Experimental method for evaluating two-qubit gate infidelity due to noise

The ability to entangle two qubits with high-fidelity gates is fundamental for realizing quantum computing systems. However, these gates are performed in environments where the experimental parameters not only can suffer from static offsets, but also change over time. The detrimental effect of time-varying noise on the coherence can be quantitatively described with the filter function formalism, although it cannot effectively capture the gate infidelity for noise frequencies lower than the inverse of the gate time [1].

For evaluating the impact of noise on the gate fidelity, we implemented a method based on the measure of the system spectral response when controlled noise is injected on purpose. We applied it on two trapped Ca+ ions as qubits, entangled by a light shift gate. Specifically, we injected noise both on the amplitude and the frequency of the laser driving the gate, thus generating both commuting and non-commuting terms with respect to the two-qubit gate hamiltonian. We measured the fidelity drop as a function of the applied noise frequency and used it to quantify the infidelity contribution due to noise.

[1] M. Kang, Phys. Rev. Applied 19, 014014 (2023)