In a groundbreaking stride towards scalable quantum computing, researchers have unveiled a sophisticated crossbar chip designed to benchmark semiconductor spin qubits with unprecedented precision and coherence. This pioneering development pivots on a germanium-based quantum device architecture—QARPET—that ingeniously encodes singlet-triplet (ST) and single-hole spin qubits in a tightly packed crossbar geometry. The elaborate charge stability and spin dynamic characteristics mapped in this device showcase a new level of control and measurement fidelity that could chart a transformative path for quantum processor scalability.
At the heart of this innovation lies the meticulous tuning of double quantum dots within a single tile of the QARPET device, exhibiting a fully functional interdot barrier critical for coherent quantum operations. By constructing a charge stability diagram as a function of detuning and chemical potential axes, the researchers successfully isolate and manipulate the (1,1) charge configuration—the essential regime for hosting ST qubits. The ability to finely tune gate voltages to optimize the Pauli spin blockade effect enables measurement of spin states through charge read-out, demonstrating robust spin-to-charge conversion mechanisms that are vital for qubit readout.
Crucially, the team performed a spin funnel experiment by initializing the system in a (0,2) singlet state and pulsing towards the (1,1) configuration across various detunings and magnetic field strengths. This approach revealed the singlet-triplet minus (S–T⁻) anticrossing, a hallmark feature confirming successful spin readout with the Pauli blockade method. The extensive mapping of spin dynamics under varying magnetic fields reveals intricate spin splitting behavior, which verifies the precise control over qubit transitions necessary for high-fidelity quantum gate operations.
Further advancing the utility of this quantum platform, the researchers have demonstrated coherent ST₀ oscillations, confirming the viability of this architecture for dynamic qubit manipulations. By pulsing the system diabatically from a (0,2) singlet into the (1,1) regime and varying the evolution time, the experiment tracks the oscillatory behavior of singlet return probability as a function of the spin qubit’s control parameters, including detuning and magnetic field. The oscillation frequency fits yield key parameters such as the effective g-factor difference Δg and the exchange energy J at zero detuning, offering deep insights into the spin-orbit coupling and interdot tunnel coupling—elements pivotal for qubit coherence and gate speeds.
Expanding beyond the ST qubit realm, the device also enables the coherent control of two single-hole spin qubits, Q₁ and Q₂, forming localized dot qubits (LD qubits). The electric-dipole spin resonance (EDSR) spectra for these qubits were carefully measured by applying microwaves tuned to their resonance frequencies while pulsing the system through the (1,1) charge region. The observed g-factors, g₁ = 0.30 and g₂ = 0.36, are consistent with prior reports for planar germanium hole systems, underscoring the material’s potential for realizing robust qubits with significant spin coherence.
Pushing the device’s capability further, the team characterized the coherence properties of these LD qubits through Ramsey and Hahn-echo experiments. Ramsey interferometry at a moderate in-plane magnetic field (50 mT) uncovered dephasing times T₂* of several microseconds, highlighting the low-noise environment and the effectiveness of the crossbar architecture in suppressing decoherence mechanisms. Hahn-echo protocols extended the coherence times further up to nearly 13 microseconds, demonstrating significant mitigation of quasi-static noise. These measurements place the QARPET platform at the forefront of hole spin qubit research, aligning with the best demonstrated coherence benchmarks in germanium-based spin qubit devices.
The experimental sequences incorporated precise pulsing schemes for initialization, manipulation, and readout of the spin states, utilizing voltage pulses to shuttle the system between (0,2) and (1,1) charge configurations. The refined voltage control enables deterministic preparation of singlet states and controlled spin rotations via detuning modulations and microwave drives. This high level of control is essential for scalable quantum computing architectures where qubit addressing and coherent operations must be finely orchestrated within densely packed arrays.
One distinguishing aspect of the QARPET device is its use of a crossbar array architecture, which holds promise for overcoming key challenges in scaling semiconductor spin qubits. The layout reduces the wiring overhead and enhances the integration density by enabling multiplexed control and readout of qubits, a critical bottleneck in existing quantum processors. Coupled with the robust spin qubit performance demonstrated, this approach advances the practical feasibility of constructing large-scale quantum processors with uniform qubit characteristics.
Moreover, the carefully calibrated interplay between spin states and charge configurations, mediated by adjustable tunnel barriers and gate voltages, underpins the tunability and reproducibility of the system. The extracted exchange coupling energy and g-factor differences are foundational parameters for engineering qubit-qubit interactions and gate operations, providing a versatile platform for exploring two-qubit gates, entanglement generation, and error correction protocols.
The reported spin funnel and coherent oscillation experiments further elucidate the nuanced spin-dependent energy landscape in this device. The detailed mapping of singlet probabilities as functions of magnetic field, detuning, and evolution time illustrate control over spin blockade phenomena and spin mixing mechanisms—key elements for constructing high-fidelity quantum logic gates. This experimental characterization also serves as a critical benchmark for validating theoretical models of spin dynamics in complex quantum dot systems.
From a materials perspective, the use of planar germanium heterostructures enables strong spin-orbit coupling and favorable electrical properties, which are harnessed in the QARPET devices to realize fast and coherent spin manipulation. The observed coherence times and g-factor values resonate well with established literature, reinforcing germanium’s position as an advantageous semiconductor for hole spin qubits due to its intrinsic properties and compatibilities with silicon-based fabrication techniques.
The approach also exemplifies the integration of spin qubit architectures with advanced measurement modalities, including reflectometry-based charge sensing and microwave-driven spin resonance, which collectively enhance readout fidelity and qubit control bandwidth. These instrumental techniques constitute essential elements in progressing towards fault-tolerant quantum computation, where precise and rapid qubit state discrimination is imperative.
By demonstrating simultaneous operation of ST and LD qubits within the same device tile, the researchers highlight the architectural flexibility of the QARPET crossbar design. This dual encoding capability transcends conventional qubit implementations and allows exploration of hybrid qubit systems that can potentially leverage the unique advantages of each encoding scheme, providing a fertile ground for innovative quantum algorithms and error mitigation strategies.
The overall results present a compelling proof of principle for utilizing QARPET chips as a scalable testbed for statistical analysis and benchmarking of spin qubit coherence. The platform’s performance benchmarks stand as a testament to the promise of integrating scalable architectures with material-engineered qubits, progressing the quantum computing community closer to practical, large-scale semiconductor quantum processors.
Importantly, the work also sheds light on operational conditions, such as optimal magnetic field orientations and pulse timings, that critically influence qubit performance. The optimization strategies and experimental protocols outlined set a framework for subsequent efforts aimed at refining device design and control schemes to push coherence and gate fidelities even further.
In essence, this achievement lays a foundational stone in the pursuit of dense, robustly controlled quantum arrays capable of executing complex quantum computations. By combining advanced quantum dot engineering, meticulous device characterization, and innovative control architectures, the researchers have demonstrated a path toward scalable, high-coherence semiconductor quantum processors embedded within a well-designed crossbar chip platform.
Subject of Research: Semiconductor spin qubits, specifically singlet-triplet and single-hole spin qubits in planar germanium quantum dot arrays.
Article Title: A crossbar chip for benchmarking semiconductor spin qubits.
Article References:
Tosato, A., Elsayed, A., Poggiali, F. et al. A crossbar chip for benchmarking semiconductor spin qubits. Nat Electron (2026). https://doi.org/10.1038/s41928-026-01569-5
Image Credits: AI Generated
DOI: https://doi.org/10.1038/s41928-026-01569-5
