Fidelity benchmarks for two-qubit gates in silicon (1805.05027v3)

Abstract: Universal quantum computation will require qubit technology based on a scalable platform, together with quantum error correction protocols that place strict limits on the maximum infidelities for one- and two-qubit gate operations. While a variety of qubit systems have shown high fidelities at the one-qubit level, superconductor technologies have been the only solid-state qubits manufactured via standard lithographic techniques which have demonstrated two-qubit fidelities near the fault-tolerant threshold. Silicon-based quantum dot qubits are also amenable to large-scale manufacture and can achieve high single-qubit gate fidelities (exceeding 99.9%) using isotopically enriched silicon. However, while two-qubit gates have been demonstrated in silicon, it has not yet been possible to rigorously assess their fidelities using randomized benchmarking, since this requires sequences of significant numbers of qubit operations ($\gtrsim 20$) to be completed with non-vanishing fidelity. Here, for qubits encoded on the electron spin states of gate-defined quantum dots, we demonstrate Bell state tomography with fidelities ranging from 80% to 89%, and two-qubit randomized benchmarking with an average Clifford gate fidelity of 94.7% and average Controlled-ROT (CROT) fidelity of 98.0%. These fidelities are found to be limited by the relatively slow gate times employed here compared with the decoherence times $T_2^*$ of the qubits. Silicon qubit designs employing fast gate operations based on high Rabi frequencies, together with advanced pulsing techniques, should therefore enable significantly higher fidelities in the near future.

• The paper demonstrates high-fidelity two-qubit gate operations with 94.7% for Clifford and 98.0% for CROT gates using randomized benchmarking and Bell state tomography.
• The paper attributes observed fidelity limitations to slow gate times relative to qubit decoherence in isotopically enriched silicon quantum dots.
• The paper highlights silicon's potential for scalable, fault-tolerant quantum computing by leveraging precise gate control and established semiconductor technologies.

Fidelity Benchmarks for Two-Qubit Gates in Silicon

The technological advancement of quantum computation necessitates high-fidelity qubit operations within a scalable architecture. This paper provides an assessment of two-qubit gate fidelities in silicon quantum dot systems, a promising platform for achieving scalable quantum computing. The research addresses a critical challenge in quantum computing: the need for quantum error correction (QEC) protocols that enforce strict thresholds on qubit operation fidelities for fault-tolerant computation.

Experimentation and Results

The authors demonstrate two-qubit randomized benchmarking (RB) and Bell state tomography using electron spin qubits in silicon quantum dots. The experiments showcase notable achievements in fidelity measurements of quantum operations. For the two-qubit Clifford gate operations, an average fidelity of 94.7% was obtained. Furthermore, a fidelity of 98.0% was reported for the Controlled-ROT (CROT) gate, indicating significant progress towards the benchmarks required for fault-tolerant quantum computation.

The fidelity limitations observed are primarily attributed to the slow gate times employed in the experiments, particularly in comparison to the decoherence times $T_2^*$ of the qubits. Bell state tomography yielded entanglement with fidelities ranging from 80% to 89%, underscoring the potential for these silicon-based systems to achieve robust quantum entanglement.

Methodological Approach

The research utilizes electron spins within gate-defined quantum dots in isotopically enriched silicon, benefiting from the reduced decoherence due to the negligible concentration of nuclear spins. The paper carefully articulates the creation and manipulation of quantum states using electric and magnetic fields, enhancing the precision of operations through sophisticated pulse sequences.

An essential methodological aspect is the implementation of randomized benchmarking, a technique that is particularly effective at assessing average gate fidelity while minimizing the impact of state preparation and measurement (SPAM) errors. By measuring decay of process fidelity across random sequences from the Clifford group, the paper provides a detailed analysis of qubit operation accuracy.

Implications and Future Directions

This work validates silicon quantum dots as a competitive platform for scalable quantum computing. With two-qubit gates demonstrating near-threshold performance for error correction, the research highlights the suitability of silicon for large-scale quantum computer manufacturing, leveraging existing semiconductor infrastructure.

Potential future directions include the development of optimized gate operations with higher Rabi frequencies and advanced pulse shaping techniques to increase gate speed and fidelity. Improving isotopic purity and implementing more sophisticated noise mitigation strategies could further enhance performance.

Given the rapid pace of development in quantum technologies, it's anticipated that higher fidelities and efficient fault-tolerant architectures in silicon will be achievable in the near future. The continued refinement of these methodologies not only fortifies the role of silicon qubits in quantum computing but also sets a precedent for integrating cutting-edge quantum systems into existing technological frameworks.