arXiv subjects

Yann-Michel Niquet

Publications and source records attributed to Yann-Michel Niquet.

At least 37 records · Page 2Linked to original sources

arXiv2023

Linear-in-momentum spin orbit interactions in planar Ge/GeSi heterostructures and spin qubits

We investigate the existence of linear-in-momentum spin-orbit interactions in the valence band of Ge/GeSi heterostructures using an atomistic tight-binding method. We show that symmetry breaking at the Ge/GeSi interfaces gives rise to a linear Dresselhaus-type interaction for heavy-holes. This interaction results from the heavy-hole/light-hole mixings induced by the interfaces and can be captured by a suitable correction to the minimal Luttinger-Kohn, four bands $\vec{k}\cdot\vec{p}$ Hamiltonian. It is dependent on the steepness of the Ge/GeSi interfaces, and is suppressed if interdiffusion is strong enough. Besides the Dresselhaus interaction, the Ge/GeSi interfaces also make a contribution to the in-plane gyromagnetic $g$-factors of the holes. The tight-binding calculations also highlight the existence of a small linear Rashba interaction resulting from the couplings between the heavy-hole/light-hole manifold and the conduction band enabled by the low structural symmetry of Ge/GeSi heterostructures. These interactions can be leveraged to drive the hole spin. The linear Dresselhaus interaction may, in particular, dominate the physics of the devices for out-of-plane magnetic fields. When the magnetic field lies in-plane, it is, however, usually far less efficient than the $g$-tensor modulation mechanisms arising from the motion of the dot in non-separable, inhomogeneous electric fields and strains.

cond-mat.mes-hall↗

arXiv2023

Hole spin driving by strain-induced spin-orbit interactions

Hole spins in semiconductor quantum dots can be efficiently manipulated with radio-frequency electric fields owing to the strong spin-orbit interactions in the valence bands. Here we show that the motion of the dot in inhomogeneous strain fields gives rise to linear Rashba spin-orbit interactions (with spatially dependent spin-orbit lengths) and g-factor modulations that allow for fast Rabi oscillations. Such inhomogeneous strains may build up spontaneously due to process and cool down stress. We discuss spin qubits in Ge/GeSi heterostructures as an illustration. We highlight that Rabi frequencies can be enhanced by one order of magnitude by shear strain gradients as small as $3\times 10^{-6}$ nm$^{-1}$ within the dots. This underlines that spin in solids can be very sensitive to strains and opens the way for strain engineering in hole spin devices for quantum information and spintronics.

cond-mat.mes-hall↗

arXiv2023

Gate reflectometry in dense quantum dot arrays

Silicon quantum devices are maturing from academic single- and two-qubit devices to industrially-fabricated dense quantum-dot (QD) arrays, increasing operational complexity and the need for better pulsed-gate and readout techniques. We perform gate-voltage pulsing and gate-based reflectometry measurements on a dense 2$\times$2 array of silicon quantum dots fabricated in a 300-mm-wafer foundry. Utilizing the strong capacitive couplings within the array, it is sufficient to monitor only one gate electrode via high-frequency reflectometry to establish single-electron occupation in each of the four dots and to detect single-electron movements with high bandwidth. A global top-gate electrode adjusts the overall tunneling times, while linear combinations of side-gate voltages yield detailed charge stability diagrams. To test for spin physics and Pauli spin blockade at finite magnetic fields, we implement symmetric gate-voltage pulses that directly reveal bidirectional interdot charge relaxation as a function of the detuning between two dots. Charge sensing within the array can be established without the involvement of adjacent electron reservoirs, important for scaling such split-gate devices towards longer 2$\times$N arrays. Our techniques may find use in the scaling of few-dot spin-qubit devices to large-scale quantum processors.

cond-mat.mes-hall↗

arXiv2023

Strong coupling between a photon and a hole spin in silicon

Spins in semiconductor quantum dots constitute a promising platform for scalable quantum information processing. Coupling them strongly to the photonic modes of superconducting microwave resonators would enable fast non-demolition readout and long-range, on-chip connectivity, well beyond nearest-neighbor quantum interactions. Here we demonstrate strong coupling between a microwave photon in a superconducting resonator and a hole spin in a silicon-based double quantum dot issued from a foundry-compatible MOS fabrication process. By leveraging the strong spin-orbit interaction intrinsically present in the valence band of silicon, we achieve a spin-photon coupling rate as high as 330~MHz largely exceeding the combined spin-photon decoherence rate. This result, together with the recently demonstrated long coherence of hole spins in silicon, opens a new realistic pathway to the development of circuit quantum electrodynamics with spins in semiconductor quantum dots.

cond-mat.mes-hall↗

arXiv2023

Modelling of spin decoherence in a Si hole qubit perturbed by a single charge fluctuator

Spin qubits in semiconductor quantum dots are one of the promizing devices to realize a quantum processor. A better knowledge of the noise sources affecting the coherence of such a qubit is therefore of prime importance. In this work, we study the effect of telegraphic noise induced by the fluctuation of a single electric charge. We simulate as realistically as possible a hole spin qubit in a quantum dot defined electrostatically by a set of gates along a silicon nanowire channel. Calculations combining Poisson and time-dependent Schrödinger equations allow to simulate the relaxation and the dephasing of the hole spin as a function of time for a classical random telegraph signal. We show that dephasing time $T_2$ is well given by a two-level model in a wide range of frequency. Remarkably, in the most realistic configuration of a low frequency fluctuator, the system has a non-Gaussian behavior in which the phase coherence is lost as soon as the fluctuator has changed state. The Gaussian description becomes valid only beyond a threshold frequency $ω_{th}$, when the two-level system reacts to the statistical distribution of the fluctuator states. We show that the dephasing time $T_{2}(ω_{th})$ at this threshold frequency can be considerably increased by playing on the orientation of the magnetic field and the gate potentials, by running the qubit along "sweet" lines. However, $T_{2}(ω_{th})$ remains bounded due to dephasing induced by the non-diagonal terms of the stochastic perturbation Hamiltonian. Our simulations reveal that the spin relaxation cannot be described cleanly in the two-level model because the coupling to higher energy hole levels impacts very strongly the spin decoherence. This result suggests that multi-level simulations including the coupling to phonons should be necessary to describe the relaxation phenomenon in this type of qubit.

cond-mat.mes-hall↗

arXiv2023

Electrical manipulation of a single electron spin in CMOS with micromagnet and spin-valley coupling

For semiconductor spin qubits, complementary-metal-oxide-semiconductor (CMOS) technology is the ideal candidate for reliable and scalable fabrication. Making the direct leap from academic fabrication to qubits fabricated fully by industrial CMOS standards is difficult without intermediate solutions. With a flexible back-end-of-line (BEOL) new functionalities such as micromagnets or superconducting circuits can be added in a post-CMOS process to study the physics of these devices or achieve proof of concepts. Once the process is established it can be incorporated in the foundry-compatible process flow. Here, we study a single electron spin qubit in a CMOS device with a micromagnet integrated in the flexible BEOL. We exploit the synthetic spin orbit coupling (SOC) to control the qubit via electric field and we investigate the spin-valley physics in the presence of SOC where we show an enhancement of the Rabi frequency at the spin-valley hotspot. Finally, we probe the high frequency noise in the system using dynamical decoupling pulse sequences and demonstrate that charge noise dominates the qubit decoherence in this range.

cond-mat.mes-hall↗

arXiv2023

Bilinear magnetoresistance in HgTe topological insulator: opposite signs at opposite interfaces demonstrated by gate control

Spin-orbit effects appearing in topological insulators (TI) and at Rashba interfaces are currently revolutionizing how we can manipulate spins and have led to several newly discovered effects, from spin-charge interconversion and spin-orbit torques to novel magnetoresistance phenomena. In particular, a puzzling magnetoresistance has been evidenced, bilinear in electric and magnetic fields. Here, we report the observation of bilinear magnetoresistance (BMR) in strained HgTe, a prototypical TI. We show that both the amplitude and sign of this BMR can be tuned by controlling, with an electric gate, the relative proportions of the opposite contributions of opposite surfaces. At magnetic fields of 1 T, the magnetoresistance is of the order of 1 \% and has a larger figure of merit than previously measured TIs. We propose a theoretical model giving a quantitative account of our experimental data. This phenomenon, unique to TI, offers novel opportunities to tune their electrical response for spintronics.

cond-mat.mes-hall↗

arXiv2022

Hole spin manipulation in inhomogeneous and non-separable electric fields

The usual models for electrical spin manipulation in semiconductor quantum dots assume that the confinement potential is separable in the three spatial dimensions and that the AC drive field is homogeneous. However, the electric field induced by the gates in quantum dot devices is not fully separable and displays significant inhomogeneities. Here, we address the electrical manipulation of hole spins in semiconductor heterostructures subject to inhomogeneous vertical electric fields and/or in-plane AC electric fields. We consider Ge quantum dots electrically confined in a Ge/GeSi quantum well as an illustration. We show that the lack of separability between the vertical and in-plane motions gives rise to an additional spin-orbit coupling mechanism (beyond the usual linear and cubic in momentum Rashba terms) that modulates the principal axes of the hole gyromagnetic g-matrix. This non-separability mechanism can be of the same order of magnitude as Rashba-type interactions, and enables spin manipulation when the magnetic field is applied in the plane of the heterostructure even if the dot is symmetric (disk-shaped). More generally, we show that Rabi oscillations in strongly patterned electric fields harness a variety of g-factor modulations. We discuss the implications for the design, modeling and understanding of hole spin qubit devices.

cond-mat.mes-hall↗

arXiv2022

Probing charge noise in few electron CMOS quantum dots

Charge noise is one of the main sources of environmental decoherence for spin qubits in silicon, presenting a major obstacle in the path towards highly scalable and reproducible qubit fabrication. Here we demonstrate in-depth characterization of the charge noise environment experienced by a quantum dot in a CMOS-fabricated silicon nanowire. We probe the charge noise for different quantum dot configurations, finding that it is possible to tune the charge noise over two orders of magnitude, ranging from 1 ueV^2 to 100 ueV^2. In particular, we show that the top interface and the reservoirs are the main sources of charge noise and their effect can be mitigated by controlling the quantum dot extension. Additionally, we demonstrate a novel method for the measurement of the charge noise experienced by a quantum dot in the few electron regime. We measure a comparatively higher charge noise value of 40 ueV^2 at the first electron, and demonstrate that the charge noise is highly dependent on the electron occupancy of the quantum dot.

cond-mat.mes-hall↗

arXiv2022

Investigation of Lasing in Highly Strained Germanium at the Crossover to Direct Band Gap

Efficient and cost-effective Si-compatible lasers are a long standing wish of the optoelectronic industry. In principle, there are two options. For many applications, lasers based on III-V compounds provide compelling solutions, even if the integration is complex and therefore costly. However, where low costs and also high integration density are crucial, group-IV-based lasers - made of Ge and GeSn, for example - could be an alternative, provided their performance can be improved. Such progresses will come with better materials but also with the development of a profounder understanding of their optical properties. In this work, we demonstrate, using Ge microbridges with strain up to 6.6%, a powerful method for determining the population inversion gain and the material and optical losses of group IV lasers. This is made by deriving the values for the injection carrier densities and the cavity losses from the measurement of the change of the refractive index and the mode linewidth, respectively. We observe a laser threshold consistent with optical gain and material loss values obtained from a tight binding calculation. Lasing in Ge - at steady-state - is found to be limited to low temperatures in a narrow regime of tensile strain at the crossover to the direct band gap bandstructure. We explain this observation by parasitic intervalence band absorption that increases rapidly with higher injection densities and temperature. N-doping seems to reduce the material loss at low excitation but does not extend the lasing regime. We also discuss the impact of the optically inactive carriers in the L-valley on the linewidth of group IV lasers.

physics.optics↗

arXiv2021

Two-body Wigner molecularization in asymmetric quantum dot spin qubits

Coulomb interactions strongly influence the spectrum and the wave functions of few electrons or holes confined in a quantum dot. In particular, when the confinement potential is not too strong, the Coulomb repulsion triggers the formation of a correlated state, the Wigner molecule, where the particles tend to split apart. We show that the anisotropy of the confinement potential strongly enhances the molecularization process and affects the performances of quantum-dot systems used as spin qubits. Relying on analytical and numerical solutions of the two-particle problem -- both in a simplified single-band approximation and in realistic setups -- we highlight the exponential suppression of the singlet-triplet gap with increasing anisotropy. We compare the molecularization effects in different semiconductor materials and discuss how they specifically hamper Pauli spin blockade readout and reduce the exchange interactions in two-qubit gates.

cond-mat.mes-hall↗

arXiv2021

Variability of electron and hole spin qubits due to interface roughness and charge traps

Semiconductor spin qubits may show significant device-to-device variability in the presence of spin-orbit coupling mechanisms. Interface roughness, charge traps, layout or process inhomogeneities indeed shape the real space wave functions, hence the spin properties. It is, therefore, important to understand how reproducible the qubits can be, in order to assess strategies to cope with variability, and to set constraints on the quality of materials and fabrication. Here we model the variability of single qubit properties (Larmor and Rabi frequencies) due to disorder at the Si/SiO$_2$ interface (roughness, charge traps) in metal-oxide-semiconductor devices. We consider both electron qubits (with synthetic spin-orbit coupling fields created by micro-magnets) and hole qubits (with intrinsic spin-orbit coupling). We show that charge traps are much more limiting than interface roughness, and can scatter Rabi frequencies over one order of magnitude. We discuss the implications for the design of spin qubits and for the choice of materials.

cond-mat.mes-hall↗

arXiv2021

Non-reciprocal Pauli Spin Blockade in a Silicon Double Quantum Dot

Spin qubits in gate-defined silicon quantum dots are receiving increased attention thanks to their potential for large-scale quantum computing. Readout of such spin qubits is done most accurately and scalably via Pauli spin blockade (PSB), however various mechanisms may lift PSB and complicate readout. In this work, we present an experimental observation of a new, highly prevalent PSB-lifting mechanism in a silicon double quantum dot due to incoherent tunneling between different spin manifolds. Through dispersively-detected magnetospectroscopy of the double quantum dot in 16 charge configurations, we find the mechanism to be energy-level selective and non-reciprocal for neighbouring charge configurations. Additionally, using input-output theory we report a large coupling of different electron spin manifolds of 7.90 $μ$eV, the largest reported to date, indicating an enhanced spin-orbit coupling which may enable all-electrical qubit control.

cond-mat.mes-hall↗

arXiv2021

Spin-valley coupling anisotropy and noise in CMOS quantum dots

One of the main advantages of silicon spin qubits over other solid-state qubits is their inherent scalability and compatibility with the 300 mm CMOS fabrication technology that is already widely used in the semiconductor industry, whilst maintaining high readout and gate fidelities. We demonstrate detection of a single electron spin using energy-selective readout in a CMOS-fabricated nanowire device with an integrated charge detector. We measure a valley splitting of 0.3 meV and 0.16 meV in two similar devices. The anisotropy of the spin-valley mixing is measured and shown to follow the dependence expected from the symmetry of the local confinement, indicating low disorder in the region of the quantum dot. Finally the charge noise in the spin-valley coupling regime is investigated and found to induce fluctuations in the qubit energy in the range of $0.6GHz/\sqrt{Hz}$.

cond-mat.mes-hall↗

arXiv2021

Dispersively probed microwave spectroscopy of a silicon hole double quantum dot

Owing to ever increasing gate fidelities and to a potential transferability to industrial CMOS technology, silicon spin qubits have become a compelling option in the strive for quantum computation. In a scalable architecture, each spin qubit will have to be finely tuned and its operating conditions accurately determined. In this prospect, spectroscopic tools compatible with a scalable device layout are of primary importance. Here we report a two-tone spectroscopy technique providing access to the spin-dependent energy-level spectrum of a hole double quantum dot defined in a split-gate silicon device. A first GHz-frequency tone drives electric-dipole spin resonance enabled by the valence-band spin-orbit coupling. A second lower-frequency tone (approximately 500 MHz) allows for dispersive readout via rf-gate reflectometry. We compare the measured dispersive response to the linear response calculated in an extended Jaynes-Cummings model and we obtain characteristic parameters such as g-factors and tunnel/spin-orbit couplings for both even and odd occupation.

cond-mat.mes-hall↗

arXiv2021

Longitudinal and transverse electric field manipulation of hole spin-orbit qubits in one-dimensional channels

Holes confined in semiconductor nanostructures realize qubits where the quantum mechanical spin is strongly mixed with the quantum orbital angular momentum. The remarkable spin-orbit coupling allows for fast all electrical manipulation of such qubits. We study an idealization of a CMOS device where the hole is strongly confined in one direction (thin film geometry), while it is allowed to move more extensively along a one-dimensional channel. Static electric bias and $ac$ electrical driving are applied by metallic gates arranged along the channel. In quantum devices based on materials with a bulk inversion symmetry, such as silicon or germanium, there exists different possible spin-orbit coupling based mechanisms for qubit manipulation. One of them, the $g$-tensor magnetic resonance ($g$-TMR), relies on the dependence of the effective $g$-factors on the electrical confinement. In this configuration the hole is driven by an $ac$ field parallel to the static electric field and perpendicular to the channel (transverse driving). Another mechanism, which we refer to here as iso-Zeeman electric dipole spin resonance (IZ-EDSR), is due to the Rashba spin-orbit coupling that leads to an effective time-dependent magnetic field experienced by the pseudo-spin oscillating along the quantum channel (longitudinal driving). We compare these two modes of operation and we describe the conditions where the magnitudes of the Rabi frequencies are the largest. Different regimes can be attained by electrical tuning where the coupling to the $ac$ electric field is made either weak or strong...

cond-mat.mes-hall↗

arXiv2020

Hole-phonon interactions in quantum dots: Effects of phonon confinement and encapsulation materials on spin-orbit qubits

Spin-phonon interactions are one of the mechanisms limiting the lifetime of spin qubits made in semiconductor quantum dots. At variance with other mechanisms such as charge noise, phonons are intrinsic to the device and can hardly be mitigated. They set, therefore fundamental limits to the relaxation time of the qubits. Here we introduce a general framework for the calculation of the spin (and charge) transition rates induced by bulk (3D) and strongly confined 1D or 2D phonons. We discuss the particular case of hole spin-orbit qubits described by the 6 bands kp model. We next apply this theory to a hole qubit in a silicon-on-insulator device. We show that spin relaxation in this device is dominated by a band mixing term that couples the holes to transverse acoustic phonons through the valence band deformation potential d, and optimize the bias point and magnetic field orientation to maximize the number of Rabi oscillations Q that can be achieved within on relaxation time T1. Despite the strong spin-orbit coupling in the valence band, the phonon-limited Q can reach a few tens of thousands. We next explore the effects of phonon confinement in 1D and 2D structures, and the impact of the encapsulation materials on the relaxation rates. We show that the spin lifetimes can depend on the structure of the device over micrometer-long length scales and that they improve when the materials around the qubit get harder. Phonon engineering in semiconductor qubits may therefore become relevant once the extrinsic sources of relaxation have been reduced.

cond-mat.mes-hall↗

arXiv2020

Charge detection in an array of CMOS quantum dots

The recent development of arrays of quantum dots in semiconductor nanostructures highlights the progress of quantum devices toward large scale. However, how to realize such arrays on a scalable platform such as silicon is still an open question. One of the main challenge resides in the detection of charges within the array. It is a prerequisite functionality to initialize a desired charge state and readout spins through spin-to-charge conversion mechanisms. In this paper, we use two methods based on either a single-lead charge detector, or a reprogrammable single electron transistor. Thanks to these methods, we study the charge dynamics and sensitivity by performing single shot detection of the charge. Finally, we can probe the charge stability at any node of a linear array and assess the Coulomb disorder in the structure. We find an electrochemical potential fluctuation induced by charge noise comparable to that reported in other silicon quantum dots.

cond-mat.mes-hall↗