Current-Biased SET-HBT Cryogenic Preamplifier for High-Fidelity Single-Shot Spin Readout

A technique known as “enhanced latching readout” (ELR) produced single-shot readout fidelities of singlet and triplet states as high as 99.86% [Harvey-Collard et al., arXiv:1703.02651 (2017)]. In this case, the readout fidelity was limited by the circuit response time, which was ~100 μs. We present single-shot ELR with an improved circuit response time of microseconds. This is close to a factor of 100 times faster than the previous result and should enable higher fidelity readout. The faster response time is accomplished with a heterojunction-bipolar-transistor (HBT) as a cryogenic preamplifier, which is located at the mixing-chamber stage of a dilution refrigerator. A single-electron-transistor (SET), used to detect the charge state of the qubit, is connected directly to the base junction of the HBT. The SET-HBT current-biased configuration is very low power (e.g., ~0.1 to 1 μW) and combines high gain with potentially lower noise and electron temperature than other AC-coupled configurations.