Fig. 1. Entanglement genesis by ZZ parity measurement and Pauli frame update.

(A) Quantum circuit for a parity measurement of the data qubits via coherent operations with ancilla Q_A and Q_A measurement. Tomography reconstructs the data-qubit output density matrix (ρ). Echo pulses (orange) are applied halfway the Q_A measurement when performing tomography sequential to the Q_A measurement. (B) Bloch sphere representation of the even-parity subspace with a marker on ∣Φ⁺⟩. (C to F) Plots of ρ with fidelity to the Bell states (indicated by frames) for tomography simultaneous with Q_A measurement (C to E) and sequetial to Q_A measurement (F). (C)[(D)] Conditioning on M_A = +1[ −1] ideally generates ∣Φ⁺⟩ [∣Ψ⁺⟩] with equal probability P. (E)[(F)] PFU applies bit-flip correction (X on Q_DH) for M_A = −1 and reconstructs ρ using all data for simultaneous [sequential] tomography.