Fig. 3
Spectral footprint of the CNOT gate for K = 50 and Nsamples = 8192. Even though the CNOT gate has only two (degenerate) eigenvalues, we find that the spectrum of the noisy gate can be best described using four distinct eigenvalues. The fact that none of them are real suggests that the data cannot be due to the repeated execution of the same noisy gate. In section “Frame mismatch accumulation,” we propose a simple coherent non-Markovian model that offers a possible mechanism for the absence of real eigenvalues
