Control of the polarization of microwave fields is a key experimental capability for a number of atomic physics platforms. However, producing high-fidelity microwaves requires a well-controlled microwave environment, where reflections that distort the polarization must be avoided or well characterized, a constraint that often conflicts with other experimental design considerations. Here, we demonstrate a microwave control system capable of producing high-fidelity microwave polarizations in a Rydberg-ensemble experiment. We use three sets of in-vacuum dc electrodes, repurposed as microwave antennas, to produce imperfect and initially unknown polarizations. Each source is driven with independent phase and amplitude control to generate the desired microwave fields. We probe the fields produced at the position of the atoms using Rydberg-electromagnetically induced transparency spectroscopy of the microwave-induced avoided crossings. We produce ${\sigma }_{-}$, $\pi$, and ${\sigma }_{+}$ polarized microwaves with $>99\%$ fidelity and generate their combinations. We extend our purification techniques to frequencies away from Rydberg resonances by utilizing an auxiliary microwave field, generating two-photon microwave resonances. The techniques developed here will facilitate the engineering of dipolar interactions in atomic and molecular physics experiments.

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• Electromagnetically induced transparency
• Optoelectronics
• Polarization of light
• Quantum sensing
• Atomic ensemble
• Rydberg atoms & molecules
• Microwave techniques
• RF, microwave, & terahertz sources