Electric-field induced dipole blockade with Rydberg atoms |
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| Daniel Comparat | |
| Laboratoire Aimé Cotton, CNRS, Orsay, F | |
| Rydberg atoms have long been known to have huge polarizabilities leading to exaggerated collisional properties of room temperature atoms, in particular, large cross sections and long interaction times. These properties have stimulated great interest in the possibility to control the strong long-range interactions between cold atoms, which could be particularly exciting for quantum information applications. One interesting process is the possibility of the dipole blockade in the Rydberg excitation of atoms, due to the dipole-dipole interaction shifting the Rydberg energy from its isolated atomic value. The use of the dipole blockade of the excitation has been proposed as a very efficient realization of a scalable quantum logic gate. In a large ensemble of atoms, the first excited Rydberg atoms shift the resonance for their non-excited neighbors and prevent their excitation with a narrow-bandwidth laser. A partial, or local, blockade of the Rydberg excitation is expected in a high resolution spectroscopy. If the volume of the laser excitation is small enough, no two-atom collective excitation can occur, producing an atomic ensemble in a singly excited collective state. Two different configurations for laser excitation of states, np, have been studied in the case of cesium. A first approach considers Förster resonances corresponding to the resonant energy transfer of the np + np -> ns + (n+1)s reaction. Here the dipole - dipole interaction is tuned on and off by adding a small electric field with relatively low n, smaller than 42. The effects due to saturation in laser excitation, to Penning ionization result of cold Rydberg collisions or to the presence of eventual spurious ions can perturb the blockade. They are carefully analyzed and efficient dipole blockade has been demonstrated. A second evidence of the dipole blockade consists has been obtain using high resolution laser Stark excitation of np (60 < n < 85) Rydberg states of the ultra-cold cesium atoms. Here the efficient blockade of the excitation is attributed to long-range (permanent) dipole-dipole interaction. The dipole blockade effect is observed as a quenching of the Rydberg excitation depending on the value of the dipole moment induced by the external electric field. Effects of eventual ions which could match the dipole blockade effect will be discussed in detail. Analytic and Monte-Carlo simulations of the excitation of an ensemble of interacting Rydberg atoms agree with the experiments indicates a major role of the nearest neighboring Rydberg atom. Kinetic Monte Carlo modelisation of the dynamical experiment will be also presented. The application of the dipole blockade effect in the realization of scalable quantum gates will finally be discussed. |