Boosting Axion Searches with Quantum Sensing

The axion, a pseudoscalar particle originally introduced by Peccei, Quinn, Weinberg, and Wilczek to solve the "strong CP problem", is a well motivated dark-matter candidate with a mass lying in a broad range from peV to few meV. The last decade witnessed an increasing interest in axions and axion-like particles with many theoretical works published and many new experimental proposals. A major challenge for cosmological-axion discovery is the detection of the faint signal expected in detectors with power as low as a fraction of yoctowatt corresponding to a single microwave photon per minute. Early experiments used GaAs field-effect-transistors as well as pioneering technology such as Rydberg-atom single-photon detection. In the attempt of reducing the noise temperature superconductive devices were soon introduced. Microstrip SQUID Amplifiers allowed SQUIDs to operate at frequencies of a few GHz with a noise temperature reaching the standard quantum limit. Josephson Parametric Amplifiers have been recently employed extending the search to higher frequency while boradband Traveling Wave Parametric Amplifiers are now under study. The ultimate sensitivity, beyond the quantum limit, is however expected from single microwave-photon detectors. Solutions for quantum sensing include quantum non-demolition measurements and switching detectors based on superconducting qubits and hot-electron detectors. A further improvement in signal sensitivity and noise reduction is expected exploiting arrays of superconducting qubits as proposed by the SUPERGALAX project.