# Reverse isolation and backaction of the SLUG microwave amplifier

## TL;DR

This paper demonstrates that a SLUG microwave amplifier can effectively isolate quantum circuits from noise and backaction, achieving better reverse isolation than commercial isolators and enabling high-fidelity qubit readout without additional cryogenic components.

## Contribution

The study shows that a SLUG amplifier provides superior reverse isolation and minimal backaction, enabling qubit measurement without cryogenic isolators or circulators.

## Key findings

- SLUG achieves reverse isolation better than commercial cryogenic isolators.
- Backaction is mainly due to thermal emission from dissipative elements.
- Qubit performance remains unaffected when measured with the SLUG in pulsed mode.

## Abstract

An ideal preamplifier for qubit measurement must not only provide high gain and near quantum-limited noise performance, but also isolate the delicate quantum circuit from noisy downstream measurement stages while producing negligible backaction. Here we use a Superconducting Low-inductance Undulatory Galvanometer (SLUG) microwave amplifier to read out a superconducting transmon qubit, and we characterize both reverse isolation and measurement backaction of the SLUG. For appropriate dc bias, the SLUG achieves reverse isolation that is better than that of a commercial cryogenic isolator. Moreover, SLUG backaction is dominated by thermal emission from dissipative elements in the device. When the SLUG is operated in pulsed mode, it is possible to characterize the transmon qubit using a measurement chain that is free from cryogenic isolators or circulators with no measurable degradation of qubit performance.