Coherence Stabilization of a Two-Qubit Gate by ac Fields

We consider a cnot gate operation under the influence of quantum bit-flip noise and demonstrate that ac fields can change the qubit Hamiltonian in such a way that it approximately commutes with the bath coupling. Then the noise effectively acts as phase noise which improves coherence up to several orders of magnitude while the gate operation time remains unchanged. Within a high-frequency approximation, both purity and fidelity of the gate operation are studied analytically. The numerical treatment with a Bloch-Redfield master equation confirms the analytical results.

Figure 1

Purity loss $1-\mathcal{P}$ during the interaction time $t_J$ as a function of the driving amplitude. The driving frequency is $\Omega =32J/\hslash$ and the dissipation strength $2\pi \alpha =0.01$. For $A=0$, the undriven situation is reproduced.

Figure 2

(a) Purity loss during the interaction time $t_J$ as a function of the driving frequency. The driving amplitude $A\approx 2.4\hslash \Omega$ is adjusted such that $1-\mathcal{P}$ assumes its first minimum; cf. Fig. 1. The dotted lines mark the analytical estimate $1-\mathcal{P}(t_J)\approx -\dot{\mathcal{P}}(0)t_J$. (b) Corresponding fidelity defect $1-\mathcal{F}$.