[Paper Review] Production of a rubidium Bose-Einstein condensate in a hybrid trap with light induced atom desorption

Dezhi Xiong, Fudong Wang+3 more|arXiv (Cornell University)|Mar 2, 2013

Cold Atom Physics and Bose-Einstein Condensates3 citations

TL;DR

This paper demonstrates the production of a ⁸⁷Rb Bose-Einstein condensate (BEC) with over 10⁵ atoms in a compact, single-glass-cell vacuum system using light-induced atomic desorption (LIAD) to maintain ultra-high vacuum. The hybrid trap—combining a magnetic quadrupole trap for initial evaporative cooling and an optical dipole trap (ODT) for final cooling—achieves high evaporation efficiency (α = 2.7) and condensate formation in just 9.5 seconds of evaporation, enabled by a phase-space density of 3×10⁻³ at ODT loading.

ABSTRACT

We report on the production of a rubidium Bose-Einstein condensate in a simplified vacuum apparatus. Magneto-optical traps with large numbers and ultra-high vacuum for moderately long conservative trap lifetimes of 16 seconds are sequentially obtained with light induced rapid atomic vapor pressure modulation. Subsequent evaporative cooling is carried out in two stages in a hybrid magnetic quadrupole plus optical dipole trap. High evaporation efficiencies are observed in both stages and $^{87}$Rb BECs with more than 10$^5$ atoms can be reliably produced with total evaporation time of only 9.5 seconds.

Motivation & Objective

• To develop a simplified, low-cost BEC apparatus suitable for new research groups.
• To overcome the conflict between high vacuum for evaporative cooling and sufficient atomic density for magneto-optical trap (MOT) loading.
• To achieve reliable BEC production in a single glass cell with moderate vacuum conditions.
• To demonstrate high evaporation efficiency in a hybrid magnetic-optical trap configuration.

Proposed method

• A single rectangular glass cell with no anti-reflection coating is used as the vacuum chamber, connected to a CF35 flange.
• Light-induced atomic desorption (LIAD) is implemented using a 365 nm UV LED to periodically replenish Rb atoms and maintain ultra-high vacuum.
• A magnetic quadrupole trap is used for initial evaporative cooling, followed by transfer to a crossed-beam optical dipole trap (ODT) for final cooling.
• The ODT is formed by a far-red-detuned laser beam focused below the quadrupole trap center, shifting the potential minimum away from the magnetic field zero to prevent Majorana losses.
• Evaporative cooling is performed in two stages: forced microwave evaporation in the hybrid trap and further cooling in the ODT using a power ramping protocol P(t) = Pᵢ(1 + t/τ)⁻ᵝ.
• The trap depth and beam focus are adjusted to minimize thermal drift effects from the Pyrex cell wall, with evaporation efficiency defined as α = -log(PSD/PSD₀)/log(N/N₀).

Experimental results

Research questions

• RQ1 Can a BEC be reliably produced in a simplified vacuum setup with a single glass cell and moderate vacuum conditions?
• RQ2 How effective is light-induced atomic desorption (LIAD) in maintaining ultra-high vacuum while enabling high atomic loading in a MOT?
• RQ3 What is the evaporation efficiency of a hybrid magnetic quadrupole and optical dipole trap configuration under limited conservative trap lifetime?
• RQ4 To what extent do thermal lensing effects from the glass cell wall impact evaporation dynamics and trap stability?

Key findings

• A ⁸⁷Rb BEC with more than 10⁵ atoms was reliably produced in a single 100×40×40 mm glass cell with a conservative trap lifetime of 16 seconds.
• The initial phase-space density after quadrupole trap evaporation reached 3×10⁻³, enabling rapid final evaporation in just 3.5 seconds.
• Evaporation efficiency in the optical dipole trap was measured at α = 2.7, indicating high cooling efficiency despite moderate vacuum conditions.
• The overall evaporation process, including both hybrid trap and ODT stages, lasted only 9.5 seconds, with condensate formation observed at 240 nK and 2.7×10⁵ atoms.
• The transfer efficiency from the quadrupole trap to the ODT was approximately 15%, and the final trap frequencies were measured as 2π×(98, 112, 61) Hz along x, y, and z axes.
• Thermal drift from the 3 mm Pyrex cell wall caused beam focus shifts, but the effect was reproducible and did not prevent successful condensate production.