Towards Long-Distance Atom-Photon Entanglement

We report the observation of entanglement between a single trapped atom and a single photon at remote locations. The degree of coherence of the entangled atom-photon pair is verified via appropriate local correlation measurements, after communicating the photon via an optical fiber link of 300 m length to a receiver 3.5 m apart. In addition, we measured the temporal evolution of the atomic density matrix after projecting the atom via a state measurement of the photon onto several well-defined spin states. We find that the state of the single atom dephases on a time scale of $150\mu \mathrm{s}$, which represents an important step towards long-distance quantum networking with individual neutral atoms.

Figure 1

Schematic of long-distance atom-photon entanglement. (a) During the spontaneous decay of a single optically trapped ${}^{87}\mathrm{Rb}$ atom on the transition $5^2{P}_{3/2}$, $F^{\prime }=0\to 5^2{S}_{1/2}$, $F=1$ the polarization of the emitted photon gets entangled with the final spin state of the atom. (b) The emitted photon is coupled into a single-mode optical fiber and communicated to a remote location where a polarization analysis is performed. To overcome thermally and mechanically induced fluctuations of the fiber birefringence, an active polarization compensation is used. Therefore reference laser pulses are sent through the optical fiber and the output polarizations are characterized in a reference polarimeter. With the help of a software algorithm and a dynamic polarization controller it is ensured that input and output polarizations are identical.

Figure 2

Verification of atom-photon entanglement. Probability of detecting the atomic qubit in (a) $|\downarrow {⟩}_x≔1/\sqrt{2}(|\uparrow {⟩}_z-|\downarrow {⟩}_z)$ and (b) $|\downarrow {⟩}_y≔1/\sqrt{2}(|\uparrow {⟩}_z-i|\downarrow {⟩}_z)$ conditioned on the detection of the photon in detector ${\mathrm{APD}}_1$ or ${\mathrm{APD}}_2$, where the photonic qubit is projected onto the states $1/\sqrt{2}(|{\sigma }^{+}⟩\pm e^{2i\beta }|{\sigma }^{-}⟩)$. The phase $\beta$ can be set with a rotatable $\lambda /2$ wave plate in front of a polarizing beam splitter (PBS). For $\beta =0°$, respectively, $\beta /2=22.5°$ the photon is analyzed in the complementary measurement bases ${\sigma }_x$ and ${\sigma }_y$.

Figure 3

Spin precession of the atomic states $\{|\uparrow {⟩}_x≔1/\sqrt{2}(|\uparrow {⟩}_z+|\downarrow {⟩}_z)$, $|\downarrow {⟩}_x≔1/\sqrt{2}(|\uparrow {⟩}_z-|\downarrow {⟩}_z)\}$, as a guiding field of 5.5 mG is applied along the quantization axis $z$. After the temporal evolution the population $P$ of the $|\downarrow {⟩}_x$ state is measured.

Figure 4

Temporal evolution of the lower bound of the purity parameter $r$ as the atom is prepared initially in the states (a) $\{|\uparrow {⟩}_x≔1/\sqrt{2}(|\uparrow {⟩}_z+|\downarrow {⟩}_z),|\downarrow {⟩}_x≔1/\sqrt{2}(|\uparrow {⟩}_z-|\downarrow {⟩}_z)\}$, (b) $\{|\uparrow {⟩}_z≔|1,+1⟩,|\downarrow {⟩}_z≔|1,-1⟩\}$. Each measurement point results from a partial quantum state tomography of the spin-1 ground level $5^2{S}_{1/2}$, $F=1$.