Fast Ground State Manipulation of Neutral Atoms in Microscopic Optical Traps

We demonstrate Rabi flopping at MHz rates between ground hyperfine states of neutral ${}^{87}\mathrm{Rb}$ atoms that are trapped in two micron sized optical traps. Using tightly focused laser beams we demonstrate high fidelity, site specific Rabi rotations with cross talk on neighboring sites separated by $8\mu \mathrm{m}$ at the level of ${10}^{-3}$. Ramsey spectroscopy is used to measure a dephasing time of $870\mu \mathrm{s}$, which is $\approx 5000\mathrm{\text{times}}$ longer than the time for a $\pi /2$ pulse.

Figure 1

The experimental setup. Two tightly focused beams that spatially overlap with a MOT form closely spaced optical traps. The MOT beams are not shown for simplicity. Inset (a) shows a fluorescence image of the atoms that are trapped in the $2\mu \mathrm{m}$ sized FORTs (termed sites A and B, respectively). Inset (b) shows a photoelectron count histogram from 1500 loading experiments into a single trapping site, demonstrating the ability to load and measure single atoms. Inset (c) shows the relevant energy level structure of ${}^{87}\mathrm{Rb}$. Two laser beams whose frequency difference equals the hyperfine transition frequency implement Rabi rotations on the qubit states. The Rabi rotation beams are steered to either site A or site B with the use of an acousto-optic (AO) modulator.

Figure 2

Fast rabi rotations on one site with negligible cross talk to the other site. In plot (a), we start with all the atoms in state $|0⟩$ and measure the fraction of atoms in state $|1⟩$ as a function of the duration of the two-photon pulse. We observe Rabi flopping between two-qubit states at a rate of ${\Omega }_R=2\pi \times 1.36\mathrm{MHz}$. This rate corresponds to a $\pi /2$ rotation time of 183 ns (see text for details). Plot (b) shows that the cross talk to the other site is negligible and demonstrates our ability to individually address the two sites.

Figure 3

Ramsey spectroscopy on the $|0⟩\to |1⟩$ hyperfine transition. We apply two $\pi /2$ pulses with a delay $T$ and measure the fraction of atoms in state $|1⟩$ as a function of the two-photon detuning $\delta$.

Figure 4

The contrast of the Ramsey fringes of Fig. 3 as a function of the time delay between two pulses, $T$. The solid line is an exponential fit to the data.