2025
Sweet-spot protection of hole spins in sparse arrays via spin-dependent magneto-tunneling
Abstract
Recent advances in the scaling of spin qubits have led to the development of sparse architectures where spin qubits are distributed across multiple quantum dots. This distributed approach enables qubit manipulation through hopping and flopping modes, as well as protocols for spin shuttling to entangle spins beyond nearest neighbors. Therefore, understanding spin tunneling across quantum dots is fundamental for the improvement of sparse array encodings. Here, we develop a microscopic theory of a minimal sparse array formed by a hole in a double quantum dot. We show the existence of spin-dependent magnetic corrections to the tunnel couplings that help preserve existing sweet spots, even for quantum dots with different -factors, and introduce new ones that are not accounted for in the simplest models. Our analytical and numerical results explain observed sweet spots in state-of-the-art shuttling and cQED experiments, are relevant to hopping and flopping modes, and apply broadly to sparse array encodings of any size.
Laser-Induced Spin Precession in Topological Insulator Devices
Abstract
Controlling spin currents in topological insulators (TIs) is crucial for spintronics but challenged by the robustness of their chiral edge states, which impedes the spin manipulation required for devices such as spin-field effect transistors (SFETs). We theoretically demonstrate that this challenge can be overcome by synergistically applying circularly polarized light and gate-tunable Rashba spin–orbit coupling (rSOC) to a 2D TI. Laser irradiation provides access to Floquet sidebands where rSOC induces controllable spin precession, leading to the generation of one-way, switchable spin-polarized photocurrents, a non-equilibrium effect forbidden in TIs under time-reversal symmetric (equilibrium) conditions. This mechanism effectively realizes SFET functionality within a driven TI, specifically operating within a distinct Floquet replica, offering a new paradigm for light-based control in topological spintronics.
Hole spin qubits in unstrained Germanium layers
Abstract
Strained germanium heterostructures are one of the most promising material for hole spin qubits but suffer from the strong anisotropy of the gyromagnetic factors that hinders the optimization of the magnetic field orientation. The figures of merit (Rabi frequencies, lifetimes…) can indeed vary by an order of magnitude within a few degrees around the heterostructure plane. We propose to address this issue by confining the holes at the interface of an unstrained, bulk Ge substrate or thick buffer. We model such structures and show that the gyromagnetic anisotropy is indeed considerably reduced. In addition, the Rabi frequencies and quality factors can be significantly improved with respect to strained heterostructures. This extends the operational range of the qubits and shall ease the scale-up to many-qubit systems.
Dressed basis sets for the modeling of exchange interactions in double quantum dots
Abstract
We discuss the microscopic modeling of exchange interactions between double semiconductor quantum dots used as spin qubits. Starting from a reference full configuration interaction (CI) calculation for the two-particle wave functions, we build a reduced basis set of dressed states that can describe the ground-state singlets and triplets over the whole operational range with as few as 100 basis functions (as compared to a few thousands for the full CI). This enables fast explorations of the exchange interactions landscape as well as efficient time-dependent simulations. We apply this methodology to a double hole quantum dot in germanium and discuss the physics of exchange interactions in this system. We show that the net exchange splitting results from a complex interplay among interdot tunneling, Coulomb exchange, and correlations. We analyze, moreover, the effects of confinement, strains, and Rashba interactions on the anisotropic exchange and singlet-triplet mixings at finite magnetic field. We finally illustrate the relevance of this methodology for time-dependent calculations on a singlet-triplet qubit.
Strain engineering in \(\mathrm{Ge/Ge-Si}\) spin-qubit heterostructures
Abstract
The heavy-holes in Ge/GeSi heterostructures show highly anisotropic gyromagnetic response with in-plane \(g\)-factors \(g_{x,y}\leq 0.3\) and out-of-plane g-factor \(g_z\geq 10\). As a consequence, Rabi hot spots and dephasing sweet lines are extremely sharp and call for a careful alignment of the magnetic field in Ge spin qubit devices. We investigate how the \(g\)-factors can be engineered by strains. We show that uniaxial strains can raise in-plane \(g\)-factors above unity while leaving \(g_z\) essentially constant. We discuss how the etching of an elongated mesa in a strained buffer can actually induce uniaxial (but inhomogeneous) strains in the heterostructure. This broadens the operational magnetic field range and enables spin manipulation by shuttling holes between neighboring dots with different \(g\)-factors.
Unifying Floquet Theory of Longitudinal and Dispersive Readout