Synchronized High-Fidelity Two-Qubit Gates in Double Quantum Dots*

While single-qubit gates with fidelities exceeding 99% for spin qubits in natural silicon and 99.9% in isotopically purified silicon have been demonstrated [1], the fidelity of two-qubit gates is still subject to several limitations. Motivated by recent experiments [2], we theoretically describe a high-fidelity controlled-NOT gate using the exchange interaction between the spins in neighboring quantum dots subject to a magnetic field gradient. We find an optimal gate sequence and present a synchronization method which avoids detrimental spin flips when the control qubit is in state 0. Furthermore, we identify systematic phase mismatches accumulated during the gate. We show that by synchronizing the resonant and off-resonant transitions and compensating unintended phases, the overall gate fidelity can be increased significantly. Numerical simulations also demonstrate a high tolerance against charge noise due to a partial intrinsic refocussing mechanism.

[1] J. Yoneda et al., arXiv:1708.01454.
[2] D. M. Zajac, A. J. Sigillito, M. Russ, F. Borjans, J. M. Taylor, G. Burkard, and J. R. Petta, arXiv:1708.03530.