Hybrid atom traps for high-sensitivity quantum sensors in compact volumes
Atomic quantum sensors based on matter-wave interferometry have become a versatile tool for precisely measuring accelerations and rotations but are also utilized in the field of fundamental physics, e.g., testing the foundations of gravity. In contrast to its classical counterpart, a quantum sensor relies on the quantum mechanical superposition of atomic matter waves which are brought to interference for readout of the observable of interest . Since the sensitivity scales with the time the atoms are in superposition, these sensors benefit from extended interrogation times enabled by low expansion rates of ultra-cold atomic ensembles with effective temperatures of a few nK. To this end, matter-wave interferometers are operated with Bose-Einstein Condensates (BECs) in extended free fall in microgravity environments [2,3] or levitating in waveguides for ground-based operation . Here, deployment in microgravity or in field brings stringent demands on the size, weight and power budgets of quantum sensors.
As a tool for efficient preparation and precise control of BECs for quantum sensing, either one of two types of matter-wave sources are typically used that rely on fundamentally different atom traps featuring complementary advantages:
- Magnetic traps based on atom chips  micro-fabricated chip with complex conductor structure placed in ultra-high vacuum (UHV) for magnetic trapping close to the chip’s surface, allows for generation of high magnetic field gradients with low power consumptions und thus efficient production of BECs of high atom numbers with rapid production times.
- Optical dipole traps  enable spin-independent trapping of atoms and mixture studies of dual-species gases, allow for the realization of optical waveguides to suspend atoms in gravity and feature more symmetric potentials compared to atom-chip generated magnetic traps.
To keep the compactness of an atom chip but additionally allow for transfer in purely optical traps or hybrid magneto-optical traps, we are working on optical concepts to realize on-chip single beam magneto-optical traps or crossed beam optical dipole traps (ODT) . The prospective assemblies require very high stability and accuracy and thus call for an integration of the optical elements on or near the atom chip within the vacuum system. Since the 1/e lifetime of the atomic ensemble is dominated by collisions with residual atoms in the background, an UHV environment and thus very low outgassing of integration techniques and components are mandatory [8, 9]. Furthermore, the prospective setups have to withstand the mechanical and thermal loads relevant for ground-based and microgravity operation.
A prototype design of a crossed beam ODT next to an atom chip assembly is depicted in Fig. 1. After a polarization-maintaining, single-mode fiber collimator, 1064 nm light is guided into a beam conditioning system, where the diameter is increased from 600 µm to 2 mm using a telescope. Subsequently, the beam is separated into two orthogonal polarized parts with equal intensity. In both beams a spherical focusing lens with 35 mm focal length is used to realize the crossed beam close to the atom chip surface. With the size and geometry shown, the setup can be mounted to the side of an atom chip assembly and thus inside a compact UHV system. Within the chip center, the beams have a waist radius of 20 µm and form a crossed beam ODT with an incidence angle of 45°. In order to realize such a design, established integration technologies at FBH will be used.
Conceptual studies on the preparation and manipulation of a BEC in a hybrid trap geometry consisting of an atom-chip trap and a single beam ODT in a cage-system based setup have been carried out in a project within the QUANTUS („Quantengase unter Schwerelosigkeit“) collaboration. Fig. 2 shows absorption images of a BEC initially prepared inside the magnetic atom-chip trap and after lossless transfer into a single-beam ODT for all-optical trapping. Right after switching off the magnetic fields, the BEC starts propagating inside an optical waveguide-like configuration alongside the propagation of the dipole beam. Lifetime measurements of the optically trapped atoms reveal a 1/e lifetime of 10 s comparable to the magnetically trapped atoms for holding times larger than 1 s. With no evidence for atom losses imposed by the trapping laser, these results underline the importance of low outgassing components and assembly techniques for in-vacuum realizations as residual atoms in the background gas being the limiting factor.
Funding information and acknowledgment
The presented work is supported by the German Space Agency (DLR) with funds provided by the Federal Ministry of Economic Affairs and Energy (BMWi) due to an enactment of the German Bundestag under Grant No. 50WM1953 (QUANTUS-V-Fallturm), 50WM1949 (KACTUS-II), 50RK1978 (QCHIP).
The QUANTUS project is a collaboration of LU Hannover, HU Berlin, U Hamburg, U Ulm, TU Darmstadt, and ZARM at U Bremen with the aim to establish ultra-cold atoms as high-precision quantum sensors in microgravity. The KACTUS and QCHIP projects are a collaboration of LU Hannover, FBH, and SIEGERT WAFER GmbH with the goal to achieve a further integration and miniaturization of atomic quantum sensors for applications in the field and microgravity.
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