An addressable quantum dot qubit with fault-tolerant control fidelity (1407.1950v1)

Abstract: Exciting progress towards spin-based quantum computing has recently been made with qubits realized using nitrogen-vacancy (N-V) centers in diamond and phosphorus atoms in silicon, including the demonstration of long coherence times made possible by the presence of spin-free isotopes of carbon and silicon. However, despite promising single-atom nanotechnologies, there remain substantial challenges in coupling such qubits and addressing them individually. Conversely, lithographically defined quantum dots have an exchange coupling that can be precisely engineered, but strong coupling to noise has severely limited their dephasing times and control fidelities. Here we combine the best aspects of both spin qubit schemes and demonstrate a gate-addressable quantum dot qubit in isotopically engineered silicon with a control fidelity of 99.6%, obtained via Clifford based randomized benchmarking and consistent with that required for fault-tolerant quantum computing. This qubit has orders of magnitude improved coherence times compared with other quantum dot qubits, with T_2* = 120 mus and T_2 = 28 ms. By gate-voltage tuning of the electron g*-factor, we can Stark shift the electron spin resonance (ESR) frequency by more than 3000 times the 2.4 kHz ESR linewidth, providing a direct path to large-scale arrays of addressable high-fidelity qubits that are compatible with existing manufacturing technologies.

• The paper presents a silicon-based quantum dot qubit achieving 99.6% control fidelity using Clifford gate randomized benchmarking.
• It demonstrates significantly improved coherence times with T2* of 120 µs and T2 of 28 ms, reducing nuclear spin dephasing through isotopic enrichment.
• The study achieves precise qubit tunability with an 8 MHz Stark shift via gate-voltage control, offering compatibility with CMOS technologies for scalability.

An Addressable Quantum Dot Qubit with Fault-Tolerant Control Fidelity

The paper presents a significant advancement in the development of quantum dot qubits through the demonstration of a gate-addressable quantum dot qubit in isotopically enriched silicon. The research addresses the integration of the strengths from spin qubits employing nitrogen-vacancy (N-V) centers in diamond and phosphorus atoms in silicon, with those of lithographically defined quantum dots, to overcome existing challenges in quantum computing. This approach focuses on achieving high coherence times, low dephasing, and fault-tolerant control fidelities for promising scalability in quantum systems.

Key Contributions and Results

1. Control Fidelity: The silicon-based quantum dot qubit shows a control fidelity of 99.6%, verified via Clifford gate randomized benchmarking—meeting the stringent requirements for fault-tolerant quantum computing.
2. Coherence Times: The reported qubits significantly outperform previous quantum dot implementations, showcasing a $T_2^*$ of 120 microseconds and a $T_2$ of 28 milliseconds. These values demonstrate orders of magnitude improvements over existing technologies, marking a pivotal achievement in coherence time enhancements due to the reduction in nuclear spin bath dephasing by employing isotopically pure $^{28}$Si.
3. Gate-Voltage Tunability: Utilizing gate-voltage tuning, the paper achieves a Stark shift of the electron spin resonance frequency by more than 8 MHz, effectively tuning the operation frequency over 3000 times the minimum ESR linewidth. This enables precise and large-scale addressability of qubits on a single chip without significant detriment to coherence times.
4. Scalability and Compatibility: The approach, which uses existing metal-oxide-semiconductor manufacturing technologies, provides a foundational framework for the potential integration of multiple qubits that are controllable via global AC magnetic fields, with quantum operations controlled by gate voltages.

Implications and Future Directions

The demonstrated ability to integrate high-fidelity, tunable quantum dots using silicon technology has important implications for both the theoretical and practical advancements in quantum computing. The demonstrated fault-tolerant control fidelities and long coherence times signal the capability of such systems to satisfy requirements for quantum error correction using surface codes.

Looking forward, increasing coherence times may be possible by mitigating thermal noise in on-chip ESR lines, paralleling the work on phosphorus donors in silicon. The research opens avenues for developing faster qubit operations via singlet-triplet qubits, with implications for further enhancing coherence and operational speed. Also, the qubit’s tunability provides a pathway to construct scalable architectures leveraging standard CMOS processes.

This paper contributes an important step towards the realization of robust, scalable, and manufacturable quantum computing systems, positioning quantum dot-based qubits as viable contenders for large-scale quantum computational frameworks. Future research may focus on two-qubit gate operations with high fidelities and addressing the intricate challenges associated with wiring and control for densely packed qubit systems.