Experimental Bell Inequality Violation with an Atom and a Photon

We report the measurement of a Bell inequality violation with a single atom and a single photon prepared in a probabilistic entangled state. This is the first demonstration of such a violation with particles of different species. The entanglement characterization of this hybrid system may also be useful in quantum information applications.

Figure 1

The experimental procedure. (a) The atom is weakly excited with a 50 ns ${\sigma }^{+}$-polarized laser pulse at 214.5 nm, resulting in spontaneous emission to either the $|0{\rangle }_S$ state while emitting a $|0{\rangle }_P$ polarized photon or to the $|1{\rangle }_S$ state while emitting a $|1{\rangle }_P$ polarized photon. (b) A microwave ($\mu \mathrm{w}$) pulse resonant with the $|1,1\rangle \leftrightarrow |0,0\rangle$ transition coherently transfers the population in the $|1{\rangle }_S$ qubit state to the $|0,0\rangle \equiv |\tilde{1}{\rangle }_S$ qubit state. A Zeeman splitting between the ${}^{2}S_{1/2}F=1$ levels of $\delta \approx 2\pi$ (1.0 MHz) is provided by the magnetic field defining the quantization axis. (c) A second microwave pulse, the qubit rotation pulse, $R_S({\theta }_S,{\varphi }_S)$, prepares the atomic qubit for measurement in any basis.

Figure 2

The experimental apparatus. Following excitation of the atom from a 50 ns laser pulse, the scattered photons are collected by an imaging lens, directed to a polarizing beam splitter, and registered by one of two photon-counting photomultiplier tubes (PMTs). A $\lambda /2$ wave plate is used to rotate the photon polarization for photonic qubit measurements in different bases, and the microwaves are similarly used on the ion to drive coherent transitions between the atomic qubit hyperfine ground states at a frequency near 14.5 GHz. Following application of the microwaves, a $125\mu \mathrm{s}$ laser pulse is directed on the ion for atomic fluorescence qubit state detection.