Ultrafast spin-motion entanglement and interferometry in single atomic qubits*

We report entanglement between the hyperfine spin state and motional dynamics of a single atom on a timescale of 15 ns. We extract single pulses from a picosecond mode-locked laser and split them into short pulse trains tailored to create the desired spectrum by tuning the relative delays and frequency shifts appropriately. The resulting interaction imparts a momentum transfer of $2\hbar k$ to each of the two spin states in opposite directions. We apply pairs of momentum kicks to create an interferometer and probe the collapse and revival of spin coherence as the motional wavepacket is split and recombined. This technique holds promise for applications such as interferometry [1] and scalable entangling gates [2,3]. \\[4pt] [1] J.F. Poyatos et al., PRA 54, 1532 (1996)\\[0pt] [2] J.J. Garcia-Ripoll et al., PRL 91, 157901 (2003)\\[0pt] [3] L.-M. Duan, PRL 93, 100502 (2004).