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We examine a silicon-germanium heterojunction bipolar transistor (HBT) for cryogenic preamplification of a single electron transistor (SET). The SET current modulates the base current of the HBT directly. The HBT-SET circuit is immersed in liquid helium, and its frequency response from low frequency to several MHz is measured. The current gain and the noise spectrum with the HBT result in a signal-to-noise-ratio (SNR) that is a factor of 10-100 larger than without the HBT at lower frequencies. The transition frequency defined by SNR=1 has been extended by as much as a factor of 10 compared to without the HBT amplification. The power dissipated by the HBT cryogenic pre-amplifier is approximately 5 nW to 5 μW for the investigated range of operation. The circuit is also operated in a single electron charge read-out configuration in the time-domain as a proof-of-principle demonstration of the amplification approach for single spin read-out. [ABSTRACT FROM AUTHOR]

High-fidelity single-shot readout of spin qubits requires distinguishing states much faster than the T1 time of the spin state. One approach to improving readout fidelity and bandwidth (BW) is cryogenic amplification, where the signal from the qubit is amplified before noise sources are introduced and room-temperature amplifiers can operate at lower gain and higher BW. We compare the performance of two cryogenic amplification circuits: a current-biased heterojunction bipolar transistor circuit (CB-HBT), and an AC-coupled HBT circuit (AC-HBT). Both circuits are mounted on the mixing-chamber stage of a dilution refrigerator and are connected to silicon metal oxide semiconductor (Si-MOS) quantum dot devices on a printed circuit board (PCB). The power dissipated by the CB-HBT ranges from 0.1 to 1 μW whereas the power of the AC-HBT ranges from 1 to 20 μW. Referred to the input, the noise spectral density is low for both circuits, in the 15 to 30 fA/ Hz range. The charge sensitivity for the CB-HBT and AC-HBT is 330 μe/ Hz and 400 μe/ Hz , respectively. For the single-shot readout performed, less than 10 μs is required for both circuits to achieve bit error rates below 10−3, which is a putative threshold for quantum error correction. [ABSTRACT FROM AUTHOR]