Quantum Suppression of Microwave Ionization of Rydberg Atoms at High Scaled Frequency

Microwave ionization of Rydberg atoms is well described as the onset of classical chaos when the microwave frequency $\omega$ is less than the Kepler frequency $1/n^3$. However, when $\omega >1/n^3$, i.e., at high scaled frequency $\Omega =\omega n^3>1$, classical ionization is predicted to be suppressed by quantum interference, an analogue to Anderson localization in a solid. Using 17.55 GHz microwave fields we have observed the ionization of Sr Rydberg atoms in the regime $1\le \Omega \le 5$ . Our measurements demonstrate the quantum suppression of classical diffusive ionization and show the ionization field to be $n$ independent in this regime.

Figure 1

Experimental arrangement. (a) Excitation scheme of Sr $5snd$ Rydberg states. (b) Timing sequence of laser pulses, MW pulse, field ionization pulse (bold solid line), and $n_c$ depression pulse (dotted line).

Figure 2

Signal from atoms surviving fixed amplitude MW pulses obtained by scanning the frequency of the second laser. The upper trace shows the transmission of the laser light through the etalon. The data are plotted vs binding energy. Scans at field amplitudes, (a) 0 V/cm, (b) 5.1 V/cm, (c) 7.7 V/cm, (d) 9.4 V/cm, and (e) 11.0 V/cm are shown. The vertical scales are the same for (a) through (e), although the traces are vertically offset from each other. Taking vertical slices through such data allows us to determine the 10%, 50%, and 90% ionization fields.

Figure 3

Scaled 50% ionization fields for $0.4<\Omega <2.5$. Fields obtained with a cutoff $n$, $n_c=270$, by varying the MW field amplitude (•) and by scanning the second laser frequency with different fixed field amplitudes as shown in Fig. 2 (○). The substantially suppressed 50% ionization fields obtained when $n_c$ is lowered to $n_c=145$ (▵) and $n_c=120$ (×). Data points are connected by straight lines to guide the eye. The horizontal bars at the top of the figure indicate the resolution $\Delta \Omega$ from our dye laser linewidth.

Figure 4

Scaled 50% ionization fields for $0.9<\Omega <5.5$. The classical prediction of Eq. (1) (dashed line) is far below the data. The localization theory prediction of Eq. (2) (dotted line) fits reasonably well for $\Omega \approx 2.5$ but does not appear to have the correct functional form. The prediction of Eq. (4) (solid line), which corresponds to an $n$ independent field $F=6.3\mathrm{V}/\mathrm{c}\mathrm{m}$, seems to be in quite good agreement with the data. The horizontal bars at the top of the figure indicate the resolution $\Delta \Omega$ from our dye laser linewidth.