Fig. 2: Entangling scheme with time-to-polarization conversion. | npj Quantum Information

Fig. 2: Entangling scheme with time-to-polarization conversion.

From: Scalable spin–photon entanglement by time-to-polarization conversion

Fig. 2

The ground states \(\left|0\right\rangle\) and \(\left|-1\right\rangle\) constitute the matter qubit. The laser is resonant with the \(\left|0\right\rangle\) ↔ \(\left|0_{e}\right\rangle\) transition and far-detuned from other transitions, ensuring negligible cross-excitation. Path “0” (“−1”) display the evolution for initialization of the spin in the state \(\left|0\right\rangle\) (\(\left|-1\right\rangle\)), as the entangling sequence is applied to the initial spin superposition \((\left|0\right\rangle -\left|-1\right\rangle )/\sqrt{2}\). In panel , a laser excitation leads to the generation of a photon in path “0”, which is guided via a switch into a channel that rotates the polarization to an horizontal orientation. No photon is generated in path “−1” since no resonant transition is available from the state \(\left|-1\right\rangle\). In panel , the spin state is inverted in both paths by a microwave π-pulse and the classical optical routing is switched to a vertically polarized channel. In panel , a further excitation cycle is triggered with another laser pulse. Owing to the route switching, this excitation results in the generation of a vertically polarized photon in the evolution path “ −1”. This time, no photon is generated in path \(\left|0\right\rangle\). Although the photon is emitted with the same polarization in both evolution paths “0” and “ −1”, the classical photonic routing and polarization elements enable the entanglement of the spin with the photon polarization. The path information is erased with a polarizing beam splitter (PBS) where the possible photon trajectories are overlapped, resulting again in the desired entangled state of the type \((\left|0,V\right\rangle +{e}^{i\phi }\left|1,H\right\rangle )/\sqrt{2}\) (see text).

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