Ultrafast Creation of Overlapping Rydberg Electrons in an Atomic BEC and Mott-Insulator Lattice

We study an array of ultracold atoms in an optical lattice (Mott insulator) excited with a coherent ultrashort laser pulse to a state where single-electron wave functions spatially overlap. Beyond a threshold principal quantum number where Rydberg orbitals of neighboring lattice sites overlap with each other, the atoms efficiently undergo spontaneous Penning ionization resulting in a drastic change of ion-counting statistics, sharp increase of avalanche ionization, and the formation of an ultracold plasma. These observations signal the actual creation of electronic states with overlapping wave functions, which is further confirmed by a significant difference in ionization dynamics between a Bose-Einstein condensate and a Mott insulator. This system is a promising platform for simulating electronic many-body phenomena dominated by Coulomb interactions in the condensed phase.

Figure 1

Schematic of the experiment. (a) Calculated Rydberg orbitals of the neighboring lattice sites for principal quantum numbers $\nu =38$, 42, 46, 50, 52, 53, 55, and 57 (from the bottom to the top), where the lattice spacing is 532 nm. (b) Rydberg excitation and ion detection. (c) Two-step mechanism of the ionization of interacting Rydberg atoms. See the main text for the details of (b) and (c).

Figure 2

Comparison of ion productions from the BEC and MI. The initial state is the ${}^{87}\mathrm{Rb}$ ground state $5S_{1/2}$ $|F=1,m_F=1⟩$. The graphs show the results of fifty independent measurements under similar conditions ($\sim 30000\mathrm{atoms}$, volume). (a) Arrival-time distribution of the ion signals. The ordinate represents the number of TTL pulses of ion counting summed over the fifty measurements (see Ref. [22] for the details of ion counting). The ion number indicated in each figure is the one detected at arrival times shorter than $60\mu \mathrm{s}$ and averaged over the fifty measurements. These numbers are rounded to an integer. (b) Statistical distribution of ion numbers produced from the BEC (top graph) and MI (bottom graph). The bin size is 30 ions. The arrows indicate the mean number of ions. The upper abscissa shows the number of ions normalized to the average number of Rydberg atoms created initially. The $Q$ parameter quantifying the deviation from a Poissonian distribution is 36 for the BEC and 12 for the MI, respectively.

Figure 3

Statistics of ions produced after the ultrafast Rydberg excitation of a MI for different principal quantum number $\nu$. Experimental conditions and graphical representation are identical to Fig. 2, except the initial state which is now the $5S_{1/2}$ $|F=2,m_F=2⟩$ state. (a) Statistical distribution of produced ion numbers for fifty independent measurements. The arrows denote the measured mean values. (b)–(d) The median normalized to the average number of initial Rydberg atoms, $Q$ parameter, and fraction of realizations where the normalized ion number is larger than $2/3$ as a function of principal quantum number $\nu$. The blue crosses and red plus signs in (b)–(d) represent two datasets of fifty measurements made on different days, respectively. Those two datasets have been merged to make one set of 100 data. The relevant quantities of this merged dataset have been represented by the black circles.