A high-flux source system for matter-wave interferometry exploiting tunable interactions

TL;DR

This work addresses the need for high atomic flux and ultra-low expansion in precision atom interferometry by delivering a fast, all-optical 39K source with dynamically tunable interactions using magnetic Feshbach resonances. The authors demonstrate six-line evaporative cooling ramps that sustain a nearly constant flux (~3×10^5 atoms/s) and yield large Bose-Einstein condensates, while tuning the scattering length to near zero minimizes expansion energy to the nanokelvin scale. They quantify the anticipated interferometer performance, predicting SQL-limited instabilities on the order of 3–5×10^-10 m/s for practical cycle times and highlighting potential improvements via delta-kick collimation, with competitiveness to chip-based sources. The approach promises high data-rate inertial sensing and broad applicability to other species and fundamental tests, including long-baseline and microgravity experiments.

Abstract

Atom interferometers allow determining inertial effects to high accuracy. Quantum-projection noise as well as systematic effects impose demands on large atomic flux as well as ultra-low expansion rates. Here we report on a high-flux source of ultra-cold atoms with free expansion rates near the Heisenberg limit directly upon release from the trap. Our results are achieved in a time-averaged optical dipole trap and enabled through dynamic tuning of the atomic scattering length across two orders of magnitude interaction strength via magnetic Feshbach resonances. We demonstrate BECs with more than $6\times 10^{4}$ particles after evaporative cooling for $170$ ms and their subsequent release with a minimal expansion energy of $4.5$ nK in one direction. Based on our results we estimate the performance of an atom interferometer and compare our source system to a high performance chip-trap, as readily available for ultra-precise measurements in micro-gravity environments.

Figures (5)

• Figure 1: Schematic representation of the crossed ODT setup. A single-frequency laser source [Coherent Mephisto MOPA] is split into two independent beams, allowing for up to 16W per path. Beam waist were determined with a beamcam [DataRay TaperCamD-LCM], while the crossing angle of the beams within the chamber was measured with vertical imaging. Time-averaged potentials are implemented with AODs [AA Opto-Electronic DTSXY-400-1064], allowing to independently modulate each beam's center position in the horizontal and vertical directions with a maximum modulation amplitude of 1.5mm [resp. 1.8mm for the secondary beam]. In this work, only the horizontal axes of the AODs is modulated using a software defined radio [Ettus USRP X310] to provide the waveform, while the vertical axes are driven with a constant frequency. For intensity stabilization, less than 0.1% of the optical power is detected by an amplified photodetector [Femto OE-200] and used to control the diffraction efficiency of the AOD via a homebuild PID-controller together with a voltage controlled attenuate [MiniCircuits ZX73-2500-S+].
• Figure 2:
• Figure 3: Final particle numbers in the condensate, against total evaporation time. Our previously achieved results with constant scattering length in a 1960nm trap are depicted with black pentagons, while our current results with variable scattering length in the 1064nm trap are highlighted, following the color and shape coding of Fig. \ref{['fig:Evaporation']}. The error bars are given by the standard deviation of 100 measurements of the particle number for each point. For comparison, the performance of other fast BEC sources is depicted with grey upside-down triangles.
• Figure 4: Expansion energy of the BEC at different scattering lengths in horizontal (a) and vertical (b) direction. For each data point we perform a TOF series, determining the ensembles size in between 10ms and 26ms of free fall with 1ms spacing and at least four measurements per point. Contrary to the measurements performed in section \ref{['sec:Evaporativ_cooling']} we do not measure below 10ms to avoid the resolution limitation of our detection system. The insets show the TOF-series for the data taken at 308a_0, which are used together with the measurements at 203a_0 to determine the trap frequencies, by fitting a scaling approach, shown as solid lines, with the error band corresponding to the stated trap frequency error. The error bars of the energy measurements originate as one-sigma deviation from the fit error of the expansion velocity and from the magnetic field uncertainty for the scattering length. Error bands of the simulations are obtained via a Monte-Carlo method within the trap frequency interval.
• Figure 5: Calculated instability of a Mach-Zehnder atom interferometer at the standard quantum limit using different source configurations with our evaporation sequence. We show the expected instability for different pulse separation times for an immediate release from the trap (solid lines) and compare them to an interferometer after performing an additional matter-wave collimation to 50pK (dashed lines). The shaded areas show the respective performance of a chip-based source system with values taken from Ref. Rudolph15NJP for the case of an immediate release from the trap and from Ref. Deppner21PRL for the DKC case.