[Paper Review] Stark spectroscopy of Rydberg atoms in an atom-ion hybrid trap

Shinsuke Haze, Joschka Wolf+4 more|arXiv (Cornell University)|Jan 30, 2019

Cold Atom Physics and Bose-Einstein Condensates5 citations

TL;DR

This study uses Rydberg spectroscopy in a hybrid atom-ion trap to probe electric fields experienced by ultracold 87Rb atoms, revealing Stark shifts indicating fields up to 250 V/cm. Despite evidence pointing to trapped ions as the source, spectral data suggest contributions from the Paul trap’s RF fields, highlighting a critical ambiguity in field origin that requires further investigation using pulsed measurements or frequency-dependent non-linearity analysis.

ABSTRACT

We report on Rydberg spectroscopy of ultracold atoms in an atom-ion hybrid trap for probing the electric fields in a mixture of atoms and ions. We obtain spectra which exhibit excitation gaps corresponding to avoided level crossings in the Stark map. From these measurements we can conclude that the ground state atoms experience electrical fields of up to 250 V/cm. There is, however, a difficulty in interpreting the results, because some data indicate that the electrical fields are produced by the ions while other data indicate that they stem from the Paul trap. We discuss possible scenarios for explaining the measured data, provide first measurements to check these scenarios, and propose methods to finally solve this puzzle.

Motivation & Objective

• To probe local electric fields in a mixed ultracold atom-ion system using Rydberg spectroscopy.
• To determine the origin of strong electric fields (up to 250 V/cm) observed in Rydberg spectra, distinguishing between contributions from trapped ions and the Paul trap.
• To resolve a contradiction in experimental data where some measurements suggest ion-originated fields, while spectral line shapes imply Paul trap fields.
• To develop and propose methods for unambiguously identifying the source of electric fields in hybrid atom-ion systems.

Proposed method

• A dual-trap system combines an optical dipole trap (ODT) for ultracold 87Rb atoms and a linear Paul trap for Rb+ ions, enabling simultaneous trapping.
• Single-photon Rydberg excitation is performed via a frequency-doubled dye laser at ~298.5 nm, exciting atoms from the 5S1/2 ground state to the 27P state.
• Atom loss due to Rydberg excitation and subsequent ionization is measured via absorption imaging to infer ion accumulation and field effects.
• Spectra are acquired by scanning the UV laser frequency, with excitation gaps indicating avoided level crossings in the Stark map.
• Controlled displacement of the ODT relative to the Paul trap center allows measurement of field dependence at varying distances.
• Numerical simulations of electric field distributions and Stark shifts are used to compare with experimental spectra, validating field strength estimates.

Experimental results

Research questions

• RQ1 What is the origin of the electric fields (up to 250 V/cm) experienced by ultracold Rydberg atoms in the hybrid trap?
• RQ2 Why do some measurements suggest ion-originated fields while spectral line shapes point to Paul trap fields?
• RQ3 Can the non-linearity coefficient κ of atom loss with time be used to distinguish between field sources?
• RQ4 How do the spatial and temporal inhomogeneities of the Paul trap’s RF field affect Rydberg spectroscopy?
• RQ5 Can pulsed spectroscopy during zero-field phases of a digital ion trap resolve the field origin ambiguity?

Key findings

• Rydberg spectroscopy reveals excitation gaps corresponding to avoided crossings in the Stark map, indicating electric fields of up to 250 V/cm in the atomic cloud.
• The non-linearity coefficient κ of atom loss with time increases with radial trapping frequency ωr of the Paul trap, indicating higher ion density and stronger field effects.
• κ saturates at ωr ≈ 2π × 230 kHz, likely due to reduced ionic cloud compressibility from Coulomb repulsion.
• Measurements in the absence of ions show that the Paul trap’s RF field alone causes significant broadening and distortion of the 27P3/2 resonance line, spanning ~4 GHz at d = 97 μm.
• Numerical simulations of the electric field distribution and Stark shifts show good agreement with experimental spectra, validating the model of field-induced Rydberg excitation.
• The discrepancy between ion-origin hypotheses and spectral data suggests that the Paul trap’s RF field may be the dominant source of the observed Stark shifts, challenging initial assumptions.