Observation of collective excitation of two individual atoms in the Rydberg blockade regime

When two quantum systems interact strongly with each other, their simultaneous excitation by the same driving pulse may be forbidden. The phenomenon is known as blockade of excitation. Recently, extensive studies have been devoted to the so-called Rydberg blockade between neutral atoms, which appears when the atoms are in highly excited electronic states, owing to the interaction induced by the accompanying large dipole moments. Rydberg blockade has been proposed as a basic tool in quantum-information processing with neutral atoms1–5, and can be used to deterministically generate entanglement of several atoms. Here, we demonstrate Rydberg blockade between two atoms individually trapped in optical tweezers at a distance of 4μm. Moreover, we show experimentally that collective two-atom behaviour, associated with the excitation of an entangled state between the ground and Rydberg levels, enhances the allowed single-atom excitation. These observations should be a crucial step towards the deterministic manipulation of entanglement of two or more atoms, with possible implications for quantum-information science, as well as for quantum metrology, the study of strongly correlated systems in many-body physics, and fundamental studies in quantum physics. 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. 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 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 gates1 or to entangle mesoscopic ensembles, provided that the blockade is effective over the whole sample2. Blockade effects have been observed in systems where interactions are strong such as systems of electrons using the Coulomb force6 or the Pauli effective interaction7, as well as with photons and atoms coupled to an optical cavity8. 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 collisions9. An alternative approach uses the comparatively strong interaction between two atoms excited to Rydberg states. This strong interaction gives rise to the so-called Rydberg blockade, which has been observed in clouds of cold atoms10–15 as well as in a Bose condensate16. A collective behaviour associated with the blockade has been reported in an ultracold atomic cloud17. Recently, an experiment demonstrated the blockade