arXiv subjects

Y. -M. Niquet

Publications and source records attributed to Y. -M. Niquet.

14 recordsLinked to original sources

arXiv2026

A hole spin resilient to dipole-induced thermal effects

Recent advances in scaling up spin-based quantum processors have revealed unanticipated issues related to thermal effects. Microwave pulses required to manipulate and read the qubits are found to overheat the spins environment, which unexpectedly induces Larmor frequency shifts, reducing thereby gate fidelities. In this study, we shine light on these elusive thermal effects, by experimentally characterizing the temperature dependence of the Larmor frequency for a single hole spin in silicon. Our results unambiguously reveal an electrical origin underlying the thermal susceptibility, stemming from the spin-orbit-induced electric susceptibility. We perform an accurate modeling of the spin electrostatic environment and gyromagnetic properties, allowing us to pinpoint electric dipoles which unfreeze as the temperature increases, as responsible for these frequency shifts. Surprisingly, we find that the thermal susceptibility can be tuned with the magnetic field angle and can even cancel out, unveiling a sweet spot where the hole spin is rendered immune to thermal effects. These findings reveal the microscopic origin of longitudinal thermal susceptibility, providing key insight for the design of thermally robust semiconductor qubits.

cond-mat.mes-hall↗

arXiv2026

Spin-cQED with bulk germanium spin qubits

Unstrained bulk germanium is a particularly attractive material for circuit quantum electrodynamics with spins (spin-cQED). We show, through systematic modeling and comparison with state-of-the-art strained germanium heterostructures, that hole spins in bulk germanium double quantum dots readily reach the strong-coupling regime with superconducting microwave resonators, achieving spin-photon coupling strengths $g_s/2π\gtrsim100$\,MHz. This enhancement originates from large spin-orbit interactions beyond the perturbative regime. In addition, the coupling is much less sensitive to the orientation of the applied magnetic field, which shall ease operation and limit the impact of device-to-device variability. Our results establish bulk germanium as a compelling platform for scalable spin-cQED.

cond-mat.mes-hall↗

arXiv2023

A new FDSOI spin qubit platform with 40nm effective control pitch

Operating Si quantum dot (QD) arrays requires homogeneous and ultra-dense structures with aggressive gate pitch. Such a density is necessary to separately control the QDs chemical potential (i.e. charge occupation of each QD) from the exchange interaction (i.e. tunnel barriers between each QD). We present here a novel Si quantum device integration that halves the effective gate pitch and provides full controllability in 1D FDSOI QD arrays. The major advantages of this architecture are explored through numerical simulations. Functionality of the fabricated structure is validated via 300K statistical electrical characterization, while tunnel-coupling control is demonstrated at cryogenic temperature.

cond-mat.mes-hall↗

arXiv2023

Tunable hole spin-photon interaction based on g-matrix modulation

We consider a spin circuit-QED device where a superconducting microwave resonator is capacitively coupled to a single hole confined in a semiconductor quantum dot. Thanks to the strong spin-orbit coupling intrinsic to valence-band states, the gyromagnetic g-matrix of the hole can be modulated electrically. This modulation couples the photons in the resonator to the hole spin. We show that the applied gate voltages and the magnetic-field orientation enable a versatile control of the spin-photon interaction, whose character can be switched from fully transverse to fully longitudinal. The longitudinal coupling is actually maximal when the transverse one vanishes and vice-versa. This "reciprocal sweetness" results from geometrical properties of the g-matrix and protects the spin against dephasing or relaxation. We estimate coupling rates reaching ~ 10 MHz in realistic settings and discuss potential circuit-QED applications harnessing either the transverse or the longitudinal spin-photon interaction. Furthermore, we demonstrate that the g-matrix curvature can be used to achieve parametric longitudinal coupling with enhanced coherence.

cond-mat.mes-hall↗

arXiv2022

A single hole spin with enhanced coherence in natural silicon

Semiconductor spin qubits based on spin-orbit states are responsive to electric field excitation allowing for practical, fast and potentially scalable qubit control. Spin-electric susceptibility, however, renders these qubits generally vulnerable to electrical noise, which limits their coherence time. Here we report on a spin-orbit qubit consisting of a single hole electrostatically confined in a natural silicon metal-oxide-semiconductor device. By varying the magnetic field orientation, we reveal the existence of operation sweet spots where the impact of charge noise is minimized while preserving an efficient electric-dipole spin control. We correspondingly observe an extension of the Hahn-echo coherence time up to 88 $μ$s, exceeding by an order of magnitude the best reported values for hole-spin qubits, and approaching the state-of-the-art for electron spin qubits with synthetic spin-orbit coupling in isotopically-purified silicon. This finding largely enhances the prospects of silicon-based hole spin qubits for scalable quantum information processing.

cond-mat.mes-hall↗

arXiv2021

Light-hole states in a strained quantum dot: numerical calculation and phenomenological models

Starting from the numerical solution of the 6-band \textbf{k.p} description of a lattice-mismatched ellipsoidal quantum dot situated inside a nanowire, including a spin Zeeman effect with values appropriate to a dilute magnetic semiconductor, we propose and test phenomenological models of the effect of the built-in strain on the heavy hole, light hole and exciton states. We test the validity and the limits of a description restricted to a ($Γ_8$) quadruplet of ground states and we demonstrate the role of the interactions of the light-hole state with light-hole excited states. We show that the built-in axial strain not only defines the character, heavy-hole or light-hole, of the ground state, but also mixes significantly the light-hole state with the split-off band's states: Even for a spin-orbit energy as large as 1 eV, that mixing induces first-order modifications of properties such as the spin value and anisotropy, the oscillator strength, and the electron-hole exchange, for which we extend the description to the light-hole exciton. CdTe/ZnTe quantum dots are mainly used as a test case but the concepts we discuss apply to many heterostructures, from mismatched II-VI and III-V quantum dots and nanowires, to III-V nanostructures submitted to an applied stress and to silicon nanodevices with even smaller residual strains.

cond-mat.mes-hall↗

arXiv2020

Spin readout of a CMOS quantum dot by gate reflectometry and spin-dependent tunnelling

Silicon spin qubits are promising candidates for realising large scale quantum processors, benefitting from a magnetically quiet host material and the prospects of leveraging the mature silicon device fabrication industry. We report the measurement of an electron spin in a singly-occupied gate-defined quantum dot, fabricated using CMOS compatible processes at the 300 mm wafer scale. For readout, we employ spin-dependent tunneling combined with a low-footprint single-lead quantum dot charge sensor, measured using radiofrequency gate reflectometry. We demonstrate spin readout in two devices using this technique, obtaining valley splittings in the range 0.5-0.7 meV using excited state spectroscopy, and measure a maximum electron spin relaxation time ($T_1$) of $9 \pm 3$ s at 1 Tesla. These long lifetimes indicate the silicon nanowire geometry and fabrication processes employed here show a great deal of promise for qubit devices, while the spin-readout method demonstrated here is well-suited to a variety of scalable architectures.

cond-mat.mes-hall↗

arXiv2019

Gate reflectometry for probing charge and spin states in linear Si MOS split-gate arrays

We fabricated linear arrangements of multiple splitgate devices along an SOI mesa, thus forming a 2xN array of individually controllable Si quantum dots (QDs) with nearest neighbor coupling. We implemented two different gate reflectometry-based readout schemes to either probe spindependent charge movements by a coupled electrometer with single-shot precision, or directly sense a spin-dependent quantum capacitance. These results bear significance for fast, high-fidelity single-shot readout of large arrays of foundrycompatible Si MOS spin qubits.

cond-mat.mes-hall↗

arXiv2019

Towards scalable silicon quantum computing

We report the efforts and challenges dedicated towards building a scalable quantum computer based on Si spin qubits. We review the advantages of relying on devices fabricated in a thin film technology as their properties can be in situ tuned by the back gate voltage, which prefigures tuning capabilities in scalable qubits architectures.

cond-mat.mes-hall↗

arXiv2019

All-Electrical Control of a Hybrid Electron Spin/Valley Quantum Bit in SOI CMOS Technology

We successfully demonstrated experimentally the electrical-field-mediated control of the spin of electrons confined in an SOI Quantum Dot (QD) device fabricated with a standard CMOS process flow. Furthermore, we show that the Back-Gate control in SOI devices enables switching a quantum bit (qubit) between an electrically-addressable, yet charge noise-sensitive configuration, and a protected configuration.

cond-mat.mes-hall↗

arXiv2019

SOI technology for quantum information processing

We present recent progress towards the implementation of a scalable quantum processor based on fully-depleted silicon-on-insulator (FDSOI) technology. In particular, we discuss an approach where the elementary bits of quantum information - so-called qubits - are encoded in the spin degree of freedom of gate-confined holes in p-type devices. We show how a hole-spin can be efficiently manipulated by means of a microwave excitation applied to the corresponding confining gate. The hole spin state can be read out and reinitialized through a Pauli blockade mechanism. The studied devices are derived from silicon nanowire field-effect transistors. We discuss their prospects for scalability and, more broadly, the potential advantages of FDSOI technology.

cond-mat.mes-hall↗

arXiv2015

Uniaxially stressed germanium with fundamental direct band gap

We demonstrate the crossover from indirect- to direct band gap in tensile-strained germanium by temperature-dependent photoluminescence. The samples are strained microbridges that enhance a biaxial strain of 0.16% up to 3.6% uniaxial tensile strain. Cooling the bridges to 20 K increases the uniaxial strain up to a maximum of 5.4%. Temperature-dependent photoluminescence reveals the crossover to a fundamental direct band gap to occur between 4.0% and 4.5%. Our data are in good agreement with new theoretical computations that predict a strong bowing of the band parameters with strain.

cond-mat.mtrl-sci↗

arXiv2015

Topological states in multi-orbital HgTe honeycomb lattices

Research on graphene has revealed remarkable phenomena arising in the honeycomb lattice. However, the quantum spin Hall effect predicted at the K point could not be observed in graphene and other honeycomb structures of light elements due to an insufficiently strong spin-orbit coupling. Here we show theoretically that 2D honeycomb lattices of HgTe can combine the effects of the honeycomb geometry and strong spin-orbit coupling. The conduction bands, experimentally accessible via doping, can be described by a tight-binding lattice model as in graphene, but including multi-orbital degrees of freedom and spin-orbit coupling. This results in very large topological gaps (up to 35 meV) and a flattened band detached from the others. Owing to this flat band and the sizable Coulomb interaction, honeycomb structures of HgTe constitute a promising platform for the observation of a fractional Chern insulator or a fractional quantum spin Hall phase.

cond-mat.mes-hall↗

arXiv2004

Describing static correlation in bond dissociation by Kohn-Sham density functional theory

We show that density functional theory within the RPA (random phase approximation for the exchange-correlation energy) provides a correct description of bond dissociation in H$_2$ in a spin-restricted Kohn-Sham formalism, i.e. without artificial symmetry breaking. We present accurate adiabatic connection curves both at equilibrium and beyond the Coulson-Fisher point. The strong curvature at large bond length implies important static (left-right) correlation, justifying modern hybrid functional constructions but also demonstrating their limitations. Although exact at infinite and accurate around the equilibrium bond length, the RPA dissociation curve displays unphysical repulsion at larger but finite bond lengths. Going beyond the RPA by including the exact exchange kernel (RPA+X), we find a similar repulsion. We argue that this deficiency is due to the absence of double excitations in adiabatic linear response theory. Further analyzing the H$_2$ dissociation limit we show that the RPA+X is not size-consistent, in contrast to the RPA.

cond-mat.other↗