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...

Chemical reaction rates often depend strongly on stereodynamics, namely the orientation and movement of molecules in three-dimensional space [1][2][3]. An ultracold molecular gas, with a temperature below 1 µK, provides a highly unusual regime for chemistry, where polar molecules can easily be oriented using an external electric field and where, moreover, the motion of two colliding molecules is strictly quantized. Recently, atom-exchange reactions were observed in a trapped ultracold gas of KRb molecules [4]. In an external electric field, these exothermic and barrierless bimolecular reactions, KRb+KRb→ K 2 +Rb 2 , occur at a rate that rises steeply with increasing dipole moment [5]. Here we show that the quantum stereodynamics of the ultracold collisions can be exploited to suppress the bimolecular chemical reaction rate by nearly two orders of magnitude. We use an optical lattice trap to confine the fermionic polar molecules in a quasi-two-dimensional, pancake-like geometry, with the dipoles oriented along the tight confinement direction [6,7]. With the combination of sufficiently tight confinement and Fermi statistics of the molecules, two polar molecules can approach each other only in a "side-by-side" collision, where the chemical reaction rate is suppressed by the repulsive dipole-dipole interaction. We show that the suppression of the bimolecular reaction rate requires quantum-state control of both the internal and external degrees of freedom of the molecules. The suppression of chemical reactions for polar molecules in a quasi-two-dimensional trap opens the way for investigation of a dipolar molecular quantum gas. Because of the strong, long-range character of the dipole-dipole interactions, such a gas brings fundamentally new abilities to quantum-gas-based studies of strongly correlated many-body physics, where quantum phase transitions and new states 2 of matter can emerge [8][9][10][11][12][13].Two colliding polar molecules interact via long-range dipole-dipole forces well before they reach the shorter distance scales where chemical forces become relevant. Therefore, the spatial anisotropy of the dipolar interaction can play an essential role in the stereochemistry of bimolecular reactions of polar molecules. In general, one expects the attraction between oriented dipoles in a "head-to-tail" collision to be favorable for chemical reactions, while the repulsion between two oriented polar molecules in a "side-by-side" collision presents an obstacle for reactions. Up to now, however, large center-of-mass collision energies have precluded the direct control of chemical reactions via dipolar interactions. In a cold collision regime, where tens of scattering partial waves contribute, one can begin to exert control of intermolecular dynamics through the dipolar effect [14]. An ultracold gas, however, provides an optimum environment in which to fully investigate the dipolar effects [5,15,16]. Here, the molecules can be prepared in identical internal quantum states, with the dipoles oriented using an external...

The methods producing cold molecules from cold atoms tend to leave molecular ensembles with substantial residual internal energy. For instance, Cs 2 molecules initially formed via photoassociation of cold Cs atoms are in several vibrational levels, v, of the electronic ground state. Here we apply a broadband femtosecond laser that redistributes the vibrational population in the ground state via a few electronic excitation -spontaneous emission cycles. The laser pulses are shaped to remove the excitation frequency band of the v = 0 level, preventing re-excitation from that state. We observe a fast and efficient accumulation, ∼ 70% of the initially detected molecules, in the lowest vibrational level, v = 0, of the singlet electronic state. The validity of this incoherent depopulation pumping method is very general and opens exciting prospects for laser cooling and manipulation of molecules.