The dipole blockade between Rydberg atoms has been proposed as a basic tool in quantum information processing with neutral atoms. Here we demonstrate experimentally the Rydberg blockade of two individual atoms separated by 4 µm. Moreover, we show that, in this regime, the single atom excitation is enhanced by a collective two-atom behavior associated with the excitation of an entangled state. This observation is a crucial step towards the deterministic manipulation of entanglement of two or more atoms using the Rydberg dipole interaction.PACS numbers: 32.80. Rm, 03.67.Lx, 32.80.Pj, 42.50.Ct A large experimental effort is nowadays devoted to the production of entanglement, that is quantum correlations, between individual quantum objects such as atoms, ions, superconducting circuits, spins, or photons. Entangled states are important in many areas of physics such as quantum information and quantum metrology, the study of strongly correlated systems in many-body physics, and more fundamentally in the understanding of quantum physics.There are several ways to engineer entanglement in a quantum system. Here, we focus on a method that relies on a blockade mechanism where the strong interaction between different parts of a system prevents their simultaneous excitation by the same driving pulse. Single excitation is still possible, but it is delocalized over the whole system, and results in the production of an entangled state. This approach to entanglement is deterministic and can be used to realize quantum gates [1] or to entangle mesoscopic ensembles, provided that the blockade is effective over the whole sample [2]. Blockade effects have been observed in systems where interactions are strong such as systems of electrons using the Coulomb force [3] or the Pauli effective interaction [4], as well as with photons and atoms coupled to an optical cavity [5]. Recently, atoms held in the ground state of the wells of an optical lattice have been shown to exhibit interaction blockade, due to s-wave collisions [6].An alternative approach uses the comparatively strong interaction between two atoms excited to Rydberg states, which have very large dipole moments. This strong interaction gives rise to the so-called Rydberg blockade, which has been observed in clouds of cold atoms [7,8,9,10,11,12] as well as in a Bose condensate [13]. A collective behavior associated with the blockade has been reported in an ultra-cold atomic cloud [14]. Recently, an experiment demonstrated the blockade between two atoms 10 µm apart, by showing that when one atom is excited to a Rydberg state, the excitation of the second one is greatly suppressed [15].In the present work, we study two individual atoms, held at a distance of ∼ 4 µm by two optical tweezers. We demonstrate that under this condition, the atoms are in the Rydberg blockade regime since only one atom can be excited. Furthermore, we show that the single atom excitation is enhanced by a collective two-atom behavior, associated with the production of a two-atom entangled state between th...

We report the generation of entanglement between two individual 87Rb atoms in hyperfine ground states |F=1,M=1> and |F=2,M=2> which are held in two optical tweezers separated by 4 microm. Our scheme relies on the Rydberg blockade effect which prevents the simultaneous excitation of the two atoms to a Rydberg state. The entangled state is generated in about 200 ns using pulsed two-photon excitation. We quantify the entanglement by applying global Raman rotations on both atoms. We measure that 61% of the initial pairs of atoms are still present at the end of the entangling sequence. These pairs are in the target entangled state with a fidelity of 0.75.

We study in detail the mechanisms causing dephasing of hyperfine coherences of cesium atoms confined by a far off-resonant standing wave optical dipole trap [S. Kuhr et al., Phys. Rev. Lett. 91, 213002 (2003)]. Using Ramsey spectroscopy and spin echo techniques, we measure the reversible and irreversible dephasing times of the ground state coherences. We present an analytical model to interpret the experimental data and identify the homogeneous and inhomogeneous dephasing mechanisms. Our scheme to prepare and detect the atomic hyperfine state is applied at the level of a single atom as well as for ensembles of up to 50 atoms.