Single-shot qubit state readout using JTWPA idler mode: experiments and applications

Josephson Travelling Wave Parametric Amplifiers (JTWPAs) are essential components in superconducting qubit readout chains, providing broadband gain and quantum-limited noise performance. When a weak signal co-propagates with a strong pump through the nonlinear medium formed by Josephson junctions, wave mixing transfers energy from the pump to the signal and generates an idler tone. The idler frequency is fixed by energy and momentum conservation, and because it is intrinsically correlated with the signal, it carries equivalent information about the qubit state.

We demonstrate high-fidelity dispersive readout of superconducting qubits using the idler tone from a JTWPA, implemented with commercially available arbitrary waveform generators. Our setup allows simultaneous measurement of the signal and idler outputs, enabling direct investigation of their correlations. By controlling the relative phase between pump and signal, we observe signatures of two-mode phase-sensitive amplification. We discuss potential applications of this dual-tone readout scheme, including intrinsic reverse isolation and new pathways toward scalable quantum computing architectures.