  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.
