We consider hybrid systems consisting of a hole-doped semiconductor coupled to electronic states of finite-size superconductors, where the opposite sign of the masses in the two subsystems gives rise to insulating gaps at subband anticrossings. Consequently, increasing the coupling strength to the superconductor can paradoxically suppress the proximity-induced superconductivity in the semiconductor by enhancing these insulating gaps. We demonstrate that the presence of such induced insulating gaps leads to a characteristic atypical behavior of the critical supercurrent in Josephson junctions based on these hybrid structures. Our findings provide important insights for the design of robust quantum computing platforms utilizing hybrid superconductor-hole systems.

Qubit readout schemes often deviate from ideal projective measurements, introducing critical issues that limit quantum computing performance. In this Letter, we model charge-sensing-based readout for semiconductor spin qubits in double quantum dots, and identify key error mechanisms caused by the backaction of the charge sensor. We quantify how the charge noise of the sensor, residual tunneling, and g-tensor modulation degrade readout fidelity, induce a mixed postmeasurement state, and cause leakage from the computational subspace. For state-of-the-art systems with strong spin-orbit interaction and electrically tunable g tensors, we identify a readout sweet spot, that is, a special device configuration where readout is closest to projective. Our framework provides a foundation for developing effective readout error mitigation strategies, with broad applications for optimizing readout performance for a variety of charge-sensing techniques, advancing quantum protocols, and improving adaptive circuits for error correction.

The strong spin-orbit interaction in silicon and germanium hole quantum dots enables all-electric microwave control of single spins but is unsuited for multispin exchange-only qubits that rely on scalable discrete signals to suppress crosstalk and heating effects in large quantum processors. Here, we propose an exchange-only spin-orbit qubit that utilizes spin-orbit interactions to implement qubit gates and keeps the beneficial properties of the original encoding. Our encoding is robust to significant local variability in hole spin properties and, because it operates with two degenerate states, it eliminates the need for the rotating frame, avoiding the technologically demanding constraints of fast clocks and precise signal calibration. Unlike current exchange-only qubits, which require complex multistep sequences prone to leakage, our qubit design enables low-leakage two-qubit gates in a single step, addressing critical challenges in scaling spin qubits.

Strained germanium ((Formula presented.) -Ge) and strained silicon ((Formula presented.) -Si) buried quantum wells have enabled advanced spin-qubit quantum processors. However, in the absence of suitable lattice-matched substrates, (Formula presented.) -Ge and (Formula presented.) -Si are deposited on defective, metamorphic SiGe buffers, which may impact device performance and scaling. Here an alternative platform is introduced based on the heterojunction between bulk unstrained Ge and a lattice-matched strained silicon-germanium ((Formula presented.) -SiGe) barrier, eliminating the need for metamorphic buffers altogether. In a structure with a 52-nm-thick (Formula presented.) -SiGe barrier, a low-disorder two-dimensional hole gas is demonstrated with a high-mobility of (Formula presented.) and a low percolation density of (Formula presented.). Quantum transport shows that holes confined in the buried unstrained Ge channel have a strong density-dependent in-plane effective mass and out-of-plane (Formula presented.) -factor, pointing to a significant heavy-hole–light-hole mixing in agreement with theory. Measurements of Zeeman-split levels in quantum point contacts further highlight this character, showing a two-fold larger in-plane (Formula presented.) -factor in Ge than in (Formula presented.) -Ge. The prospects of strong spin–orbit interaction, isotopic purification, and of hosting superconducting pairing correlations make this platform appealing for fast quantum hardware and hybrid quantum systems.

Quantum simulators enable studies of many-body phenomena, which are intractable with classical hardware. The manipulation of electronic spin states in devices based on semiconductor quantum dots promises precise electrical control and scalability advantages, but accessing many-body phenomena has so far been restricted by challenges in nanofabrication and simultaneous control of multiple interactions. in this study, we performed spectroscopy of up to eight interacting spins using a 2-×-4 array of gate-defined germanium quantum dots. The spectroscopy protocol is based on ramsey interferometry and adiabatic mapping of many-body eigenstates to single-spin eigenstates, enabling complete energy spectrum reconstruction. As the interaction strength exceeds magnetic disorder, we observed signatures of the crossover from localization to a chaotic phase marking a step toward the observation of many-body phenomena in quantum dot systems.

Quantum computers require the systematic operation of qubits with high fidelity. For holes in germanium, the spin-orbit interaction allows for electric, fast and high-fidelity qubit gates. However, the strong g-tensor anisotropy of holes in germanium and their sensitivity to the operational and environmental conditions challenge the operation of large qubit arrays. Here, we investigate a two-dimensional 10-spin qubit array with single-qubit gate fidelities above 99%, and obtain surprisingly uniform qubit properties. By tuning the hole occupation, we demonstrate control over the spin susceptibility, enabling fast plunger gate driving with Rabi frequencies consistently above 1.45 MHz/ (mV ⋅ T). Moreover, we probe the locality of electric dipole spin resonance and find that the configuration with three-hole occupancy driven by the associated quantum dot plunger gate reduces crosstalk, lowering it by an average factor of 2.5 to nearest neighbours, compared to single-hole plunger driving. Theoretical modelling points towards the pronounced anisotropy of p-like orbitals as the main mechanism with significant contributions through Coulomb interactions, giving directions for reproducible control of large qubit arrays.

Across leading qubit platforms, a common trade-off persists: increasing coherence comes at the cost of operational speed, reflecting the notion that protecting a qubit from its noisy surroundings also limits control over it. This speed-coherence dilemma limits qubit performance across various technologies. Here, we demonstrate a hole spin qubit in a Ge/Si core/shell nanowire that triples its Rabi frequency while simultaneously quadrupling its Hahn-echo coherence time, boosting the Q-factor by over an order of magnitude. This is enabled by the direct Rashba spin-orbit interaction, emerging from heavy-hole-light-hole mixing through strong confinement in two dimensions. Tuning a gate voltage causes this interaction to peak, providing maximum drive speed and a point where the qubit is optimally protected from charge noise, allowing speed and coherence to scale together. Our proof-of-concept shows that careful dot design can overcome a long-standing limitation, offering a new approach towards building high-performance, fault-tolerant qubits.

Quantum spintronics is an emerging field focused on developing novel applications by utilizing the quantum coherence of magnetic systems. A key challenge in this context is achieving scalable long-range quantum information transmission in magnetic systems. Here, we propose a novel transmission scheme based on topological spin textures in a hybrid architecture combining a magnetic racetrack and localized spin qubits. We demonstrate this principle by employing the domain wall (DW) - the most fundamental texture - to transport quantum signal between distant qubits. We introduce a measurement-free protocol that utilizes DW mobility to enable high-fidelity and tunable entanglement generation. Furthermore, we demonstrate that spin qubits can function as quantum stations on the racetrack, enabling flexible state transfer among fast-moving DWs on a single track. Finally, we discuss concrete material platforms to implement the proposed scheme. Our work introduces a new hybrid quantum platform that merges topological spin textures with solid-state qubits, offering a scalable architecture for quantum information processing and opening promising directions for quantum spintronics.

All-electrical baseband control of qubits facilitates scaling up quantum processors by removing issues of crosstalk and heat generation. In semiconductor quantum dots, this is enabled by multispin qubit encodings, such as the exchange-only qubit. However, their performance is limited by unavoidable leakage states that are energetically close to the computational subspace. In this Letter, we introduce an alternative, scalable spin qubit architecture that leverages strong spin-orbit interactions of hole nanostructures for baseband qubit operations while completely eliminating leakage channels and reducing the overall gate overhead. This encoding is intrinsically robust to local variability in hole spin properties and operates with two degenerate states, removing the need for precise calibration and mitigating heat generation from fast signal sources. Finally, our architecture is fully compatible with current technology, utilizing the same initialization, readout, and multiqubit protocols of state-of-The-Art spin-1/2 systems. By addressing critical scalability challenges, our design offers a robust and scalable pathway for semiconductor spin qubit technologies.

Non-reciprocal devices are key components in both classical and quantum electronics. One approach to realizing passive non-reciprocal microwave devices is through capacitive coupling between external electrodes and materials exhibiting non-reciprocal conductance. In this work, we develop an analytic framework that captures the response of such devices in the presence of dissipation while accounting for the full AC dynamics of the material. Our results yield an effective circuit model that accurately describes the device response in experimentally relevant regimes even at small dissipation levels. Furthermore, our analysis reveals counterpropagating features arising from the intrinsic AC response of the material that could be exploited to dynamically switch the non-reciprocity of the device, opening pathways for tunable non-reciprocal microwave technologies.

Hole spin qubits are emerging as the workhorse of semiconducting quantum processors because of their large spin-orbit interaction, enabling fast, low-power, all-electric operations. However, this interaction also causes non-uniformities, resulting in site-dependent qubit energies and anisotropies. Although these anisotropies enable single-spin control, if not properly harnessed, they can hinder scalability. Here, we report on microwave-driven singlet-triplet qubits in planar germanium and use them to investigate spin anisotropies. For in-plane magnetic fields, the spins are largely anisotropic and electrically tunable, allowing access to all transitions and coherence times exceeding 3 μs are extracted. For out-of-plane fields they have an isotropic response. 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 spin anisotropies, applicable to two-dimensional devices, facilitating the path towards scalable quantum processors.

Efficient adiabatic control schemes, where one steers a quantum system along an adiabatic path ensuring minimal excitations while achieving a desired final state, that enable fast, high-fidelity operations are essential for any practical quantum computation. However, current optimization protocols are not universally tractable due to stringent requirements imposed by the microscopic systems encoding the qubit, including complex energy level structures and unwanted transitions, and generally require a trade-off between speed and fidelity of the operation. Here, we address these challenges by developing a general framework for optimal control based on the quantum metric tensor. This framework allows for fast and high-fidelity adiabatic pulses, even for a dense energy spectrum, based solely on the Hamiltonian of the system instead of the time evolution propagator and independent of the size of the underlying Hilbert space. Furthermore, our framework suppresses diabatic transitions and state-dependent crosstalk effects without the need for additional control fields. As an example, we study the adiabatic charge transfer in a double quantum dot to find optimal control pulses with improved performance. We show that for the geometric protocol, the transfer fidelities are lower bounded F>99% for ultrafast 20ns pulses, regardless of the size of the anti-crossing, while being robust against miscalibration errors and quasistatic noise.

Shuttling spins with high fidelity is a key requirement to scale up semiconducting quantum computers, enabling qubit entanglement over large distances and favoring the integration of control electronics on-chip. To decouple the spin from the unavoidable charge noise, state-of-the-art spin shuttlers try to minimize the inhomogeneity of the Zeeman field. However, this decoupling is challenging in otherwise promising quantum computing platforms such as hole spin qubits in silicon and germanium, characterized by a large spin-orbit interaction and an electrically tunable qubit frequency. In this work, we show that, surprisingly, the large inhomogeneity of the Zeeman field stabilizes the coherence of a moving spin state, thus also enabling high-fidelity shuttling in these systems. We relate this enhancement in fidelity to the deterministic dynamics of the spin that filters out the dominant low-frequency contributions of the charge noise. By simulating several different scenarios and noise sources, we show that this is a robust phenomenon generally occurring at large field inhomogeneity. By appropriately adjusting the motion of the quantum dot, we also design realistic protocols enabling faster and more coherent spin shuttling. Our findings are generally applicable to a wide range of setups and could pave the way toward large-scale quantum processors.