demonstration of mode entanglement and swapping for enhanced weak signal detection

Quantum noise is the main barrier in the detection of a weak signal at an unknown frequency. Here we demonstrate a prototype detector that accelerates the detection of a weak microwave tone by 5.6 times compared to a quantum-limited detector. The detector comprises two microstrip modes of a Josephson parametric converter (JPC), where one serves as the science mode and the other is used for readout. Dynamically coupling the two modes via simultaneous entanglement and state-swapping interactions induced by two-mode squeezing (G) and frequency-conversion (C) drives with matched interaction rates results in a quantum non-demolition interaction. This backaction-evading technique allows us to extract information from one of the quadratures of the science mode more rapidly, yielding an increase in the detector bandwidth. To mimic a real axion search, we inject a synthetic axion signal comprising 1% of the power expected from vacuum fluctuations at a protocol-blinded frequency into the science mode, and we demonstrate an improvement in the signal-to-noise ratio of 2.36 times using the GC-enhanced method compared to an equivalent quantum-limited search. This improvement corresponds with a 5.6-fold speedup in the spectral scan rate of an axion search.