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arXiv:cond-mat/0702216 · Physical Review A

Spatially resolved photoionization of ultracold atoms on an atom chip

Sebastian Kraft, Andreas Günther, József Fortágh, Claus Zimmermann

Published Jun 6, 2007• 17 citations

Abstract

We report on photoionization of ultracold magnetically trapped Rb atoms on an atom chip. The atoms are trapped at $5\phantom{\rule{0.3em}{0ex}}\ensuremath{\mu}\mathrm{K}$ in a strongly anisotropic trap. Through a hole in the chip with a diameter of $150\phantom{\rule{0.3em}{0ex}}\ensuremath{\mu}\mathrm{m}$, two laser beams are focused onto a fraction of the atomic cloud. A first laser beam with a wavelength of $778\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ excites the atoms via a two-photon transition to the $5D$ level. With a fiber laser at $1080\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ the excited atoms are photoionized. Ionization leads to depletion of the atomic density distribution observed by absorption imaging. The resonant ionization spectrum is reported. The setup used in this experiment is suitable not only to investigate mixtures of Bose-Einstein condensates and ions but also for single-atom detection on an atom chip.

DOI: 10.1103/physreva.75.063605

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We report on photoionization of ultracold magnetically trapped Rb atoms on an atom chip. The atoms are trapped at

5\phantom{\rule{0.3em}{0ex}}\ensuremath{\mu}\mathrm{K}

in a strongly anisotropic trap. Through a hole in the chip with a diameter of

150\phantom{\rule{0.3em}{0ex}}\ensuremath{\mu}\mathrm{m}

, two laser beams are focused onto a fraction of the atomic cloud. A first laser beam with a wavelength of

778\phantom{\rule{0.3em}{0ex}}\mathrm{nm}

excites the atoms via a two-photon transition to the

5D

level. With a fiber laser at

1080\phantom{\rule{0.3em}{0ex}}\mathrm{nm}

the excited atoms are photoionized. Ionization leads to depletion of the atomic density distribution observed by absorption imaging. The resonant ionization spectrum is reported. The setup used in this experiment is suitable not only to investigate mixtures of Bose-Einstein condensates and ions but also for single-atom detection on an atom chip.

Photoionization of a cold atomic sample offers intriguing possibilities for observing collective effects at extremely low temperatures. Irradiation of a rubidium condensate and of cold rubidium atoms within a magneto-optical trap (MOT) with laser pulses ionizing through one-photon and two-photon absorption processes was performed. Losses and modifications in the density profile of the remaining trapped cold cloud or the remaining condensate sample were examined as functions of the ionizing laser parameters. Ionization cross sections were measured for atoms in a MOT, while in magnetic traps losses larger than those expected for ionization process were measured.

We experimentally demonstrate optical spectroscopy of magnetically trapped atoms on an atom chip. High resolution optical spectra of individual trapped clouds are recorded within a few hundred milliseconds. Detection sensitivities close to the single atom level are obtained by photoionization of the excited atoms and subsequent ion detection with a channel electron multiplier. Temperature and decay rates of the trapped atomic cloud can be monitored in real time for several seconds with only little detection losses. The spectrometer can be used for investigations of ultracold atomic mixtures and for the development of interferometric quantum sensors on atom chips.

The authors have used laser cooling and trapping techniques to investigate photoionization and cold collisions. With laser-trapped Rb, they have measured the photoionization cross section from the first excited (5P) level by observing the photoionization-induced loss rate of neutral atoms from the trap. This technique has the advantage that it directly measures the photoionization rate per atom. Knowing the ionizing laser intensity and the excited-state fraction, the measured loss rate gives the absolute cross section. Using this technique, the Rb 5P photoionization cross section at {approximately}400 nm has been determined with an uncertainty of 9%. The authors are currently attempting to extend this method to the 5D level. Using time-ordered pulses of diode-laser light (similar to the STIRAP technique), they have performed very efficient two-photon excitation of trapped Rb atoms to 5D. Finally, they will present results from a recent collaboration which combines measurements form conventional molecular spectroscopy (single photon and double resonance) with photoassociation collisions of ultracold Na atoms to yield a precise ({le}1 ppm) value for the dissociation energy of the X {Sigma}{sub g}{sub +} ground state of the Na{sub 2} molecule.

In this paper, we describe the preparation and detection of ultracold atoms on a microchip with single-atom sensitivity. The detection scheme is based on multi-photon ionization of atoms and the subsequent guiding of the generated ions by ion optics to a channel electron multiplier. We resolve single atoms with a detection efficiency above 60%. The detector is suitable for real-time observations of static and dynamic processes in ultracold quantum gases. Although the ionization is destructive, sampling a small subset of the atomic distribution is sufficient for the determination of the desired information. We take full high-resolution spectra of ultracold atoms by ionizing only 5% of the atoms. Using an additional microwave near 6.8 GHz, the detection scheme becomes energy, position and state selective. This can be used for in situ determination of the energy distribution and temperature of atom clouds inside the trap and applied for future correlation measurements.