Manipulating Rydberg atoms close to surfaces at cryogenic temperatures

Abstract

Helium atoms in Rydberg states have been manipulated coherently with microwave radiation pulses near a gold surface and near a superconducting NbTiN surface at a temperature of $3\phantom{\rule{0.28em}{0ex}}\mathrm{K}$. The experiments were carried out with a skimmed supersonic beam of metastable ${(1s)}^{1}{(2s)}^{1}\phantom{\rule{0.28em}{0ex}}{}^{1}{S}_{0}$ helium atoms excited with laser radiation to $np$ Rydberg levels with principal quantum number $n$ between 30 and 40. The separation between the cold surface and the center of the collimated beam is adjustable down to $250\phantom{\rule{0.28em}{0ex}}\ensuremath{\mu}\mathrm{m}$. Short-lived $np$ Rydberg states were coherently transferred to the long-lived $ns$ state to avoid radiative decay of the Rydberg atoms between the photoexcitation region and the region above the cold surfaces. Further coherent manipulation of the $ns$ Rydberg states with pulsed microwave radiation above the surfaces enabled measurements of stray electric fields and allowed us to study the decoherence of the atomic ensemble. Adsorption of residual gas onto the surfaces and the resulting slow buildup of stray fields was minimized by controlling the temperature of the surface and monitoring the partial pressures of ${\mathrm{H}}_{2}\mathrm{O}$, ${\mathrm{N}}_{2}$, ${\mathrm{O}}_{2}$, and $\mathrm{C}{\mathrm{O}}_{2}$ in the experimental chamber during the cool-down procedure. Compensation of the stray electric fields to levels below $100\phantom{\rule{0.28em}{0ex}}\mathrm{mV}/\mathrm{cm}$ was achieved over a region of $6\phantom{\rule{0.28em}{0ex}}\mathrm{mm}$ along the beam-propagation direction which, for the $1770\text{-m}/\mathrm{s}$ beam velocity, implies the possibility to preserve the coherence of the atomic sample for several microseconds above the cold surfaces.