Fig. 2: Nondestructive parity measurement.
From: A nondestructive Bell-state measurement on two distant atomic qubits
a, Quantum circuit diagram of the parity measurement. An antidiagonally polarized ancilla photon \(\left|A\right\rangle\) reflects sequentially from node 1 and node 2 and implements a CNOT gate with each atomic qubit. The polarization detection on the ancilla projects the atoms on a state with known parity. For the shown initial state, this results in one of the entangled states \(\left|{{{\varPhi }}}^{+}\right\rangle\) or \(\left|{{{\varPsi }}}^{+}\right\rangle\). b, Real part of the two-atom density matrices corresponding to the two possible measurement outcomes (\(\left|A\right\rangle\) or \(\left|D\right\rangle\)) of the photon polarization. The atoms are initially prepared in the state \(1/\sqrt{2}(\left|{\uparrow }_{z}\right\rangle +\left|{\downarrow }_{z}\right\rangle )\otimes 1/\sqrt{2}(\left|{\uparrow }_{z}\right\rangle +\left|{\downarrow }_{z}\right\rangle )\), as indicated in a. The two density matrices show a large overlap with the entangled states \(\left|{{{\varPhi }}}^{+}\right\rangle\) and \(\left|{{{\varPsi }}}^{+}\right\rangle\) with fidelities \({{\mathcal{F}}}_{A}=(80.8\pm 1.4) \%\) and \({{\mathcal{F}}}_{D}=(75.3\pm 1.5) \%\), respectively (the errors indicate the standard deviation of the means).
