteristic polarization selection rules of the E$_\text{2g}$ phonon mode of hexagonal Ge. Conversely, photoluminescence spectroscopy does not reveal any Ge-related emission feature. Combined transmission electron microscopy observations and atomistic modeling show that strain relaxation is governed by a limited set of dislocation mechanisms, which efficiently relieve most of the mismatch strain within a few nanometers from the interface and can involve localized cubic stacking insertions. At greater distances from the interface, the progressive loss of hexagonal order is increasingly dominated by stacking-fault disorder, particularly I3-type defects. These results establish CdS as a promising template for the planar stabilization of hexagonal Ge and provide insight into the defect mechanisms governing strain relaxation in metastable group-IV heterostructures.

cond-mat.mtrl-sci↗

arXiv2026

Germanium-Based Mid-Infrared Photonics

The mid-infrared (mid-IR) spectral range is a part of the electromagnetic spectrum in which most of the molecules have vibrational and rotational resonances. Ge-based photonic integrated circuits in this wavelength range have thus seen a burst of interest in the recent years, mainly driven by applications related with the detection of chemical and biological substances. Here we review the motivations and the recent developments in this field, from the different material platforms to active and nonlinear devices. We also discuss a few demonstrations of sensing that have already been conducted, attesting the potential applications of such devices. Finally, we conclude by discussing the challenges that have to be solved to transition from lab demonstrations to practical industrial devices.

physics.optics↗

arXiv2026

Granular aluminum induced superconductivity in germanium for hole spin-based hybrid devices

In superconductor-semiconductor hybrid structures, superconductivity and spin polarization are competing effects as magnetic fields break Cooper pairs. They can be combined using thin films and in-plane magnetic fields, an approach that enabled the pursuit of Majorana zero modes, Kitaev chains, and Andreev spin qubits (ASQs), but remains challenging for materials with small in-plane $g$-factors or when out-of-plane fields are required. We demonstrate that granular aluminium (grAl), composed of nanometre-scale aluminium grains embedded in an amorphous oxide matrix, can overcome this limitation. By depositing grAl on Ge/SiGe heterostructures, we induce a hard superconducting gap with BCS peaks at $305~\mu\mathrm{eV}$ and magnetic-field resilience for both the in-plane and out-of-plane directions, allowing Zeeman splitting of Yu-Shiba-Rusinov (YSR) states beyond $50~\mu\mathrm{eV}$ (12 GHz). Leveraging this robustness, we reveal signatures of hole physics and discuss a driving mechanism for ASQs regardless of the strength of the Rashba spin-orbit coupling in planar germanium.

cond-mat.mes-hall↗

arXiv2025

Brittle-to-ductile transition and strain relaxation in Si$_{1-x}$Ge$_x$ linearly graded buffers

The strain-relaxation mechanism of a set of Si$_{0.6}$Ge$_{0.4}$ linearly graded buffers (LGBs), grown following different temperature profiles, has been investigated by means of defect-etching and variable-temperature high-resolution X-ray diffraction (VT-HRXRD). Defect-etching experiments demonstrate that a sharp increase of threading dislocation density (TDD) from $3 \times 10^{5}$\,cm$^{-2}$ to $1.2 \times 10^{6}$\,cm$^{-2}$ takes place when the final growth temperature exceeds a critical value T$_c\approx 530^\circ$C. VT-HRXRD measurements show that in low TDD samples extra relaxation takes place for annealing temperatures larger than T$_c$, thanks to the nucleation of new dislocations. These results indicate that, below T$_c$, strain relaxation is driven by the gliding of existing dislocations while above T$_c$ new dislocations are nucleated, suggesting a link with our results and the brittle-to-ductile transition in Si$_{1-x}$Ge$_x$ alloys.

cond-mat.mtrl-sci↗

arXiv2025

Entropic costs of extracting classical ticks from a quantum clock

We experimentally realize a quantum clock by using a charge sensor to count charges tunneling through a double quantum dot (DQD). Individual tunneling events are used as the clock's ticks. We quantify the clock's precision while measuring the power dissipated by the DQD and, separately, the charge sensor in both direct-current and radio-frequency readout modes. This allows us to probe the thermodynamic cost of creating ticks microscopically and recording them macroscopically. Our experiment is the first to explore the interplay between the entropy produced by a microscopic clockwork and its macroscopic measurement apparatus. We show that the latter contribution not only dwarfs the former but also unlocks greatly increased precision, because the measurement record can be exploited to optimally estimate time even when the DQD is at equilibrium. Our results suggest that the entropy produced by the amplification and measurement of a clock's ticks, which has often been ignored in the literature, is the most important and fundamental thermodynamic cost of timekeeping at the quantum scale.

quant-ph↗

arXiv2025

Virtual Gates Enabled by Digital Surrogate of Quantum Dot Devices

Advances in quantum technologies are often limited by slow device characterization, complex tuning requirements, and scalability challenges. Spin qubits in electrostatically defined quantum dots provide a promising platform but are not exempt from these limitations. Simulations enhance our understanding of such devices, and in many cases, rapid feedback between measurements and simulations can guide the development of optimal design and control strategies. Here, we introduce a modular, graph-based simulator that acts as a digital surrogate for a semiconductor quantum dot device, where computationally expensive processes are accelerated using deep learning. We demonstrate its potential by estimating crosstalk effects between gate electrodes and applying these estimates to construct virtual gates in a quantum dot device. We validate our approach through comparison with experiments on a double quantum dot defined in a Ge/SiGe heterostructure. We envision that this simulation framework will advance semiconductor-based quantum technologies by enabling more efficient design, characterization, and control of complex devices.

cond-mat.mes-hall↗

arXiv2025

Rapid optimal work extraction from a quantum-dot information engine

The conversion of thermal energy into work is usually more efficient in the slow-driving regime, where the power output is vanishingly small. Efficient work extraction for fast driving protocols remains an outstanding challenge at the nanoscale, where fluctuations play a significant role. In this Letter, we use a quantum-dot Szilard engine to extract work from thermal fluctuations with maximum efficiency over two decades of driving speed. We design and implement a family of optimised protocols ranging from the slow- to the fast-driving regime, and measure the engine's efficiency as well as the mean and variance of its power output in each case. These optimised protocols exhibit significant improvements in power and efficiency compared to the naive approach. Our results also show that, when optimising for efficiency, boosting the power output of a Szilard engine inevitably comes at the cost of increased power fluctuations.

quant-ph↗

arXiv2025

Automated All-RF Tuning for Spin Qubit Readout and Control

Efficient tuning of spin qubits remains a major bottleneck in scaling semiconductor quantum dot-based quantum processors. A key challenge is the rapid identification of gate voltage regimes suitable for qubit initialisation, control, and readout. Here, we leverage radio-frequency charge sensing to automate spin qubit tuning, achieving a median tuning time of approximately 15 minutes. In a single continuous run, our routine identifies spin qubits at 12 distinct charge transitions in under 17 hours. Beyond tuning, our routine autonomously acquires data revealing the gate-voltage dependence of the exchange interaction, dephasing time, and quality factor -- quantities that vary substantially between charge configurations. These results represent a step change in high-throughput spin qubit tuning and provide a foundation for a systematic and automated exploration of semiconductor quantum circuits.

cond-mat.mes-hall↗

arXiv2025

On-chip pulse generation at 8 μm wavelength

The mid-infrared spectral region holds growing importance for applications such as gas sensing and spectroscopy. Although compact ultrashort pulse laser sources are essential to enable these applications, their realization in this spectral range remains an open challenge. We demonstrate an integrated approach to generate pulses in the mid-infrared based on chirped Bragg gratings engineered to compensate for the group delay dispersion of quantum cascade laser frequency comb sources. SiGe graded-index photonic circuits are used for operation around 8 μm wavelength. With this approach, pulses as short as 1.39 picoseconds were obtained, marking a key step towards fully integrated ultrashort pulse sources in the mid-infrared.

physics.optics↗

arXiv2025

Enhancing atomic-resolution in electron microscopy: A frequency-domain deep learning denoiser

Atomic resolution electron microscopy, particularly high-angle annular dark-field scanning transmission electron microscopy, has become an essential tool for many scientific fields, when direct visualization of atomic arrangements and defects are needed, as they dictate the material's functional and mechanical behavior. However, achieving this precision is often hindered by noise, arising from electron microscopy acquisition limitations, particularly when imaging beam-sensitive materials or light atoms. In this work, we present a deep learning-based denoising approach that operates in the frequency domain using a convolutional neural network U-Net trained on simulated data. To generate the training dataset, we simulate FFT patterns for various materials, crystallographic orientations, and imaging conditions, introducing noise and drift artifacts to accurately mimic experimental scenarios. The model is trained to identify relevant frequency components, which are then used to enhance experimental images by applying element-wise multiplication in the frequency domain. The model enhances experimental images by identifying and amplifying relevant frequency components, significantly improving signal-to-noise ratio while preserving structural integrity. Applied to both Ge quantum wells and WS2 monolayers, the method facilitates more accurate strain quantitative analyses, critical for assessing functional device performance (e.g. quantum properties in SiGe quantum wells), and enables the clear identification of light atoms in beam sensitive materials. Our results demonstrate the potential of automated frequency-based deep learning denoising as a useful tool for atomic-resolution nano-materials analysis.

cond-mat.mtrl-sci↗

arXiv2024

Strong hole-photon coupling in planar Ge for probing charge degree and strongly-correlated states

Semiconductor quantum dots (QDs) in planar germanium (Ge) heterostructures have emerged as front-runners for future hole-based quantum processors. Here, we present strong coupling between a hole charge qubit, defined in a double quantum dot (DQD) in planar Ge, and microwave photons in a high-impedance ($Z_\mathrm{r} = 1.3 ~\mathrm{k}Ω$) resonator based on an array of superconducting quantum interference devices (SQUIDs). Our investigation reveals vacuum-Rabi splittings with coupling strengths up to $g_0/2π= 260 ~\mathrm{MHz}$, and a cooperativity of $C \sim 100$, dependent on DQD tuning. Furthermore, utilizing the frequency tunability of our resonator, we explore the quenched energy splitting associated with strong Coulomb correlation effects in Ge QDs. The observed enhanced coherence of the strongly correlated excited state signals the presence of distinct symmetries within related spin functions, serving as a precursor to the strong coupling between photons and spin-charge hybrid qubits in planar Ge. This work paves the way towards coherent quantum connections between remote hole qubits in planar Ge, required to scale up hole-based quantum processors.

quant-ph↗

arXiv2024

Exchange anisotropies in microwave-driven singlet-triplet qubits

Hole spin qubits are rapidly emerging as the workhorse of semiconducting quantum processors because of their large spin-orbit interaction, enabling fast all-electric operations at low power. However, spin-orbit interaction also causes non-uniformities in devices, resulting in locally varying qubit energies and site-dependent anisotropies. While these anisotropies can be used to drive single-spins, if not properly harnessed, they can hinder the path toward large-scale quantum processors. Here, we report on microwave-driven singlet-triplet qubits in planar germanium and use them to investigate the anisotropy of two spins in a double quantum dot. We show two distinct operating regimes depending on the magnetic field direction. For in-plane fields, the two spins are largely anisotropic, and electrically tunable, which enables to measure all the available transitions; coherence times exceeding 3 $μ$s are extracted. For out-of-plane fields, they have an isotropic response but preserve the substantial energy difference required to address the singlet-triplet qubit. Even in this field direction, where the qubit lifetime is strongly affected by nuclear spins, we find 400 ns coherence times. Our work adds a valuable tool to investigate and harness the anisotropy of spin qubits and can be implemented in any large-scale NxN device, facilitating the path towards scalable quantum processors.

cond-mat.mes-hall↗

arXiv2024

Artificial Intelligence-Assisted Workflow for Transmission Electron Microscopy: From Data Analysis Automation to Materials Knowledge Unveiling

(Scanning) transmission electron microscopy ((S)TEM) has significantly advanced materials science but faces challenges in correlating precise atomic structure information with the functional properties of devices due to its time-intensive nature. To address this, we introduce an analytical workflow for the holistic characterization, modelling, and simulation of device heterostructures. This workflow automates the experimental (S)TEM data analysis, providing an in-depth characterization of crystallographic information, 3D orientation, elemental composition, and strain distribution. It reduces a process that typically takes days for a trained human into an automatic routine solved in minutes. Utilizing a physics-guided artificial intelligence model, it generates representative descriptions of materials and samples. The workflow culminates in creating digital twins, 3D finite element and atomic models of millions of atoms, enabling simulations that provide crucial insights into device behaviour in practical applications. Demonstrated with SiGe planar heterostructures for scalable spin qubits, the workflow links digital twins to theoretical properties, revealing how atomic structure impacts materials and functional properties such as spatially-resolved phononic or electronic characteristics, or (inverse) spin orbit lengths. The versatility of our workflow is demonstrated through its application to a wide array of materials systems, device configurations, and sample morphologies.

cond-mat.mtrl-sci↗

arXiv2024

Strong Charge-Photon Coupling in Planar Germanium Enabled by Granular Aluminium Superinductors

High kinetic inductance superconductors are gaining increasing interest for the realisation of qubits, amplifiers and detectors. Moreover, thanks to their high impedance, quantum buses made of such materials enable large zero-point fluctuations of the voltage, boosting the coupling rates to spin and charge qubits. However, fully exploiting the potential of disordered or granular superconductors is challenging, as their inductance and, therefore, impedance at high values are difficult to control. Here we have integrated a granular aluminium resonator, having a characteristic impedance exceeding the resistance quantum, with a germanium double quantum dot and demonstrate strong charge-photon coupling with a rate of $g_\text{c}/2π= (566 \pm 2)$ MHz. This was achieved due to the realisation of a wireless ohmmeter, which allows \emph{in situ} measurements during film deposition and, therefore, control of the kinetic inductance of granular aluminium films. Reproducible fabrication of circuits with impedances (inductances) exceeding 13 k$Ω$ (1 nH per square) is now possible. This broadly applicable method opens the path for novel qubits and high-fidelity, long-distance two-qubit gates.

cond-mat.mes-hall↗

arXiv2024

A gate tunable transmon qubit in planar Ge

Gate-tunable transmons (gatemons) employing semiconductor Josephson junctions have recently emerged as building blocks for hybrid quantum circuits. In this study, we present a gatemon fabricated in planar Germanium. We induce superconductivity in a two-dimensional hole gas by evaporating aluminum atop a thin spacer, which separates the superconductor from the Ge quantum well. The Josephson junction is then integrated into an Xmon circuit and capacitively coupled to a transmission line resonator. We showcase the qubit tunability in a broad frequency range with resonator and two-tone spectroscopy. Time-domain characterizations reveal energy relaxation and coherence times up to 75 ns. Our results, combined with the recent advances in the spin qubit field, pave the way towards novel hybrid and protected qubits in a group IV, CMOS-compatible material.

cond-mat.mes-hall↗

arXiv2024

Tunable on-chip electro-optic frequency-comb generation at 8 μm wavelength

Dual-comb spectroscopy is a powerful technique to measure optical spectra in a wide spectral range with high-frequency resolution. The development of compact systems operating in the long-wave infrared wavelength range is of high interest for spectroscopic and sensing applications. Amongst the different techniques to obtain optical frequency-combs, electro-optic frequency-comb generation presents major advantages thanks to the tunable repetition rate only limited by the bandwidth of the used electro-optical modulator. However, the development of integrated and efficient electro-optical modulators operating in a wide long-wave infrared spectral band is still at its infancy, and electro-optical frequency-comb has not been demonstrated so far beyond the telecom band. In this work, a Schottky-based modulator embedded in a Ge-rich graded SiGe waveguide is used for electro-optic frequency-comb generation. Considering the limited efficiency of the modulator, harmonically-rich RF signals are used to enhance the generation of comb lines around the optical carrier. Interestingly, this allows us to demonstrate the generation of electro-optical combs spanning over 2.4 GHz around 8 μm wavelength. This paves the way towards fully integrated and tunable mid-infrared electro-optic frequency-comb generation systems.

physics.optics↗

arXiv2023

Parity-conserving Cooper-pair transport and ideal superconducting diode in planar Germanium

Superconductor/semiconductor hybrid devices have attracted increasing interest in the past years. Superconducting electronics aims to complement semiconductor technology, while hybrid architectures are at the forefront of new ideas such as topological superconductivity and protected qubits. In this work, we engineer the induced superconductivity in two-dimensional germanium hole gas by varying the distance between the quantum well and the aluminum. We demonstrate a hard superconducting gap and realize an electrically and flux tunable superconducting diode using a superconducting quantum interference device (SQUID). This allows to tune the current phase relation (CPR), to a regime where single Cooper pair tunneling is suppressed, creating a $\sin \left( 2 φ\right)$ CPR. Shapiro experiments complement this interpretation and the microwave drive allows to create a diode with 100% efficiency. The reported results open up the path towards integration of spin qubit devices, microwave resonators and (protected) superconducting qubits on a silicon technology compatible platform.

cond-mat.mes-hall↗

arXiv2023

Automated long-range compensation of an rf quantum dot sensor

Charge sensing is a sensitive technique for probing quantum devices, of particular importance for spin qubit readout. To achieve good readout sensitivities, the proximity of the charge sensor to the device to be measured is a necessity. However, this proximity also means that the operation of the device affects, in turn, the sensor tuning and ultimately the readout sensitivity. We present an approach for compensating for this cross-talk effect allowing for the gate voltages of the measured device to be swept in a 1 V x 1 V window while maintaining a sensor configuration chosen by a Bayesian optimiser. Our algorithm is a key contribution to the suite of fully automated solutions required for the operation of large quantum device architectures.

cond-mat.mes-hall↗