Figure 1

(a) Three-step scheme of excitation for $n{p}_{3/2}$ Rydberg levels. (b) Excitation of a pair of atoms at the Förster resonance. The dipole-dipole coupling $W\sim \mu {\mu }^{\prime }{R}^{-3}$ leads to an avoided crossing between the energy levels of the pair of atoms $pp\equiv |n{p}_{3/2}⟩\otimes |n{p}_{3/2}⟩$ and $s{s}^{\prime }\equiv |ns⟩\otimes |(n+1)s⟩$. At resonance, the eigenstates are $|+⟩=(pp+s{s}^{\prime })/\sqrt{2}$ and $|-⟩=(pp-s{s}^{\prime })/\sqrt{2}$. (c) Internuclear energy dependence and excitation of the states $|+⟩$ and $|-⟩$ states corresponding to mutual repulsive and attractive forces between the two atoms, respectively.Reuse & Permissions

Figure 2

Dipole blockade for the $38p_{3/2}$ level versus the applied electric field for a $7s$-atom density $\sim 8\pm 3\times {10}^9{\mathrm{cm}}^{-3}$. For each recorded point of the curves, the Ti:Sa laser is resonant with the transition $7s\to 38p_{3/2}$, with a power $\sim 8\mathrm{mW}$ corresponding to an average intensity of $45\mathrm{W}/{\mathrm{cm}}^2$. (a) Total number of Rydberg atoms, (b) number of $38s$ atoms, and (c) number of ions. (a)–(c) Error bars indicate measurement uncertainties.Reuse & Permissions

Figure 3

Dipole blockade for the $36p_{3/2}$ level. Similar conditions as Fig. 2 are used in (a) and (b). (a) Field sweep of the FR. The signal is normalized to one at $0.86\mathrm{V}/\mathrm{cm}$. (b) Excitation spectral lines for two different fields: (1) at FR ($F=2.52\mathrm{V}/\mathrm{cm}$) and (2) off FR at $0.86\mathrm{V}/\mathrm{cm}$. (c) Total number of Rydberg atoms at FR versus the one off FR ($F^{\prime }=1.37\mathrm{V}/\mathrm{cm}$), for the $7s$-atom densities $D\sim 8\pm 3\times {10}^9{\mathrm{cm}}^{-3}$ (solid circles) and $D/3$ (empty stars). The Ti:Sa power ranges 0–14 and 0–67 mW for the $D$ and $D/3$ densities, respectively. (d) Linewidths of the $7s\to 36p_{3/2}$ spectral line, versus the Ti:Sa power $P$ at both fields $F$ (upper curve) and $F^{\prime }$ (lower curve). (c) and (d) Solid line arrows indicate the beginning of the dipole blockade, dashed arrows the beginning of the line broadening due to the power saturation. (c) A double down arrow indicates the complete power saturation of the excitation [$P(\mathrm{Ti}\mathrm{\text{:}}\mathrm{Sa})>60\mathrm{mW}$] in the case of the $D/3$ density (the blockade is no longer observed). A similar result is obtained for the $D$ density when the number of Rydberg atoms reaches $\sim 12000$.Reuse & Permissions

Figure 4

Normalized total number of excited $41p_{3/2}$ Rydberg atoms versus the Stark shift ($\mathrm{V}/\mathrm{cm}$ and MHz) of the $41p_{3/2}$ Stark level. The electric field has been converted to frequency using the experimental Stark shifts, in agreement with calculated ones. The two curves correspond to two different Rydberg atom number at Förster resonance, 3000 (circles) and 5000 (squares), obtained for laser powers 8 and 20 mW, respectively. The normalization factor is the number of atoms excited at a higher frequency (200 MHz). The fits use an empirical law not given here. The widths (FWHM) of the above Förster resonances are $20\pm 5$ and $90\pm 30\mathrm{MHz}$, respectively.Reuse & Permissions