A >99.9%-fidelity quantum-dot spin qubit with coherence limited by charge noise

Abstract: Recent advances towards spin-based quantum computation have been primarily fuelled by elaborate isolation from noise sources, such as surrounding nuclear spins and spin-electric susceptibility, to extend spin coherence. In the meanwhile, addressable single-spin and spin-spin manipulations in multiple-qubit systems will necessitate sizable spin-electric coupling. Given background charge fluctuation in nanostructures, however, its compatibility with enhanced coherence should be crucially questioned. Here we realise a single-electron spin qubit with isotopically-enriched phase coherence time (20 microseconds) and fast electrical control speed (up to 30 MHz) mediated by extrinsic spin-electric coupling. Using rapid spin rotations, we reveal that the free-evolution dephasing is caused by charge (instead of conventional magnetic) noise featured by a 1/f spectrum over seven decades of frequency. The qubit nevertheless exhibits superior performance with single-qubit gate fidelities exceeding 99.9% on average. Our work strongly suggests that designing artificial spin-electric coupling with account taken of charge noise is a promising route to large-scale spin-qubit systems having fault-tolerant controllability.

• The paper presents a single-electron spin qubit achieving >99.9% gate fidelity using rapid electrical control and precise noise management.
• It employs extrinsic spin-electric coupling in a Si/SiGe quantum dot to extend phase coherence up to 20 µs while mitigating charge noise effects.
• The findings identify charge noise as the main dephasing factor, setting a new direction for scalable, fault-tolerant quantum computing.

Fidelity Enhancement in Quantum-Dot Spin Qubits with Controlled Charge Noise

The paper presents a significant advancement in the domain of spin-based quantum computation, particularly focusing on quantum-dot (QD) spin qubits. This study demonstrates the achievement of a single-electron spin qubit with gate fidelities that exceed 99.9%, realized through a balance between rapid electrical control and phase coherence predominantly limited by charge noise. The performance metrics are achieved by employing an extrinsic spin-electric coupling (SEC), finely engineered to minimize interference with the coherence of the qubit.

Key Contributions and Methodology

The research outlines the realization of a single-electron spin qubit achieving an isotopically enriched phase coherence time of 20 microseconds and a fast electrical control speed of up to 30 MHz. The study posits that charge noise, rather than magnetic noise, is the principal factor limiting dephasing, which marks a paradigm shift from conventional understanding where magnetic noise was predominantly considered. This is evidenced by a noise spectrum analysis that shows a 1/f charge noise spectrum extending across seven decades of frequency, in contrast to magnetic-noise-associated spectral exponents.

Experimental Setup and Results:

• A Si/SiGe quantum dot was utilized, isolated from nuclear spins to avoid traditional magnetic noise sources.
• The introduction of "artificial" SEC fields via local magnets facilitated rapid spin manipulation without significant degradation of coherence, which is a common issue associated with SEC in narrow-bandgap semiconductors.
• The SEC was employed both in transverse and longitudinal configurations to enhance control fidelity and manipulate qubit frequency effectively.

The findings suggest that the approach introduced, particularly with the incorporation of SEC fields that are cognizant of charge-induced fluctuations, offers a promising path for developing scalable, fault-tolerant spin qubits. The exceptional average single-qubit gate fidelity demonstrated (99.928% from randomized benchmarking) supports the prospect of universal quantum computation within this architecture.

Implications and Future Directions

The implications of this research extend to both theoretical and practical frameworks in quantum computing. The demonstration of high-fidelity qubit control, while mitigating the impact of charge noise, illustrates the potential for realizing robust quantum computing systems. A pivotal takeaway is the detailed characterization of charge noise and its role in spin dephasing, encouraging further studies on charge noise mitigation in different qubit systems.

The approach sets a precedent for addressing the coherence-controllability trade-off that is emblematic of QD spin qubits, suggesting that tailored SEC can enhance operational speeds without compromising coherence. The practical utility of such advancements lies in their potential integration into larger quantum-information processing units where single-qubit and multi-qubit operations can be conducted with high precision.

Future developments as speculated from the findings could involve more advanced micromagnet design, superior charge noise management techniques, and further enhancement of SEC field design to optimize qubit performance. The study encourages interdisciplinary research converging materials science, physics, and electrical engineering to further advance quantum-dot-based quantum computing technologies. Thus, this paper contributes a significant leap in achieving scalable quantum computing platforms through optimized quantum-dot spin qubits.