  Robust, low-loss photonic packaging of on-chip nanophotonic circuits is a key
enabling technology for the deployment of integrated photonics in a variety of
classical and quantum technologies including optical communications and quantum
communications, sensing, and transduction. To date, no process has been
established that enables permanent, broadband, and cryogenically-compatible
coupling with sub-dB losses from optical fibers to nanophotonic circuits. Here
we report a technique for reproducibly generating a permanently packaged
interface between a tapered optical fiber and nanophotonic devices with a
record-low coupling loss < 1 dB per facet at near-infrared wavelengths (~730
nm) that remains stable from 300 K to 30 mK. We further demonstrate the
compatibility of this technique with etched lithium niobate on insulator
waveguides. The technique lifts performance limitations imposed by scattering
as light transfers between photonic devices and optical fibers, paving the way
for scalable integration of photonic technologies at both room and cryogenic
temperatures.
