Quantum non-demolition measurement of a microwave cavity via a remote Josephson converter

Superconducting circuits have recently been used to accelerate the search for dark matter axions. However, protecting these circuits from the strong magnetic fields required in such experiments makes the implementation of quantum-sensing techniques challenging. Here, we demonstrate a remote quantum-enhanced sensing method in a system that emulates an axion-search apparatus. A Josephson parametric converter, positioned 50 cm away from a microwave cavity and coupled by a transmission line, enables a quantum-non-demolition measurement of a single cavity quadrature. Despite the presence of parasitic transmission line modes, the scheme achieves 17 dB of cavity-signal amplification with minimal added noise. This approach extends the detector’s sensitivity bandwidth, thereby improving the quantum-limited search rate.