Fig. 3: Single-excitation quantum state transfer in a 2D 6 × 6 qubit network with optimized couplings.

a left, shows the measured couplings of the 6 × 6 qubit network; center, the corresponding time evolution of Q₁ and Q₃₆ excited-state populations, which shows a transfer fidelity of 0.902 ± 0.006 at about 250 ns; right, the quantum state tomography in the subspace of the initial and target qubits, Q₁ and Q₃₆. b Fidelity dynamics for the QST using a Bell state initially encoded in qubits Q₁ and Q₂; here, the quantum state tomography at tJ = 0 is shown in the (Q₁, Q₂) subspace whereas at time tJ ≈ π/2 (J/2π = 1 MHz) is shown in (Q₃₅, Q₃₆) with a fidelity of 0.840 ± 0.006. The fidelity here is a generalization of the probability to the Bell case (see text), where we have two basis states in our initial and final wavefunctions to characterize the QST transfer. Lines (circles) are the numerical (experimental) evolution with the measured couplings. Solid bars (gray frames) represent experimental (ideal) values of density matrix elements. Error bars come from the standard deviation of five experimental repetitions. t_ΔE is the minimum time for a perfect QST set by “quantum speed limit” arguments (see Methods). See Supplementary Figs. 5 and 6 for the specific values of experimentally measured couplings.