Critical role of tunneling noise on spin decoherence in a two-qubit gate

Rapid progress in semiconductor spin qubits enables experimental demonstrations of a two-qubit logic gate, which is necessary for universal quantum computing. Here, we study the decoherence of two electron-spin qubits due to 1/f charge noise in a silicon double quantum dot used for a two-qubit logic gate. We find that, even though the amplitude of tunneling fluctuation is small, its effect on the spin qubit is first order in the charge admixture in comparison with an effect, second order in the admixture, due to detuning fluctuation. As a consequence, the tunneling noise can dominate over detuning noise under conditions typical for accumulation mode quantum dots. The different orders of contributions also result in the different detuning dependencies of the decoherence for detuning and tunneling noise, which enables the identification of noise sources. By comparing with a recent two-qubit experiment, we find that the decoherence was dominated by tunneling fluctuation from charge noise instead of detuning fluctuation. When tunneling noise dominates, more, rather than less, asymmetry in the device detuning can increase the number of two-qubit operations, pointing to the importance of considering tunneling noise to design optimal operation of spin qubits.