High-fidelity entangling gate for double-quantum-dot spin qubits (1608.04258v1)

Abstract: Electron spins in semiconductors are promising qubits because their long coherence times enable nearly 10^9 coherent quantum gate operations. However, developing a scalable high-fidelity two-qubit gate remains challenging. Here, we demonstrate an entangling gate between two double-quantum-dot spin qubits in GaAs by using a magnetic field gradient between the two dots in each qubit to suppress decoherence due to charge noise. When the magnetic gradient dominates the voltage-controlled exchange interaction between electrons, qubit coherence times increase by an order of magnitude. Using randomized benchmarking and self-consistent quantum measurement, state, and process tomography, we measure single-qubit gate fidelities of approximately 99% and an entangling gate fidelity of 90%. In the future, operating double quantum dot spin qubits with large gradients in nuclear-spin-free materials, such as Si, should enable a two-qubit gate fidelity surpassing the threshold for fault-tolerant quantum information processing.

High-Fidelity Entangling Gate for Double-Quantum-Dot Spin Qubits

The paper under evaluation presents a paper on the implementation of a high-fidelity entangling gate for double-quantum-dot (DQD) spin qubits, specifically utilizing GaAs quantum dots. The research is driven by the imperative need to enhance the coherence times and operational fidelity of spin qubits for their potential application in scalable quantum computing. The authors introduce a methodology leveraging a magnetic field gradient to reduce charge noise, thereby demonstrating substantial improvement in the coherence of quantum operations.

Key Contributions and Methodology

The central contribution of this work is the demonstration of an entangling gate between two DQD spin qubits. The researchers capitalize on the magnetic field gradient, which significantly mitigates decoherence caused by charge noise—a prevalent issue in semiconductor qubit platforms. By ensuring that the magnetic gradient dominates over the voltage-controlled exchange interaction, the authors achieve a notable extension of the coherence times, approximately by an order of magnitude.

Using techniques such as randomized benchmarking and self-consistent quantum measurement, state, and process tomography, the authors report single-qubit gate fidelities nearing 99% and entangling gate fidelities reaching 90%. This represents a significant enhancement over previous implementations, particularly for entangling gates between singlet-triplet qubits. Notably, they emphasize that such fidelities are feasible in materials with nuclear-spin-free environments, suggesting future implementations in silicon-based systems.

Implications and Future Directions

The implications of this research are multifaceted. Practically, the findings provide a viable path towards achieving the fault-tolerance threshold necessary for quantum information processing. Theoretical developments underscore the potential of using magnetic gradients effectively within spin qubits to achieve high coherence and operational fidelity.

The work opens several avenues for future exploration. Firstly, the proposed techniques could benefit significantly from further refinement within silicon-based heterostructures or utilizing advanced materials that provide intrinsic low noise levels. Additionally, advancements in dynamic nuclear polarization methods might further augment coherence times, thus propelling entanglement fidelities beyond current limitations. An exciting direction lies in integrating these high-fidelity operations into larger qubit systems, thereby making strides towards practical quantum computing architectures.

Ultimately, this research demonstrates how targeted engineering of quantum dot environments can overcome significant hindrances in quantum gate fidelity, bringing us closer to scalable quantum computation using spin qubits. As methods to increase magnetic field gradients and reduce inherent material noise continue to advance, it is reasonable to expect further improvements in the fidelity of such quantum operations.