The continual success of superconducting photon-detection technologies in quantum photonics asserts cryogenic-compatible systems as a cornerstone of full quantum photonic integration. Here, we present a way to reversibly fine-tune the optical properties of individual waveguide structures through local changes to their geometry using solidified xenon. Essentially, we remove the need for additional on-chip calibration elements, effectively zeroing the power consumption tied to reconfigurable elements, with virtually no detriment to photonic device performance. We enable passive circuit tuning in pressure-controlled environments, locally manipulating the cladding thickness over portions of optical waveguides. We realize this in a cryogenic environment, through controlled deposition of xenon gas and precise tuning of its thickness using sublimation, triggered by on-chip resistive heaters. $pi$ phase shifts occur over a calculated length of just $L_{pi}$ = 12.3$pm$0.3 $mu m$. This work paves the way towards the integration of compact, reconfigurable photonic circuits alongside superconducting detectors, devices, or otherwise.
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