Cryogenic packaging of nanophotonic devices with a low coupling loss <1 dB

Robust, low-loss photonic packaging of on-chip nanophotonic circuits is a key enabling technology for the deployment of integrated photon- ics in a variety of classical and quantum technologies including optical communications and quantum communications, sensing, and trans- duction. 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 on diamond with a record-low coupling loss <1 dB per facet at near-infrared wave- lengths ($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 devi- ces and optical fibers, paving the way for scalable integration of photonic technologies at both room and cryogenic temperatures.