We review here the studies performed about interactions in an assembly of cold Rydberg atoms. We focus especially on the review of the dipole–dipole interactions and on the effect of the dipole blockade in the laser Rydberg excitation, which offers attractive possibilities for quantum engineering. We present first the various interactions between Rydberg atoms. The laser Rydberg excitation of such an assembly is then described with the introduction of the dipole blockade phenomenon. We report recent experiments performed in this subject by starting with the case of a pair of atoms allowing the entanglement of the wave-functions of the atoms and opening a fascinating way for the realization of quantum bits and quantum gates. We consider then several works on the blockade effect in a large assembly of atoms for three different configurations: blockade through electric-field induced dipole, through Forster resonance, and in van der Waals interaction. The properties of coherence and cooperativity are analyzed. Finally, we treat the role of dipole–dipole interactions between Rydberg atoms responsible for Penning ionization. The perturbation of the dipole blockade by ions and the evolution of the Rydberg towards an ultracold plasma are discussed.

Dipole blockade in a cold Rydberg atomic sample [Invited]

Ultracold neutral plasmas

We review experimental and theoretical tools to excite, study and understand strongly interacting Rydberg gases. The focus lies on the excitation of dense ultracold atomic samples close to, or within quantum degeneracy, high-lying Rydberg states. The major part is dedicated to highly excited S-states of rubidium, which feature an isotropic van der Waals potential. Nevertheless, the setup and the methods presented are also applicable to other atomic species used in the field of laser cooling and atom trapping. (Some figures may appear in colour only in the online journal)

https://iopscience.iop.org/article/10.1088/0953-4075/45/11/113001/pdf

An experimental and theoretical guide to strongly interacting Rydberg gases

We report quantum degeneracy in a gas of ultracold fermionic (87)Sr atoms. By evaporatively cooling a mixture of spin states in an optical dipole trap for 10.5 s, we obtain samples well into the degenerate regime with T/T(F)=0.26(-0.06)(+0.05). The main signature of degeneracy is a change in the momentum distribution as measured by time-of-flight imaging, and we also observe a decrease in evaporation efficiency below T/T(F) ∼0.5.

/pdf/degenerate-fermi-gas-of-87-sr-r9t3r3vjph.pdf

Degenerate Fermi Gas of 87 Sr

We report optical absorption imaging of ultracold neutral strontium plasmas. The ion absorption spectrum determined from the images is Doppler broadened and thus provides a quantitative measure of the ion kinetic energy. For the particular plasma conditions studied, ions heat rapidly as they equilibrate during the first 250 ns after plasma formation. Equilibration leaves ions on the border between the weakly coupled gaseous and strongly coupled liquid states. On a longer time scale of microseconds, pressure exerted by the trapped electron gas accelerates the ions radially.

/pdf/using-absorption-imaging-to-study-ion-dynamics-in-an-1ybkk1gits.pdf

Using absorption imaging to study ion dynamics in an ultracold neutral plasma.

We study equilibration of strongly coupled ions in an ultracold neutral plasma produced by photoionizing laser-cooled and trapped atoms. By varying the electron temperature, we show that electron screening modifies the equilibrium ion temperature. Even with few electrons in a Debye sphere, the screening is well described by a model using a Yukawa ion-ion potential. We also observe damped oscillations of the ion kinetic energy that are a unique feature of equilibration of a strongly coupled plasma.

/pdf/electron-screening-and-kinetic-energy-oscillations-in-a-3g0t7l53p3.pdf

Electron Screening and Kinetic-Energy Oscillations in a Strongly Coupled Plasma

We report the magnetic trapping of metastable {sup 3}P{sub 2} atomic strontium. Atoms are cooled in a magneto-optical trap (MOT) operating on the dipole-allowed {sup 1}S{sub 0}-{sup 1}P{sub 1} transition at 461 nm. Decay via {sup 1}P{sub 1}{yields}{sup 1}D{sub 2}{yields}{sup 3}P{sub 2} continuously loads a magnetic trap formed by the quadrupole magnetic field of the MOT. Over 10{sup 8} atoms at a density of 8x10{sup 9} cm{sup -3} and temperature of 1 mK are trapped. The atom temperature is significantly lower than what would be expected from the kinetic and potential energies of atoms as they are transferred from the MOT. This suggests the occurrence of thermalization and evaporative cooling in the magnetic trap.

https://arxiv.org/pdf/physics/0212024.pdf

Magnetic trapping of metastable 3P2 atomic strontium

We study the expansion of ultracold neutral plasmas in the regime in which inelastic collisions are negligible. The plasma expands due to the thermal pressure of the electrons, and for an initial spherically symmetric Gaussian density profile, the expansion is self-similar. Measurements of the plasma size and ion kinetic energy using fluorescence imaging and spectroscopy show that the expansion follows an analytic solution of the Vlasov equations for an adiabatically expanding plasma.

/pdf/experimental-realization-of-an-exact-solution-to-the-vlasov-hxlcs9x2hc.pdf

Experimental Realization of an Exact Solution to the Vlasov Equations for an Expanding Plasma

We have used the free expansion of ultracold neutral plasmas as a time-resolved probe of electron temperature. A combination of experimental measurements of the ion expansion velocity and numerical simulations characterize the crossover from an elastic-collision regime at low initial {gamma}{sub e}, which is dominated by adiabatic cooling of the electrons, to the regime of high {gamma}{sub e} in which inelastic processes drastically heat the electrons. We identify the time scales and relative contributions of various processes, and we experimentally show the importance of radiative decay and disorder-induced electron heating for the first time in ultracold neutral plasmas.

Electron-temperature evolution in expanding ultracold neutral plasmas.

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