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arXiv:2505.15302 · EPJ Quantum Technology

Robust and compact single-lens crossed-beam optical dipole trap for Bose-Einstein condensation in microgravity

J. S. Haase, A. Fieguth, I. Bröckel, J. Hamann, J. Kruse, Carsten Klempt

Published Sep 29, 2025• 1 citation
arXivHTMLPDF

Abstract

Abstract We present a novel concept for a compact and robust crossed-beam optical dipole trap (cODT) based on a single lens, designed for the efficient generation of Bose-Einstein condensates (BECs) under dynamic conditions. The system employs two independent two-dimensional acousto-optical deflectors (AODs) in combination with a single high-numerical-aperture lens to provide full three-dimensional control over the trap geometry, minimizing potential misalignments and ensuring long-term operational stability. By leveraging time-averaged potentials, rapid and efficient evaporative cooling sequences toward BECs are enabled. The functionality of the cODT under microgravity conditions has been successfully demonstrated in the Einstein-Elevator in Hannover, Germany, where the beam intersection was shown to remain stable throughout the microgravity phase of the flight. In addition, the system has been implemented in the sensor head of the INTENTAS project to verify BEC generation. Additional realization of one-, two-, and three-dimensional arrays of condensates through dynamic trap shaping was achieved. This versatile approach allows for advanced quantum sensing applications in mobile and space-based environments based on all-optical BECs.

DOI
10.1140/epjqt/s40507-025-00421-7

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Abstract We present a novel concept for a compact and robust crossed-beam optical dipole trap (cODT) based on a single lens, designed for the efficient generation of Bose-Einstein condensates (BECs) under dynamic conditions. The system employs two independent two-dimensional acousto-optical deflectors (AODs) in combination with a single high-numerical-aperture lens to provide full three-dimensional control over the trap geometry, minimizing potential misalignments and ensuring long-term operational stability. By leveraging time-averaged potentials, rapid and efficient evaporative cooling sequences toward BECs are enabled. The functionality of the cODT under microgravity conditions has been successfully demonstrated in the Einstein-Elevator in Hannover, Germany, where the beam intersection was shown to remain stable throughout the microgravity phase of the flight. In addition, the system has been implemented in the sensor head of the INTENTAS project to verify BEC generation. Additional realization of one-, two-, and three-dimensional arrays of condensates through dynamic trap shaping was achieved. This versatile approach allows for advanced quantum sensing applications in mobile and space-based environments based on all-optical BECs.
Atom interferometers serve as high-precision sensors for quantities like acceleration, rotation, or magnetic fields. The sensitivity of atom interferometers is greatly enhanced by long interrogation times, as they are available in spaceborne applications. Second-long interrogation times favor the employment of atomic Bose-Einstein condensates (BECs) with their well-controlled spatial mode and their slow expansion rates. The sensitivity can be increased even further by employing squeezed atomic ensembles that enable measurements beyond the standards quantum limit. The INTENTAS project develops a source of entangled atoms that can be tested under microgravity conditions. Microgravity is provided by Hannover’s Einstein Elevator, which offers up to 4s of free fall. A key feature is the fast all-optical BEC generation which is performed in a crossed-beam optical dipole trap. In this poster, the status of the project will be presented, which includes operation in microgravity.
In microgravity environments, the evaporative cooling time required to prepare a Bose–Einstein condensate (BEC) in a crossed optical dipole trap is significantly longer than that observed in ground-based experiments using identical apparatus. This slowdown arises from the simultaneous decrease in both trap depth and trap frequency of the trap under microgravity conditions, which results in an extended duration necessary for achieving thermal equilibrium. To address this issue, we propose incorporating a saddle-shaped blue-detuning light field to create a composite optical trap. This approach effectively mitigates the decline of trap frequency while simultaneously reducing the depth of the composite trap, thereby facilitating rapid evaporative cooling. Numerical simulations indicate that this strategy allows the atomic phase-space density to approach the Bose–Einstein condensation threshold in less than 4 s under microgravity. Such performance represents an order-of-magnitude improvement in the evaporative cooling rate of optical dipole traps operating in microgravity. This advancement not only facilitates rapid BEC preparation under orbital conditions but also opens new avenues for analogous experiments in ground-based drop towers.
Abstract We present a simple recipe for the rapid formation of spinor Bose–Einstein condensates of 87 Rb atoms in a three-beam crossed optical dipole trap (ODT). Two laser Gaussian beams were crossed at a small angle to form an effective single potential in the intersection region with an independent and widely tunable trap volume and aspect ratio, for both efficient atom loading and number density enhancement. A third Gaussian beam was then introduced to create a potential dimple at the trap center to further increase the atom number density and collision rate, which enabled rapid evaporation to a Bose condensation. In this configuration, 2 × 10 6 atoms were loaded into the three-beam crossed trap from a vapor cell magneto-optical trap, and condensates of 3 × 10 4 – 4.5 × 10 4 atoms were created after 3–5 s of rapid evaporation. The demonstration of efficient and robust BEC production in this ODT suggests the applicability of this trapping geometry to lasers with any far off-resonant wavelengths. To prepare the spinor condensates with a wide tunability of spin population, control of the magnetic fields was first employed during evaporation to polarize the spin states, and the tuning mechanism is discussed. Spinor condensates with an arbitrary spin population were then created via multiple microwave pulses driving the ground hyperfine transitions.
Quantum technologies extensively use laser light for state preparation, manipulation, and readout. For field applications, these systems must be robust and compact, driving the need for miniaturized and highly stable optical setups and system integration. In this work, we present a micro-integrated crossed-beam optical dipole trap setup, the µXODT, designed for trapping and cooling 87 Rb. This fiber-coupled setup operates at 1064 nm wavelength with up to 2.5 W optical power and realizes a free-space crossed beam geometry. The µXODT precisely overlaps two focused beams ( w 0 ≈ 33 µm) at their waists in a 45° crossing angle, achieving a position difference of ≤3.4 µm and a 0.998 power ratio between both beams with long-term stability. We describe the design and assembly process in detail, along with optical and thermal tests with temperatures of up to 65 °C. The system’s volume of 25 ml represents a reduction of more than two orders of magnitude compared to typically used macroscopic setups while demonstrating exceptional mechanical robustness and thermal stability. The µXODT is integrated with an 87 Rb 3D MOT setup, trapping 3 × 10 5 atoms from a laser-cooled atomic cloud, and has shown no signs of degradation after two years of operation.
We propose a novel, to our knowledge, scheme to form a controllable double-well optical dipole trap for cold atoms (or molecules) by using an optical system composed of a binary pi-phase plate and a lens illuminated by a plane light wave. We calculate the intensity distribution of the double-well trap and derive the analytical relationships between the characteristic parameters of the double-well trap (including geometric parameters, intensity distributions, and intensity gradients and their curvatures) and the relative aperture beta of the lens system. We also extend our controllable double-well trap to its trap array by using a binary pi-phase grating combined with an array of spherical microlenses. Our study shows that, if the pi-phase plate (or the pi-phase grating) is moved along the x direction, our double-well trap (or array of double-well ones) can continuously evolve to a single-well one (or an array of single-well ones) and vice versa, which can be used to study cold collisions between two atomic (or molecular) samples and atom interference with Bose-Einstein condensation (BEC) in a double-well potential; to prepare quantum entanglement between two macroscopic atomic assembles, even to realize an all-optical double-well atomic (molecular) BEC (including an array of all-optical double-well BECs) by using optical-potential evaporative cooling; to form a novel optical lattice with a larger lattice constant; and so on.
We report the realization of Bose-Einstein condensation (BEC) of metastable helium atoms using an in-vacuum coil magnetic trap and a crossed-beam optical dipole trap. A quadrupole-Ioffe configuration magnetic trap made from in-vacuum hollow copper tubes provides fast switching times while generating traps with a 10-G bias, without compromising optical access. The bias enables in-trap one-dimensional Doppler cooling to be used, which is the only cooling stage between the magneto-optic trap and the optical dipole trap. This allows direct transfer to the dipole trap without the need for any additional evaporative cooling in the magnetic trap. The entire experimental sequence takes 3.3 s, with essentially pure BECs observed with $\ensuremath{\sim}{10}^{6}$ atoms after evaporative cooling in the dipole trap.