Fig. 5. Ancilla parity read-out and measurement-based Bell-state generation.

a, An ancilla qubit (middle) is prepared in parallel with a Bell pair, which is then used in Ramsey interrogation. A parity signal is measured either directly on the Bell pair or through ancilla-based parity read-out. b, Direct read-out of the Bell-pair parity signal (pink) is compared with a single ancilla read-out (orange) yielding a weight-2 parity signal oscillating at the same frequency and phase within error bars (Methods). c, A circuit for the measurement-based preparation of a long-range Bell pair, composed of three-qubit cluster state generation followed by MCR on the middle ancilla qubit. Conditioning on the ancilla result $\mid 0⟩$ (approximately 45% of experimental shots) projects the two exterior atoms to the Bell state $\mid {\Phi }^{+}⟩$. d, Characterization of the resulting Bell state with population overlap of 0.71(2) in the even-parity subspace $\left\{\mid 00⟩,\mid 11⟩\right\}$. The parity oscillation of the resulting state yields a contrast of 0.52(3), resulting in an overall raw Bell-state fidelity of 0.61(2). Data were conditioned on the successful shelving of the two exterior qubits (approximately 82% of experimental shots). e, A proposal for a continuous clock based on a modular architecture (see ‘Outlook’ for details).