Suppression of Excitation and Spectral Broadening Induced by Interactions in a Cold Gas of Rydberg Atoms

Abstract

We report on the observation of ultralong range interactions in a gas of cold Rubidium Rydberg atoms. The van-der-Waals interaction between a pair of Rydberg atoms separated as far as 100,000 Bohr radii features two important effects: Spectral broadening of the resonance lines and suppression of excitation with increasing density. The density dependence of these effects is investigated in detail for the S-and P-Rydberg states with main quantum numbers n ∼ 60 and n ∼ 80 excited by narrow-band continuous-wave laser light. The density-dependent suppression of excitation can be interpreted as the onset of an interaction-induced local blockade.PACS numbers: 32.80. Rm,32.80.Pj,34.20.Cf,03.67.Lx With the advances in laser cooling and trapping, new perspectives for the investigation of Rydberg atoms [1] have been opening. When cooled to very low temperatures, the core motion can be neglected for the timescales of excitation ("frozen Rydberg gas"). Unexpected effects have been discovered, such as the many-body diffusion of excitation [2,3], the population of high-angularmomentum states through free charges [4], or the spontaneous formation and recombination of ultracold plasmas [5,6]. Other fascinating features of cold, interacting Rydberg atoms have been proposed but not been observed so far, e.g. the creation of ultralong range molecules [7,8], whereas molecular crossover resonances have already been found experimentally [9]. One outstanding property of Rydberg atoms is their high polarizability, caused by the large distance between the outer electron and the core. This leads to strong electric field sensitivity and strong long-range dipole-dipole and vander-Waals (vdW) interactions are expected. First indications of interaction effects between Rydberg gases at high densities have been found in an atomic beam experiment [10] and, more recently, collisional evidence for ultralong range interactions in a cold Rydberg gas has been reported [11]. In a frozen gas these interactions make Rydberg atoms possible candidates for quantum information processing [12,13]. One promising approach is based on the concept of a dipole blockade [13], i.e. the inhibition of multiple Rydberg excitations in a confined volume due to interaction-induced energy shifts.In this Letter we report on experimental evidence for ultralong range interactions in a frozen Rydberg gas and we present high-resolution spectroscopic signatures of these interactions. citation from a cold gas [14]. Different to these findings, our experiment makes use of a tunable narrow-bandwidth continuous-wave (cw) laser for Rydberg excitation and thus allows for high-resolution spectroscopy of the resonance lines. By varying the density of Rydberg atoms in a controlled way, the influence of interactions on the strength and the shape of these lines is investigated in detail. Signatures of ultralong range interactions appear as spectral broadening of the excitation lines and saturation of the resonance peak height, the latter being an indication of the dipole blockade.To re...

Radiative and collisional processes in translationally cold samples of hydrogen Rydberg atoms studied in an electrostatic trap

“…The dominant term in the long-range electrostatic interaction series between two Rydberg atoms is the dipole-dipole interaction term [45]. This interaction is at the origin of diverse phenomena in cold Rydberg atom gases, such as resonant transitions of Rydberg-atom pairs [46,47], inelastic collisions [2], Penning ionization [48,49], Rydberg-excitation blockade [50][51][52] and the formation of Rydberg macrodimers [53][54][55]. It also plays an important role in the evolution of a cold Rydberg gas into a plasma [56][57][58].…”

Observation of Rydberg blockade between two atoms

“…Furthermore, dipolar interactions between highly excited atoms have been proposed as a mechanism for 'Rydberg blockade' 6,7 , which might provide a novel approach to a number of quantum protocols [8][9][10][11] . Dipolar interactions between Rydberg atoms were observed several decades ago 12 and have been studied recently in a many-body regime using cold atoms [13][14][15][16][17][18] . However, to harness Rydberg blockade for controlled quantum dynamics, it is necessary to achieve strong interactions between single pairs of atoms.…”

Coherent excitation of Rydberg atoms in micrometre-sized atomic vapour cells

“…We investigated states with principle quantum numbers between 30 and 50. Our results demonstrate that microcells with a size on the order of the blockade radius, ∼ 2 µm, at temperatures of 100 − 300 • C are robust, promising candidates to investigate low dimensional strongly interacting Rydberg gases, construct quantum gates and build single photon sources.Recently, the mutual interaction between highly excited Rydberg atoms in dense frozen samples has lead to the observation of Rydberg atom excitation blockade [1,2,3]. In Rydberg atom blockade, the excitation of more than one Rydberg atom within a blockade volume is suppressed as the mutual interaction between Rydberg atoms at internuclear separations on the order of micrometers shifts the atomic state out of resonance with a narrow band excitation laser.…”

“…Recently, the mutual interaction between highly excited Rydberg atoms in dense frozen samples has lead to the observation of Rydberg atom excitation blockade [1,2,3]. In Rydberg atom blockade, the excitation of more than one Rydberg atom within a blockade volume is suppressed as the mutual interaction between Rydberg atoms at internuclear separations on the order of micrometers shifts the atomic state out of resonance with a narrow band excitation laser.…”