Generation of strong ultralow-phase-noise microwave fields with tunable ellipticity for ultracold polar molecules

Microwave (MW) fields with strong field strength, ultralow phase-noise, and tunable polarization are crucial for stabilizing and manipulating ultracold polar molecules, which have emerged as a promising platform for quantum science. In this article, we present the design, characterization, and performance of a robust MW setup tailored for precise control of molecular states. This setup achieves a high electric field intensity of 6.9 kV/m in the near-field from a dual-feed waveguide antenna, enabling a Rabi frequency as high as 71 MHz for the rotational transition of sodium-potassium molecules. In addition, the low noise signal source and controlled electronics provide ultralow phase-noise and dynamically tunable polarization. Narrowband filters within the MW circuitry further reduce phase noise by more than 20 dB at 20 MHz offset frequency, ensuring prolonged one-body molecular lifetimes up to 10 s. We also show practical methods to measure the MW field strength and polarization using a simple homemade dipole probe and to characterize phase-noise down to -170 dBc/Hz using a commercial spectrum analyzer and a notch filter. Those capabilities allowed us to evaporatively cool our molecular sample to deep quantum degeneracy. Furthermore, the polarization tunability enabled the observation of field-linked resonances and facilitated the creation of field-linked tetramers. These techniques advance the study of ultracold polar molecules and broaden the potential applications of MW tools in other platforms of quantum science.