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

Universal quantum computing requires fast single- and two-qubit gates with individual qubit addressability to minimize decoherence errors during processor operation. Electron spin qubits using individual phosphorus donor atoms in silicon have demonstrated long coherence times with high fidelities, providing an attractive platform for scalable quantum computing. While individual qubit addressability has been demonstrated by controlling the hyperfine interaction between the electron and nuclear wave function in a global magnetic field, the small hyperfine Stark coefficient of 0.34 MHz/MV m(-1) achieved to date has limited the speed of single quantum gates to similar to 42 mu s to avoid rotating neighboring qubits due to power broadening from the antenna. The use of molecular 2P qubits with more than one donor atom has not only demonstrated fast (0.8 ns) two-qubit root SWAP gates and long spin relaxation times of similar to 30 s but provides an alternate way to achieve high selectivity of the qubit resonance frequency. Here, we show in two different devices that by placing the donors with comparable interatomic spacings (similar to 0.8 nm) but along different crystallographic axes, either the [110] or [310] orientations using STM lithography, we can engineer the hyperfine Stark shift from 1 MHz/MV m(-1) to 11.2 MHz/MV m(-1), respectively, a factor of 10 difference. NEMO atomistic calculations show that larger hyperfine Stark coefficients of up to similar to 70 MHz/MV m(-1) can be achieved within 2P molecules by placing the donors >= 5 nm apart. When combined with Gaussian pulse shaping, we show that fast single qubit gates with 2p rotation times of 10 ns and similar to 99% fidelity single qubit operations are feasible without affecting neighboring qubits. By increasing the single qubit gate time to similar to 550 ns, two orders of magnitude faster than previously measured, our simulations confirm that >99.99% single qubit control fidelities are achievable.