Figure 2: Fidelity of the uncollapsing protocol without storage.

(a) Measured χ/Tr(χ) (bars with colour), where χ is the non-trace-preserving quantum process tomography matrix for the sequence in Fig. 1e excluding step 2, here with p=p_u=0.75. The desired matrix, χ^ideal, corresponds to a π rotation about the Bloch sphere x axis (identified by black frames). (b) Process fidelity for both the three-qubit QRQ-based uncollapsing (blue circles) and the single-qubit partial-tunnelling version (red circles)²³, both as a function of p=p_u. Statistical errors are shown by error bars, defined as ±1 s.d., using the repeated sets of fidelity measurement. The process fidelity is above 0.9 for p≤0.8 using the QRQ swaps, while for the partial-tunnelling scheme it decreases significantly for p≥0.5. This decrease is primarily due to reduction in qubit T₂ with measurement current bias, shown in the inset; partial tunnelling occurs in the shaded region. Blue line is a simulation using κ₁=κ₃=0.985, κ₂=1, and κ_ϕ=0.95 (see Supplementary Note 3); the red line is a guide to the eye.