Low-loss thermomechanically stable packaging for cryogenic quantum photonic network devices

C55-04b-270703The photonic approach to quantum information processing and networking is promising – it is expected that the quantum internet will be substantially built on optical fiber networks, where qubits can be exchanged over long distances via photons with wavelengths in the telecom range. The field of quantum photonics is continually developing and devices are being fabricated on scalable integrated photonic platforms, including silicon-on-insulator and silicon nitride. However, the heterogeneous integration of quantum photonic devices with optical fibers presents a challenge: The inherent insertion loss or limited coupling efficiency between chip and fiber limits the exchange of quantum information. While such insertion losses are typically overcome in classical photonics by simply increasing the power of the light source, this is not an option for quantum devices that operate at the level of single photons. The fact that quantum light sources and photon detectors may operate at cryogenic temperatures further complicates the fiber packaging problem. Low-loss coupling and packaging structures optimized for room temperature operation will experience deleterious reductions in coupling coupling efficiency owing to differential thermal contraction and shifts in temperature dependent refractive indices of the package and photonic circuit materials. As a result, there is a need for new fiber-chip photonic packaging solutions that can achieve low insertion loss over a wide temperature range. At memQ, we are developing quantum internet technologies based on rare earth ion (REI) optical transitions that operate in the telecom C-band. These sources of single photons and potential quantum memories are integrated directly with silicon photonic circuits, enabling the scalable fabrication of devices for managing quantum entanglement at the network-level, e.g., quantum repeaters. We have identified high insertion loss as a critical challenge to be overcome for the development of the quantum internet, and we have teamed up with Freedom Photonics, a leading laser manufacturer in California, to tackle this challenge. In Phase I of this SBIR, we will address two engineering challenges for low-loss cryogenic photonic packaging: (1) thermomechanical stability of the fiber-chip package and (2) Optical mode mismatch between photonic waveguides and fibers. We will address these challenges using the novel packaging technique of photonic wirebonding (PWB) in which flexible waveguides can be 3D-printed between chip and fiber. Freedom Photonics operates the first PWB tool in the US. Using PWB and thermally-engineered packaging, we seek to demonstrate insertion losses less than 3 dB at 4 K, at parity with those achieved at room temperature. The focus of Phase II will be to push for smaller insertion losses, to integrate our Er-doped photonic cavity devices with this new packaging, and to network devices together in two separate cryostats. This improved packaging will enable more efficient single photon sources. Based on our discussions with over 100 stakeholders in the community, these single photon sources would be valuable to researchers and engineers working on quantum networking and computing. Further, improved optical packaging will have considerable impact on the photonics industry, which is expected to approach $1 trillion in value by 2028.