Superradiance in ultracold Rydberg gases

Abstract

Experiments in dense, ultracold gases of rubidium Rydberg atoms show a considerable decrease of the radiative excited state lifetimes compared to dilute gases. This accelerated decay is explained by collective and cooperative effects, leading to superradiance. A novel formalism to calculate effective decay times in a dense Rydberg gas shows that for these atoms the decay into nearby levels increases by up to three orders of magnitude. Excellent agreement between theory and experiment follows from this treatment of Rydberg decay behavior.In recent years, ultracold atomic gases have been used to probe a variety of many-body phenomena such as Bose-Einstein condensation [1,2] and degenerate Fermi gases [3]. In addition to collective effects due to particle statistics, other manifestations of many-body physics have been explored, such as in slow-light experiments [4] and in ultracold Rydberg gases (e.g. the diffusion of excitations through resonant collisions [5] and the blockade mechanism [6]). Another important fundamental collective effect is superradiance, in which photon exchange between atoms modifies the behavior of the sample. In particular, cooperative effects due to virtual photon exchange can lead to the formation of so called Dicke states [7]. These states are the symmetric superposition of all states with the same total excitation level for constant atom number N . Interest in Dicke states has grown recently because of their potential advantages in quantum information processing [8] and their importance in the behavior of Bose-Einstein condensates [9].In this Letter, we are interested in many-body physics involving photon exchange in an ultracold gas of Rydberg atoms. Because superradiance depends on the atomic density per cubic wavelength, and because radiative decay of Rydberg atoms takes place predominantly between the closely spaced upper levels, ultracold Rydberg gases are ideal systems to study superradiance. In fact, Rydberg atoms have many interesting properties: their size can become comparable to the atomic separation, and they have huge dipole moments ℘ ∼ n 2 , where n is the principal quantum number of the Rydberg state. In addition, for long-wavelength transitions between neighboring states of high n the "cooperative parameter" C = N λ 3 /4π 2 (where N is the density of atoms, λ is the transition wavelength), is large for Rydberg atoms, which means collective effects are much easier to obtain than for ground-state atoms [10]. This was confirmed in earlier experiments for Rydberg atoms at high [11,12] and low temperatures [13]. Note that these many-body effects may pose a limit on the measurement of lifetimes of Rydberg atoms [14] and may cause undesirable frequency shifts, for example in atomic clocks [15].The source responsible for both virtual and real photon exchange is the dipole-dipole interaction. It governs the build-up as well as the decay of coherence in a dense radiating sample. On the one hand, the virtual exchange of photons is responsible for the so-called exchange interaction. ...

Direct detection of Rydberg–Rydberg millimeter-wave transitions in a buffer gas cooled molecular beam

“…The most important dephasing mechanism, especially at high number density, is homogeneous superradiant decay [42,43]. We confirm this by measuring the linewidth as a function of average number density, as shown in Figure 4, Plot (d).…”

“…However, in a dense Rydberg gas, interparticle dipole-dipole interactions can shorten the coherence lifetime via cooperative or collision-induced dephasing [7,33,42,43]. This dephasing is more serious in Rydberg transitions with large electric dipole transition moments (µ ∼ 1000 Debye) and degrees of polarization (P ∼ 1), than typical rotational transitions (µ ∼ 1 Debye, P < 0.1).…”

Spatial correlations of one-dimensional driven-dissipative systems of Rydberg atoms

“…We assume that each atom emits into different electromagnetic modes, i.e., the decay is not superradiant. (This approximation is appropriate for relatively small N , but for large N , superradiant decay to neighboring Rydberg states dominates [21,34,35]). …”

Nonequilibrium Phase Transition in a Dilute Rydberg Ensemble

“…The emission spectrum in the high occupancy phase can be understood as a superradiant cascade to lowerlying Rydberg states [32]. Evidence for a superradiant cascade has also been observed in ultracold atoms [33,34]. When the cooperativity on a particular transition is high, the atoms emit collectively and in-phase with one another.…”