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• Ion-Photon Entanglement 

When excited by short laser pulses, single atoms emit single photons. Properties of these photons are strongly correlated with the properites of ions they left behind. In fact, these correlations are stronger than anything that can be explained by classical physics. The atom and the photon are entangled .

We excite single Ba ions with 400 femtosecond laser pulses at 455 nm which drives the 6S_1/2 to 6P_3/2 transition. The ion decays with a 6 nanosecond lifetime, emitting a single photon. We detect the decay process by observing the "shelving" - the ion decay into the 5D_5/2 state. Figure on the right shows the result. The shelving probability reaches maximum at some level of excitation as indicated by the pulse energy.