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A trapped single ion inside a Bose-Einstein condensate
Christoph Zipkes, Stefan Palzer, Carlo Sias, Michael Köhl
TL;DR
The paper asks whether a single trapped ion and a Bose-Einstein condensate can be combined as a controllable hybrid quantum system. It immerses a trapped 174Yb+ ion in ultracold 87Rb atoms, characterizes their interactions, and observes sympathetic cooling, while identifying charge exchange and micromotion as relevant processes. The results motivate further study of condensates as ion refrigerators and of hybrid-system entanglement and decoherence.
Problem
Atomic quantum gases and single trapped ions had been studied separately, leaving open whether their combination could provide a useful hybrid quantum system.
Method
The experiment immerses a single trapped 174Yb+ ion in a Bose-Einstein condensate of neutral 87Rb atoms and independently controls and measures both components.
Results
The condensate symphatically cools the ion from an initial temperature of approximately 4 K to T = 0.6 ± 0.7 K, while charge exchange is strongly suppressed for the selected elements.
Takeaways & Limitations
The demonstrated immersion enables sympathetic cooling studies relevant to ion-trap quantum information and further investigations of hybrid-system entanglement and decoherence.
Takeaways & Limitations
Ion–atom collisions can transfer energy from the trap’s driven micromotion to the collision partners.
Abstract
from arXiv · showhide
Improved control of the motional and internal quantum states of ultracold neutral atoms and ions has opened intriguing possibilities for quantum simulation and quantum computation. Many-body effects have been explored with hundreds of thousands of quantum-degenerate neutral atoms and coherent light-matter interfaces have been built. Systems of single or a few trapped ions have been used to demonstrate universal quantum computing algorithms and to detect variations of fundamental constants in precision atomic clocks. Until now, atomic quantum gases and single trapped ions have been treated separately in experiments. Here we investigate whether they can be advantageously combined into one hybrid system, by exploring the immersion of a single trapped ion into a Bose-Einstein condensate of neutral atoms. We demonstrate independent control over the two components within the hybrid system, study the fundamental interaction processes and observe sympathetic cooling of the single ion by the condensate. Our experiment calls for further research into the possibility of using this technique for the continuous cooling of quantum computers. We also anticipate that it will lead to explorations of entanglement in hybrid quantum systems and to fundamental studies of the decoherence of a single, locally controlled impurity particle coupled to a quantum environment.
Methods summary
The experiment combines a single trapped 174Yb+ ion with an ultracold 87Rb atomic cloud in a shared trapping setup, enabling independent control and temperature probing of the two components.
- Ion trapping: A single 174Yb+ ion is confined in a three-dimensional radio-frequency Paul trap with radial and axial motional frequencies specified.The characteristic frequencies are ω⊥,ion = 2π × 2 · 10^5 Hz and ωx,ion = 2π × 5 · 10^4 Hz.
- Neutral-atom preparation: Ultracold 87Rb atoms are transported into the ion-trapping volume and prepared as Bose-Einstein condensates containing up to 3 × 10^4 atoms.The condensate is formed in a far-detuned optical dipole trap using two crossed 935 nm laser beams and forced evaporation.
- Neutral-atom preparation: The optical trap has final frequencies ωx,opt = 2π × 51 Hz and ωy,opt = 2π × 144 Hz.The passage reports these final optical-trap frequencies as part of the condensate preparation.
- Temperature measurement: Ion temperature is inferred from velocity-sensitive near-resonant fluorescence collected during laser probing.The ion uses the 370 nm S1/2 → P1/2 transition, with fluorescence analyzed through the time-dependent optical response.
A. Ion trapping
The ion is confined by the ponderomotive potential of a linear Paul trap, generated by a rapidly oscillating electric quadrupole field.
- A. Ion trapping: The linear Paul trap operates at an RF frequency of Ω = 2π × 42.7 MHz, above the frequencies used for evaporative cooling of the neutral atoms.The ion-electrode distance is R = 0.5 mm, and an applied voltage of approximately 250 V produces radial confinement.
- A. Ion trapping: The applied RF voltage VRF ≈ 250 V gives rise to radial ion confinement of ω⊥,ion = 2π × 2 · 10^5 Hz.This confinement is generated by the rapidly oscillating quadrupole field of the Paul trap.
B. Preparation of Bose-Einstein condensates
The cold neutral cloud is transported into the ion trap through the end-cap electrode, while the ion is displaced to control its position relative to the incoming atoms.
- B. Preparation of Bose-Einstein condensates: The neutral atomic cloud is transported by shifting the magnetic Ioffe-trap minimum through timed changes in solenoid currents.The atoms enter through a 700 µm-wide bore in the end-cap electrode and move along the linear ion trap’s symmetry axis.
- B. Preparation of Bose-Einstein condensates: The ion is displaced by 140 µm shortly before the neutral atoms reach their final position.This displacement is used during transport to control the ion’s position relative to the neutral cloud.
- B. Preparation of Bose-Einstein condensates: The optical trap produces a weak anti-trapping harmonic potential of approximately 10 Hz through the Stark effect.This potential does not significantly interfere with atom trapping, while the optical trapping effect on the ion is weaker and neglected.
C. Temperature measurements of the ion
Ion temperature is measured from the time-dependent fluorescence response during near-resonant laser probing and extracted by fitting a modeled optical response.
- C. Temperature measurements of the ion: The fluorescence method distinguishes cold and hot ions because Doppler shifts reduce fluorescence from faster ions.Fluorescence is recorded with 25 µs bins while Doppler laser cooling changes the ion’s temperature during detection.
- C. Temperature measurements of the ion: The temperature is deduced by fitting thermally averaged solutions of the time-dependent optical Bloch equations to the fluorescence data.The fit uses a χ2 criterion applied to the time-resolved fluorescence signal.
- C. Temperature measurements of the ion: For an ion laser-cooled to the Doppler limit TD = 0.5 mK, the method returns T = 0.2 ± 0.3 K.This result is indistinguishable from an ion at the stated measurement resolution.