Microwave photon detection at parametric criticality

The detection of microwave fields at single-photon power levels is a much sought-after technology, with practical applications in nanoelectronics and quantum information science. Here we demonstrate a simple yet powerful criticality-enhanced method of microwave photon detection by operating a magnetic-field tunable Kerr Josephson parametric amplifier at the border of a first-order phase transition and close to the critical point. We obtain a 73% efficiency and a dark-count rate of 167 kHz, corresponding to a responsivity of $1.3\times {10}^{17}{\mathrm{W}}^{-1}$ and noise-equivalent power of 3.28 zW/$\sqrt{\mathrm{H}\mathrm{z}}$. We verify the single-photon operation by extracting the Poissonian statistics of a coherent probe signal.