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
∼42 μs 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 SWAP gates and long spin relaxation times of
∼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 (∼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
∼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 2π rotation times of 10 ns and ∼99% fidelity single
qubit operations are feasible without affecting neighboring qubits.
By increasing the single qubit gate time to ∼550 ns, two orders
of magnitude faster than previously measured, our simulations confirm
that >99.99% single qubit control fidelities are achievable.