Demonstration of a Stable Atom-Photon Entanglement Source for Quantum Repeaters

We demonstrate a novel way to efficiently create a robust entanglement between an atomic and a photonic qubit. A single laser beam is used to excite one atomic ensemble and two different modes of Raman fields are collected to generate the atom-photon entanglement. With the help of built-in quantum memory, the entanglement still exists after $20.5\mu \mathrm{s}$ storage time which is further proved by the violation of Clauser-Horne-Shimony-Holt type Bell’s inequality. The entanglement procedure can serve as a building block for a novel robust quantum repeater architecture [Zhao et al., Phys. Rev. Lett. 98, 240502 (2007)] and can be extended to generate high-dimensional atom-photon entanglements.

Figure 1

Illustration of the scheme of the experiment setup and the relevant energy levels of the ${}^{87}\mathrm{Rb}$ atoms. Cold ${}^{87}\mathrm{Rb}$ atoms captured by MOT are initially prepared in state $|a⟩$. A weak horizontal polarized write pulse ${\Omega }_W$ illuminates the atom cloud. The spontaneous Raman scattered anti-Stokes field ${\mathrm{AS}}_L$ and ${\mathrm{AS}}_R$ with vertical polarization are collected at $\pm 3°$ to the propagating direction of the write beam, defining the spatial mode of the atomic ensembles $L$ and $R$, respectively. The ${\mathrm{AS}}_R$ mode is rotated to be horizontal polarized, combined with ${\mathrm{AS}}_L$ mode on a polarizing beam splitter ${\mathrm{PBS}}_1$ and sent to the polarization analyzer. This creates the entanglement between the polarization of the anti-Stokes field and the spatial modes of spin excitation in the atomic ensemble. To verify the entanglement after a storage time $\tau$, a vertical polarized read pulse counter-propagating with write pulse is applied to retrieve the spin excitation to the Stokes fields $S_L$ and $S_R$. The polarization of $S_L$ is rotated by 90°, combined with $S_R$ on ${\mathrm{PBS}}_2$ and sent to the polarization analyzer.

Figure 2

Visibility of the interference fringes $V$ between Anti-Stokes fields and Stokes fields various with anti-Stokes detecting rate $p_{\mathrm{AS}}$. The solid line is the fit corresponding to Eq. (6). The dashed line shows the bound of $1/\sqrt{2}$ which mark the limit to violate the CHSH-type Bell’s inequality.

Figure 3

Decay of the $S$ parameter in the Bell’s inequality measurement with the storage time $\tau$. The dashed line shows the classical bound of $S=2$.

Figure 4

The decay of retrieve efficiency and cross correlation $g_{\mathrm{AS},S}^{(2)}$ with the storage time $\tau$. The anti-Stokes detection rate is fixed at $p_{\mathrm{AS}}=2\times {10}^{-3}$. The square dots show the decay process of the retrieve efficiency of the Stokes fields, round dots show the decay of the cross correlation $g_{\mathrm{AS},S}^{(2)}$ between anti-Stokes field and Stokes field.