Magnetic Trapping of Long-Lived Cold Rydberg Atoms

We report on the trapping of long-lived strongly magnetized Rydberg atoms. ${}^{85}\mathrm{Rb}$ atoms are laser cooled and collected in a superconducting magnetic trap with a strong bias field (2.9 T) and laser excited to Rydberg states. Collisions scatter a small fraction of the Rydberg atoms into long-lived high-angular momentum “guiding-center” Rydberg states, which are magnetically trapped. The Rydberg atomic cloud is examined using a time-delayed, position-sensitive probe. We observe magnetic trapping of these Rydberg atoms for times up to 200 ms. Oscillations of the Rydberg-atom cloud in the trap reveal an average magnetic moment of the trapped Rydberg atoms of $\approx -8{\mu }_B$. These results provide guidance for other Rydberg-atom trapping schemes and illuminate a possible route for trapping antihydrogen.

Figure 1

(a) Collisions transform laser excited low-$|m|$ Rydberg atoms into drift-state Rydberg atoms. The associated dramatic change in the nature of the atoms is qualitatively visualized by depicting classical Rydberg electron trajectories for $m=0$ and $m=-300$ ($a_0$: Bohr radius). Strongly magnetized atoms have permanent electric quadrupole moments. (b) Force diagram in a drift Rydberg atom. The Rydberg electron moves in a Penning-trap-like orbit on a cylindrical surface. In an inhomogeneous magnetic field, a magnetic-bottle force ${\mathbf{F}}_{\mathrm{B}}$ is acting on the electron’s cyclotron orbit. Since the internal atomic Coulomb forces ${\mathbf{F}}_{\mathrm{Coul}}$ cancel, the net trapping force ${\mathbf{F}}_{\mathrm{net}}$ is equivalent to the average magnetic-bottle force ${\mathbf{F}}_{\mathrm{B}}$.

Figure 2

(a)–(d) Phosphor images of electrons from field ionization at different times of Rydberg-atom detection. Each dot represents a field-ionized Rydberg atom. The decrease in the Rydberg counts is discussed in the text.

Figure 3

Rydberg-atom counts vs time of Rydberg-atom detection, averaged over 500 cycles. Trap ON: Measurement results for a typical trapping configuration. Filled circles: initial Rydberg-atom number ${10}^5$ and density ${10}^7{\mathrm{cm}}^{-3}$. Half-filled circles: initial Rydberg-atom number ${10}^4$ and density ${10}^6{\mathrm{cm}}^{-3}$. Trap OFF, open circles: Measurement result with the transverse confinement coils turned off and with an initial Rydberg-atom number ${10}^5$ and density ${10}^6{\mathrm{cm}}^{-3}$. The systematic background count is less than 0.02 per shot. Insets: The $B$-field magnitude has a minimum at the center location in the trap-on case, while it has a saddle point in the trap-off case.

Figure 4

(a) An increase in the trap depth can cause two types of oscillatory motion of the Rydberg-atom cloud. (b) The vertical profile of the cloud is obtained by integrating the phosphor screen images along the horizontal direction. One-dimensional vertical profiles of the clouds at different Rydberg-atom detection times show (c) breathing and (d) sloshing motions of the Rydberg-atom cloud. (e) Circles: standard deviations of the distributions in (c) vs Rydberg-atom detection time. Lines: simulation results (explained in the text). (f) Average positions of the distributions in (d) vs detection time.