Fig. 5
QPT of the geometric four-qubit CCCZ gate, obtained with the drive amplitude \(\Omega {\rm{/}}2\pi \approx \sqrt {6.9}\) MHz and detuning δ/2π = 4 MHz. The color code for Pauli basis {I, X, Y, Z} is shown at the top-left corner. The partially shown process matrix χ exp is reconstructed by preparing a complete set of 256 input states, and measuring both the input and output density matrices using QST. Imaginary components of χ exp are measured to be no larger than 0.062 in magnitude. The fidelity of χ exp is 0.817 ± 0.006. Numerical simulation suggests that the \(\left| 2 \right\rangle\)-state occupation probability for each qubit is no higher than 0.025
