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 remarkable noise 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 chainadded 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 report on the use of a kinetic-inductance traveling-wave parametric amplifier (KITWPA) as the first amplifier in the readout chain of a microwave superconducting quantum interference device (SQUID) multiplexer (µmux). This µmux is designed to multiplex signals from arrays of low temperature detectors such as superconducting transition-edge sensor microcalorimeters. When modulated with a periodic flux-ramp to linearize the SQUID response, the flux noise improves, on average, from 1.6 µΦ 0 / √ Hz with the KITWPA off, to 0.77 µΦ 0 / √ Hz with the KITWPA on. When statically biasing the µmux to the maximally flux-sensitive point, the flux noise drops from 0.45 µΦ 0 / √ Hz to 0.2 µΦ 0 / √ Hz. We validate this new readout scheme by coupling a transition-edge sensor microcalorimeter to the µmux and detecting background radiation. The combination of µmux and KITWPA provides a variety of new capabilities including improved detector sensitivity and more efficient bandwidth utilization.