High-fidelity, multiplexed, QND qubit measurements with a circulator-free readout chain, Part 1

High-fidelity QND qubit measurements are a cornerstone of quantum error correction schemes. In state-of-the-art superconducting quantum processors, this is achieved by combining dispersive readout with low-noise microwave measurement chains based on parametric amplifiers. However, to protect the qubits from unwanted backaction, typical systems require multiple stages of bulky magnetic circulators, a major roadblock toward scaling this approach to thousands or millions of qubits.

Here we present a solution to this problem. We experimentally demonstrate high-fidelity, multiplexed, QND qubit measurements with a circulator-free readout chain, achieving all the requirements for scalability. In the first part of this talk, we will focus on the key component of this readout chain, a multi-stage TWPA [1] with near-quantum-limited forward gain and over 35dB of built-in passive isolation.

[1] C. S. Kow and M. T. Bell, ArXiv 2505.04059 [2025]