Quantum bit controller and observer circuits in SOS-CMOS technology for gigahertz low-temperature operation

Quantum bit (qubit) control and readout requires controller-qubit-observer systems for rapid control signal generation and injection to the qubit gates, and observation of their final state projections. Conventionally, for solid-state qubits, this is achieved by generating the control signal at 300 K and transmitting it along very long coaxial cables that span from 300 K to sub-K (typically ≤ 500 mK), then reading out the response from charge proximity sensors such as single-electron transistors along similar lengths of cable. Our approach is to fabricate the classical controller and observer circuits using a commercial foundry processed silicon-on-sapphire (SOS) RFCMOS technology for operation at low temperatures (either at 4.2 K, 1 K, or sub-K). We have demonstrated SOS-CMOS NFET and PFET device operation at 4.2 K, and sub-K that showed deviations from their 300 K characteristics, but with further experiments these were shown to have minimal effects on control circuit function. Using these results, we have fabricated and demonstrated a low-power proof-of-concept SOS-CMOS controller circuit (monostable 100 ps voltage-pulse generator) that can operate at sub-K temperatures in a dilution refrigerator. We briefly discuss experimental and conceptual schemes with which we can develop qubit control systems for cryogenic and lower temperatures. These low temperature experiments also demonstrate that commercial SOS RF-CMOS technology can be feasible for other low temperature and low power applications.