Atomic Engineering of Molecular Qubits for High-Speed, High-Fidelity Single Qubit Gates

Area of Science:

• Quantum computing
• Solid-state quantum information science
• Atomic-scale device engineering

Background:

• Universal quantum computing demands high-fidelity, fast single- and two-qubit gates with individual qubit addressability to minimize decoherence.
• Phosphorus donor electron spin qubits in silicon offer long coherence times and high fidelities, making them a promising platform for scalable quantum computing.
• Current methods for individual qubit addressability using hyperfine interactions are limited by a small hyperfine Stark coefficient, restricting single-qubit gate speeds.

Purpose of the Study:

• To engineer enhanced hyperfine Stark shifts in silicon-based electron spin qubits.
• To demonstrate faster single- and two-qubit gate operations with high fidelity and selectivity.
• To explore the potential of molecular qubits for scalable quantum computing.

Main Methods:

• Utilized scanning tunneling microscopy (STM) lithography to precisely place phosphorus donors in silicon along specific crystallographic axes ([110] and [310] orientations).
• Investigated molecular 2P qubits with varying interatomic spacings (∼0.8 nm and ≥5 nm) to engineer hyperfine Stark shifts.
• Employed NEMO atomistic calculations to predict hyperfine Stark coefficients and simulated quantum gate operations with Gaussian pulse shaping.

Main Results:

• Achieved a factor of 10 increase in hyperfine Stark shift (up to 11.2 MHz/MV m^-1) by controlling donor placement along different crystallographic axes.
• NEMO calculations predict potential hyperfine Stark coefficients up to ∼70 MHz/MV m^-1 in 2P molecules with donors ≥5 nm apart.
• Demonstrated feasibility of fast single-qubit gates (10 ns rotation time) with ∼99% fidelity and potential for >99.99% control fidelity with optimized gate times, without affecting neighboring qubits.

Conclusions:

• Engineering the hyperfine Stark shift by controlling donor atom placement is a viable strategy to enhance qubit addressability and gate speeds in silicon quantum processors.
• Molecular qubits offer a pathway to significantly faster and more selective qubit operations, overcoming limitations of single-donor qubits.
• The demonstrated advancements pave the way for building high-performance, scalable quantum computers based on silicon electron spin qubits.