Germanium (Ge) quantum wells are emerging as versatile platforms for quantum devices, supporting high-quality spin qubits and integration with superconducting leads. These applications benefit from strong intrinsic spin-orbit interaction (SOI), enabling efficient electrical control and engineering of spin degrees of freedom. The most advanced Ge/SiGe heterostructures to date, based on compressively strained Ge channels within strain-relaxed silicon-germanium (SiGe) barriers, exhibit weak SOI due to the heavy-hole character of the wave function, posing challenges for spin-based quantum devices and requiring complex device designs for fast qubit manipulation. In this work, we demonstrate that concrete heterostructure modifications can overcome these limitations, enhancing SOI by up to three orders of magnitude. Specifically, we propose to enrich unstrained Ge channels by localized, strained silicon spikes. Leveraging a multi-objective Bayesian optimization, we optimize the spike profile to maximize SOI, while ensuring compatibility with current epitaxial growth processes and robustness against realistic variations of growth parameters. Our heterostructure substantially enhances device performance, yielding up to two orders of magnitude higher quantum-dot spin qubit quality factors than state-of-the-art materials. We also predict GHz-scale spin splittings for hybrid superconducting Andreev spin qubits. These novel Ge heterostructures with engineered Si concentration profiles can open pathways to scalable quantum and spintronic applications.

The large-scale integration of semiconductor spin qubits into quantum processors will require the characterization of quantum components at scale. However, such characterization is challenging and typically requires radio-frequency measurements at millikelvin temperatures and the presence of magnetic fields. Here we report a scalable architecture for characterizing spin qubits using a quantum dot crossbar array. The approach, which we term as the qubit-array research platform for engineering and testing, uses a crossbar array comprising tightly pitched spin-qubit tiles and is implemented in planar germanium, with the potential to host 1,058 single-hole spin qubits. We measure a subset of 40 tiles and demonstrate key device functionality at millikelvin temperatures, including tile addressability, threshold voltage and charge noise statistics, as well as the characterization of hole spin qubits and their coherence times in a single tile.

Silicon-quantum-dot spin qubits have become a promising platform for scalable quantum computing because of their small size and compatibility with industrial semiconductor manufacturing processes. Although Si/SiGe heterostructures are commonly used to host spin qubits due to their high mobility and low percolation density, the SiGe spacer creates a gap between the qubits and control electrodes, which limits the ability to tune the exchange coupling. As a result, residual coupling leads to unwanted single-qubit phase shifts, making multi-qubit control more difficult. In this work, we explore swapping the roles of overlapping nanogates to overcome this issue. By reconfiguring the gate voltages, we demonstrate in situ role switching while maintaining multi-qubit control. Additionally, this method improves the tunability of the exchange coupling by up to 3.6 times. This strategy reduces unintended single-qubit phase shifts and minimizes the complexity of multi-qubit control, supporting scalable growth with minimal experimental overhead.

Singlet-triplet qubits offer an attractive encoding for semiconductor quantum computing, combining ancilla-free readout, reduced sensitivity to common-mode noise, and baseband voltage control. However, the Zeeman energy difference ΔE_Z is typically fixed by local magnetic field gradients or g-factor inhomogeneities, leaving the exchange interaction J as the only dynamically tunable parameter. This always-on ΔE_Z precludes orthogonal control of the qubit's rotation axes and introduces unwanted state rotations during idling. Here we demonstrate all-electrical orthogonal control of a degenerate singlet-triplet (DST) qubit formed by two hole spins in a germanium double quantum dot. Exploiting the electrically tunable anisotropic g-factors of the two spins, we identify a regime where both ΔE_Z and J vanish, making the S and T_0 states degenerate at the idle point. By applying only baseband voltage pulses, we independently control both J and ΔE_Z, enabling fully orthogonal Z- and X-axis rotations. Randomized benchmarking yields an average physical single-qubit gate fidelity of 99.53% for a gate duration of approximately 100 ns. Finally, we electrically tune the degenerate point across a wide range of magnetic field orientations, enabling operation in a regime of enhanced coherence time and offering a route towards multi-qubit scaling under a shared global magnetic field.

Orbital energy splittings are important quantum dot parameters for the operation of hole spin qubits. They are known to depend on the lateral confinement of the quantum dots. However, when changing top, plunger gate voltages, which are the typical control parameter for qubit applications, such energy splitting changes are typically negligible, both as measured in experiment and as assumed in effective theories. Here, we study the singlet-triplet (ST) splittings, which depend on the orbital splittings, of a double quantum dot (DQD) in a Ge/SiGe heterostructure using photon-assisted tunneling (PAT) and pulsed-gate spectroscopy. We find that the ST splittings have a surprising, strong dependence on the top gate voltages, leading to anomalous PAT measurements. We combine data from both measurements in a model that well describes the linear gate-voltage dependence of the ST splittings. Finally, we show that the ST splittings of the two dots exhibit similar linear gate-voltage dependences when the device is retuned such that their ratio is significantly different.

Recent advances in coherent spin shuttling have made sparse semiconductor spin qubit arrays an appealing solid-state platform to realize quantum processors. The dynamic and long-range connectivity enabled by shuttling is also essential for many quantum error-correction (QEC) schemes. Here, we demonstrate a silicon spin-qubit device that comprises a shuttling bus for coherently transporting qubits that can interact at four isolated locations we call bus stops. We dynamically populate the array and tune all single- and two-qubit operations using shuttling and quantum non-demolition (QND) spin measurements, without access to charge sensing in most of the device. We achieve universal control of the effective five-qubit processor and select the connectivity required to form a surface-code stabilizer plaquette that supports X- and Z-type parity checks up to weight-four. We use the parity checks to generate multi-qubit entanglement between all qubit combinations in the array and report the genuine entanglement of a five-qubit Greenberger-Horne-Zeilinger (GHZ) state, constituting the largest such state ever constructed with gate-defined semiconductor spins. This work opens immediate opportunities to pursue QEC experiments with spin qubits, and the protocols developed here lay the groundwork for the modular calibration and operation of sparse spin qubit arrays.

The optical generation of an out-of-equilibrium spin population is a keystone process for quantum technologies and spintronics alike. Although this is an established technique for studying direct band-gap semiconductors, it has been proven limited in materials that possess weak oscillator strengths for the optical transitions. We address the problem by presenting an all-optical analog of the spin-pumping method. By applying this concept to a Ge-on-Si heterostructure, we observe luminescence from Si with a polarization degree as high as 9%. The progressive etching of the absorbing layer, assisted by magneto-optic experiments, allows us to ascertain that the polarized emission is determined by effective spin injection aided by the carrier lifetime shortening due to extended defects. These findings can facilitate the use of highly promising spin-dependent phenomena of Si, whose optical exploitation has been hampered by fundamental limitations due to its peculiar electronic structure.

Abstract Germanium spin qubits offer high-fidelity control via strong spin-orbit interaction, but their magnetic field sensitivity typically requires bulky superconducting vector magnets that limit scalability. This study investigates replacing these with an external permanent magnet located outside the cryostat to reclaim internal sample space. By operating in a hybrid mode (internal and external magnets), we fine-tuned the field to an in-plane orientation, achieving a dephasing time $${T}_{2}^{* }=13\,\upmu {{{\rm{s}}}}$$ T 2 * = 13 μ s and single-qubit Clifford gate fidelities exceeding 99.9%. Remarkably, we demonstrate qubit operation even with the internal superconducting magnet deactivated. In this regime, we observed extended coherence times of $${T}_{2}^{* }=31\,\upmu {{{\rm{s}}}}$$ T 2 * = 31 μ s and $${T}_{2}^{{{{\rm{H}}}}}=266\,\upmu {{{\rm{s}}}}$$ T 2 H = 266 μ s , on a natural Germanium heterostructure. These results demonstrate that room-temperature magnets allow for high qubit performance. This approach facilitates scaling by freeing cryogenic space for essential control circuitry and wiring, paving the way for large-scale quantum processors.

Constricting transport through a one-dimensional quantum point contact in the quantum Hall regime enables gate-tunable selection of the edge modes propagating between voltage probe electrodes. Here, we investigate the quantum Hall effect in a quantum point contact fabricated on low disorder strained germanium quantum wells. For increasing magnetic field, we observe Zeeman spin-split 1D ballistic hole transport evolving to integer quantum Hall states, with well-defined quantized conductance increasing in multiples of e(2)/h down to the first integer filling factor nu=1. These results establish strained germanium as a viable platform for complex experiments probing many-body states and quantum phase transitions.

Understanding scattering mechanisms in semiconductor heterostructures is crucial to reducing sources of disorder and ensuring high yield and uniformity in large spin qubit arrays. Disorder of the parent two-dimensional electron or hole gas is commonly estimated by the critical, percolation-driven density associated with the metal-insulator transition. However, a reliable estimation of the critical density within percolation theory is hindered by the need to measure conductivity with high precision at low carrier densities, where experiments are most difficult. Here, we connect experimentally percolation density and quantum Hall plateau width, in line with an earlier heuristic intuition, and offer an alternative method for characterizing semiconductor heterostructure disorder.

Controlling the energy scales of a quantum system is essential for defining robust qubits. In silicon spin qubits, the nearly degenerate conduction-band valleys create a leakage channel from the single-spin computational basis, posing a challenge to scaling and to shuttling-based architectures. Here, we measure the relevant energy scales of single-electron spin qubits in buried silicon quantum wells co-designed for low disorder and high valley splitting. Across a linear array of four quantum dots with an average orbital energy of 2.4(2) meV, we report an average single-electron valley splitting of 0.40(6) meV and an average two-electron singlet-triplet splitting of 0.24(7) meV. In three dots, we observe a strong correlation between valley splitting and orbital energy, with an average linear coefficient of ≈ 0.22 (meV/meV), demonstrating that electrostatic confinement can increase the valley splitting by several hundred microelectronvolts. In contrast, the remaining dot exhibits the highest valley splitting of 0.76(2) meV and low correlation, suggesting excellent characteristics for spin-qubit operation. Our findings demonstrate that strong confinement can be exploited in buried quantum wells to effectively enhance the valley splitting, thereby establishing a viable path toward the realization of shuttling and sparse-occupation-based architectures in low-disorder heterostructures.

In semiconductor single-spin qubits, high-fidelity quantum gates have been demonstrated; however, achieving consistent performance remains challenging due to variations in driven qubit coherence, which is less explored than free-evolution coherence such as T_2^*. Here, we report single-qubit gate fidelities above 99.999

Recent advances in coherent spin shuttling have made sparse semiconductor spin-qubit arrays an appealing solid-state platform to realize quantum processors1-7. The dynamic and long-range connectivity enabled by shuttling is also essential for many quantum error-correction schemes8-10. Here we demonstrate a silicon spin-qubit device comprising a shuttling bus for coherently transporting qubits that can interact at four isolated locations that we call bus stops. We dynamically populate the array and tune all single- and two-qubit operations using shuttling and quantum non-demolition spin measurements, without access to charge sensing in most of the device. We achieve universal control of the effective five-qubit processor and select the connectivity required to form a surface-code stabilizer plaquette that supports X- and Z-type parity checks up to weight four. We use the parity checks to generate multi-qubit entanglement between all qubit combinations in the array and report the genuine entanglement of a five-qubit Greenberger-Horne-Zeilinger state, constituting one of the largest such states constructed with gate-defined semiconductor spins. The protocols developed here lay the groundwork for modular calibration and operation of sparse spin-qubit arrays, and we highlight the feasibility of near-term quantum error-correction experiments with mobile spin qubits.

As quantum dot (QD)-based spin qubits advance toward larger, more complex device architectures, rapid, automated characterization and data analysis tools become critical. The orientation and spacing of transition lines in a charge stability diagram (CSD) contain a fingerprint of a QD device’s capacitive environment, making these measurements useful tools for device characterization. However, manually interpreting these features is time-consuming, error-prone, and impractical at scale. Here, we present the Automated Characterization of Capacitive Environment for Testing, Optimization, and Reconstruction (ACCEnTOR) protocol for extracting underlying capacitive properties from CSDs. Our method combines machine-learning-based segmentation, geometric reconstruction, and transition tracking to identify and analyze charge transitions across large CSD datasets without manual annotation. We demonstrate this method using experimentally measured data from QD devices: a strained-germanium single-quantum-well (planar) and a strained-germanium double-quantum-well (bilayer). Unlike for planar QD devices, CSDs in bilayer germanium heterostructure exhibit a larger set of transitions, including interlayer tunneling and distinct loading lines for the vertically stacked QDs, making them a powerful testbed for automation methods. By analyzing series of CSDs, ACCEnTOR estimates physically relevant quantities, including relative lever arms and capacitive couplings, enabling rapid electrostatic characterization of QD devices.

The scalability and power of quantum computing architectures depend critically on high-fidelity operations and robust and flexible qubit connectivity1-3. In this respect, mobile qubits are particularly attractive as they enable dynamic and reconfigurable qubit arrays. This approach allows quantum processors to adapt their connectivity patterns during operation, implement different quantum error correction codes on the same hardware and optimize resource use through dedicated functional zones for specific operations such as measurement or entanglement generation4-7. Such flexibility also relieves architectural constraints, as recently demonstrated in atomic systems based on trapped ions4,5 and neutral atoms manipulated with optical tweezers6,7. In solid-state platforms, highly coherent shuttling of electron spins was recently reported8,9. A key outstanding question is whether it may be possible to perform quantum gates directly on the mobile spins. Here we demonstrate two-qubit operations between two electron spins carried towards each other in separate travelling potential minima in a semiconductor device. We find that the interaction strength is highly tunable by their spatial separation. When we shuttle the two spins towards the centre by 120 nm each for a total displacement of 240 nm, we achieve an average two-qubit gate fidelity of about 99%. Furthermore, we implement conditional post-selected quantum state teleportation between qubits separated by 320 nm with an average gate fidelity of 87%, showcasing the potential of mobile spin qubits for non-local quantum information processing. We expect that operations on mobile qubits will become a universal feature of future large-scale semiconductor quantum processors.

Strained germanium (ε-Ge) quantum wells on metamorphic SiGe buffers have enabled advanced hole-based spin qubit devices. Alternatively, unstrained Ge with lattice-matched strained silicon-germanium (ε-SiGe) barriers eliminates the need for metamorphic buffers altogether. The ground state character of both these platforms is predominantly heavy-hole (HH) with a largely anisotropic spin response. We propose and study an alternative heterostructure, lattice-matched to Ge, in which both the SiGe quantum well and barriers are tensile strained, with their composition contrast providing the band offset for confinement and the tensile strain stabilizing a light-hole (LH) ground state. We show large spin-orbit coupling (SOC), both linear and cubic, along with a significantly more isotropic spin response compared to strained HH qubits. We also study the decoherence properties of the proposed device, showing an appreciable gain in the quality factor compared to their HH counterparts. Finally, we propose a bilayer heterostructure that allows for electrical switching between HH and LH ground state character.

We investigate low-frequency noise in a spin-qubit device made in isotopically purified Si/Si-Ge. Observing sizable cross-correlations among energy fluctuations of different qubits, we conclude that these fluctuations are dominated by charge noise. At low frequencies, the noise spectra are not well described by a power law; instead, they reveal the presence of a few individual two-level fluctuators (TLFs). We demonstrate that the noise cross-correlations allow one to get information on the spatial location of such individual TLFs.

Abstract Electron spins in silicon offer a promising path toward scalable, fault-tolerant quantum computing, with the potential to host millions of qubits. However, scaling up dense quantum-dot arrays and enabling qubit interconnections through shuttling are hindered by uncontrolled lateral variations of the valley splitting energy E VS. We map E VS across a 40 nm × 400 nm region of a 28Si/Si0.7Ge0.3 shuttle device and analyze the spin coherence of a single electron spin transported by conveyor-belt shuttling. We observe that the E VS varies over a wide range from 1.5 μeV to 200 μeV and is dominated by SiGe alloy disorder. In regions of low E VS and at spin-valley resonances, spin coherence is reduced and its dependence on shuttle velocity matches predictions. Rapid and frequent traversal of low-E VS regions induces a regime of enhanced spin coherence explained by motional narrowing. By selecting shuttle trajectories that avoid problematic areas on the E VS map, we achieve transport over tens of microns with coherence limited by the coupling to a static electron spin entangled with the mobile qubit. Our results provide experimental confirmation of the theory of spin decoherence of mobile electron spin-qubits and present practical strategies to integrate conveyor-mode qubit shuttling into silicon quantum chips.

Strained germanium ( ε $\varepsilon$ -Ge) and strained silicon ( ε $\varepsilon$ -Si) buried quantum wells have enabled advanced spin-qubit quantum processors. However, in the absence of suitable lattice-matched substrates, ε $\varepsilon$ -Ge and ε $\varepsilon$ -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 ( ε $\varepsilon$ -SiGe) barrier, eliminating the need for metamorphic buffers altogether. In a structure with a 52-nm-thick ε $\varepsilon$ -SiGe barrier, a low-disorder two-dimensional hole gas is demonstrated with a high-mobility of 1.33 × 10 5 cm 2 / Vs $1.33 \times 10^{5} \nobreakspace \mathrm{cm^2/Vs}$ and a low percolation density of 1.4 ( 1 ) × 10 10 cm - 2 $1.4(1)\times 10^{10} \nobreakspace \mathrm{cm^{-2}}$ . 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 g $g$ -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 g $g$ -factor in Ge than in ε $\varepsilon$ -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.

The characterization of excited-state structure in semiconductor quantum dot (QD) devices is an important component of tuning them for spin-qubit operation. We present Spectroscopy With Intelligent Feature Tracking (SWIFT), a framework that combines machine-learning (ML)-assisted feature identification with physics-informed geometric processing to extract energy-level splittings from pulsed-gate spectroscopy data. SWIFT isolates the relevant spectral features and exploits their characteristic geometry to reduce the two-dimensional spectroscopy analysis to a one-dimensional peak-detection problem. It further combines an ensemble-based confidence metric with sequential accumulation of rapid, low-SNR scans, allowing the inferred spectrum to be reevaluated as experimental evidence accumulates. Using Si/SiGe QD devices, we demonstrate SWIFT both offline and in real time, including automated tracking of QD excited states and lead resonances. Benchmarking on 255 manually labeled scans shows that SWIFT reduces the median splitting error to 0.05 mV from 0.11 mV for a classical baseline, with the largest improvement on lower-quality measurements. These results provide a path toward incorporating excited-state spectroscopy into autonomous QD characterization, tuning, and optimization, which will be essential in large-scale quantum dot devices.