Most microwave readout architectures in quantum computing or sensing rely on a semiconductor amplifier at 4 K, typically a high-electron mobility transistor (HEMT). Despite its remarkablenoise performance, a conventional HEMT dissipates several milliwatts of power, posing a practical challenge to scale up the number of qubits or sensors addressed in these architectures. As an alternative, we present an amplification chain consisting of a kinetic-inductance traveling-wave parametric amplifier (KI-TWPA) placed at 4 K, followed by a HEMT placed at 70 K, and demonstrate a chain-added noise TΣ=6.3±0.5 K between 3.5 and 5.5 GHz. While, in principle, any parametric amplifier can be quantum limited even at 4 K, in practice we find the KI-TWPA’s performance limited by the temperature of its inputs, and by an excess of noise Tex=1.9 K. The dissipation of the KI-TWPA’s rf pump constitutes the main power load at 4 K and is about one percent that of a HEMT. These combined noise and power dissipation values pave the way for the KI-TWPA’s use as a replacement for semiconductor amplifiers.
We present a superconducting microresonator thermometer based on two-level systems (TLS) that is drop-in compatible with cryogenic microwave systems. The operational temperature rangeis 50-1000~mK (which may be extended to 5~mK), and the sensitivity (50-75~μK/Hz−−−√) is relatively uniform across this range. The miniature footprint that conveniently attaches to the feedline of a cryogenic microwave device facilitates the measurement of on-chip device temperature and requires no additional thermometry wiring or readout electronics. We demonstrate the practical use of these TLS thermometers to investigate static and transient chip heating in a kinetic inductance traveling-wave parametric amplifier operated with a strong pump tone. TLS thermometry may find broad application in cryogenic microwave devices such as superconducting qubits and detectors.
We present a theoretical model and experimental characterization of a microwave kinetic inductance traveling-wave amplifier (KIT), whose noise performance, measured by a shot noisethermometer, approaches the quantum limit. Biased with a dc current, the KIT operates in a three-wave mixing fashion, thereby reducing by several orders of magnitude the power of the microwave pump tone compared to conventional four-wave mixing KIT devices. It is built in an artificial transmission line intrinsically matched to 50 Ohms, whose dispersion allows for a controlled amplification bandwidth. We experimentally measure 17.6+1.1−1.4 dB of gain across a 2 GHz bandwidth, with an input 1 dB compression power of -63 dBm within that bandwidth, in qualitative agreement with theory. Using the KIT as the first amplifier in an amplification chain, we measure a system-added noise of 0.61±0.08 K between 3.5 and 5.5 GHz, about one eighth the noise obtained when using only a representative classical amplifier. The KIT contribution to this added noise is estimated to be 0.2±0.1 K, consistent with the quantum limit on amplifier added noise. This device is therefore suitable to read large arrays of microwave kinetic inductance detectors or thousands of superconducting qubits.
We demonstrate enhanced relaxation and dephasing times of transmon qubits, up to ~ 60 mu s by fabricating the interdigitated shunting capacitors using titanium nitride (TiN). Comparedto lift-off aluminum deposited simultaneously with the Josephson junction, this represents as much as a six-fold improvement and provides evidence that previous planar transmon coherence times are limited by surface losses from two-level system (TLS) defects residing at or near interfaces. Concurrently, we observe an anomalous temperature dependent frequency shift of TiN resonators which is inconsistent with the predicted TLS model.