The requirements and details of designing a measuring cell and low-back-action deeply-cooled amplifier for quantum measurements at 10 mK are discussed. This equipment is a part of a microwave single-photon counter based on a superconducting flux qubit. The high-electron mobility transistors (HEMTs) in the amplifier operate in unsaturated microcurrent regime and dissipate only 1 μW of dc power per transistor. Simulated amplifier gain is 15 dB at 450 MHz with a high-impedance (≈ 5 kΩ) signal source and standard 50-Ω output.
The cryogenic indium phosphide (InP) high-electron-mobility transistor (HEMT) low-noise amplifier (LNA) is used for the readout amplification of qubits at
4K where cooling capabilities are limited implying that the dc power of the active circuits is an essential design constraint. In this article, the RF and noise performance of the InP HEMT under ultralow-power (ULP) operation at
4K has been characterized. The small-signal and noise parameter model of the InP HEMT was extracted down to 1
μW. The tradeoff between noise performance and dc power consumption was analyzed in terms of the drain current and drain voltage. A 4–6 GHz hybrid cryogenic HEMT LNA designed for qubit readout and optimized for lowest noise below
1mW dc power consumption was fabricated. The measured performance of the LNA at
4K attained 23.1 dB average gain and 2.0 K average noise temperature at 200
μW dc power.
The connection of RSFQ circuits with commercial room temperature electronics and amplifying SFQ pulses with high data rates require cryogenic interface amplifiers. Such amplifiers should provide extremely high bandwidth, low noise and low power consumption at the same time. Various hybrid amplifiers based on commercial p-HEMT transistors in an embedded microwave design were designed and characterized. Towards the p-HEMT transistor characterization at cryogenic temperature the biasing settings according to an optimum between voltage gain and low power consumption were determined. Thus a total power consumption of 2 mW and a voltage gain of 12 dB per single stage were achieved. For a preamplifier and an amplifier concept a multiple number of these stages were implemented in a microstrip and a coplanar transmission line design with a special matched interconnect taper towards the RSFQ components. Measurements of the amplifiers, the combination of an amplifier with a RSFQ circuit at 4.2 K showed their good performances without any disturbance of the RSFQ circuit. Due to the losses between the stages a total gain of 25 dB with a four stage amplifier was achieved; pulse rates went up to 3 Gb/s. The total power consumption was in the range of 8 mW. Further increase of data rates will be achieved by improving the matching between RSFQ output stage and amplifier.
There is a great deal of interest in amplifiers with extremely high bandwidth, low noise and low power consumption which interface RSFQ circuits with commercial room temperature electronics or amplifying SFQ pulses. In this special application a hybrid amplifier should interface an RSFQ circuitry with a Josephson junction array quantizer chip (JA-Q) at 4.2 K. The input signal for the amplifier is generated by an RSFQ signal generator and the output signal fed-in to a Josephson array quantizer. A voltage gain of about 10 4 with a pattern frequency of 2 GHz for a good transfer of the SFQ pulses is required. Additionally, the JA-Q requires a high output current at low power consumption of the amplifier at the same time. Various hybrid amplifiers based on commercially available p-HEMT transistors in an embedded microwave design were designed and characterized. For the p-HEMT transistor characterization at cryogenic temperatures the biasing settings according to an optimum between voltage gain A V and low power consumption P V were determined. Thus a power consumption of P V = 2 mW and a voltage gain of about A V = 4 per stage were achieved. For a preamplifier and an amplifier concept a number of these stages were implemented in a microstrip and a coplanar transmission line design with a special matched interconnect taper towards the RSFQ components. Measurements of the amplifiers and the combination of the amplifiers and the JA-Q at 4.2 K showed their good performance without any disturbances of the quantized voltage steps.
High-fidelity single-shot readout of superconducting qubits is essential for fault-tolerant quantum computation. Without a quantum-limited amplifier such as a Josephson parametric amplifier, the cryogenic high-electron-mobility-transistor (HEMT) low-noise amplifier (LNA) at the 4 K stage is the dominant noise source in the readout chain. HEMT LNAs are conventionally characterized by Y-factor measurements of the noise temperature
TN, independently of the quantum measurement they ultimately serve. Here we present a qubit-in-the-loop benchmarking method. The method uses the two-dimensional IQ histogram of single-shot readout as a direct figure of merit. We apply it to three state-of-the-art cryogenic InP HEMT LNAs with 53\%, 60\%, and 70\% channel indium content, together with a commercial reference amplifier. The 60\% and 70\% devices have similar
TN in Y-factor measurements, both lower than the 53\% device. The 70\% device has the highest gain, but qubit readout SNR and assignment fidelity
Fa instead peak at 60\%. Using this method, we map
Fa against HEMT LNA dc power consumption for each device.
Fa does not simply saturate with increasing power. For several devices it declines above a device-dependent power well below typical operating points. For the 60\% device, which has the highest overall
Fa, about 1~mW is needed to maintain
Fa>85%. Across all four devices,
Fa>80% is reached with as little as about 0.3~mW, roughly an order of magnitude below typical operating points. These results establish a qubit-referenced benchmarking methodology and provide practical guidance for amplifier selection and bias in power-constrained multi-qubit readout systems.
A wideband cooled measuring amplifier, which solves a wide class of search problems in the area of cryoelectronics at frequencies of 10–100 MHz, is described. The amplifier differs from the existing prototypes in the reduced power consumption and influence on the source (suppressed back action effect), serviceability in a temperature interval of 0.1–300 K, and high regulated gain (up to 40 dB). The power consumed (dissipated) by the first ultralow-temperature stage, is ∼1 μW. The circuitry components are commercially available.