Development of fabrication process for controlling phononic thermal transport to improve qubit performance

Seeqc together with LBNL is pleased to submit this phase I SBIR proposal to develop a fabrication process for improving performance of quantum sensors. In recent years, interest in research and development of superconducting quantum devices such as qubits for quantum computers has been expanding rapidly. Devices for Quantum Information Science (QIS) share a lot in common with detectors for these HEP experiments. They operate on microfabricated superconducting devices at sub-Kelvin temperature. High-energy radiation source, such as cosmic ray, has been identified as a source of error in superconducting quantum device, here we are proposing a way to engineer the environment the qubit sits to decouple from the bulk reducing the effect of the high energy phonons generated from cosmic rays. We propose to reduce cross section of superconducting quantum devices to cosmic rays by microfabricating a Micro-ElectroMechanical System (MEMS) structure that significantly reduces the interaction volume and ability for external phonons to propagate from the environment to the superconducting quantum device. Our approach will solve the joint optimization problem of minimizing phonon transport while maintaining superconducting electrical signal conductivity, mechanical sturdiness of the device and its mounting, and resonance quality of the superconducting device. We propose to develop an isolation structure to reduce cross section to cosmic ray interaction while preserving low loss dielectric structure that is required to fabricate QIS (and HEP detector) devices. Spider web bolometer sensor design was used widely by mm-wave to sub-mm wave astrophysics community. It was designed to thermally isolate a thermistor located at center of the spider web using silicon nitride membranes as a web medium. Spider structure was chosen for its structural strength as well as its small cross-section to cosmic rays. Our approach is to adapt this well-established MEMs technique for QIS devices to reduce its cross-section to cosmic rays. However, spider web bolometers used for astrophysics experiments have thermistors (NTD-Ge sensors or Transition Edge Sensors) and superconducting traces fabricated on silicon nitride membrane. QIS devices require superconducting resonators to be fabricated on low loss dielectric medium such as high resistivity silicon. We will suspend an island of high resistivity silicon with the spider web membranes such that superconducting resonators will be fabricated on the low loss medium. Spider web structure will be fabricated out of Silicon-on-Insulator (SOI) wafer and low loss silicon nitride membrane. We will fabricate co-planar waveguide based superconducting resonator and Transition Edge Sensor on the isolated high resistivity silicon island. The resonator will be used to monitor quality factor of the system, while Transition Edge Sensor will be used to make detection of cosmic ray hit. By having both resonators and TES sensors on the same isolated island, it would be possible to make coincidence measurement to corelate cosmic ray event to quality factor of resonators. We will compare performance of the isolated system against unmitigated system to study its effectiveness. During Phase II of the program, we will then develop a process to fabricate high quality qubits in place of TES to demonstrate prolonged coherent time with this technique. The introduction of a high-quality qubit process will be directly applicable to the needs of quantum computing community and hence Seeqc and will greatly increase business opportunities to the industry in general.