Fig. 4: The quantum radiometry.

a Experimental sequence for the quantum-enhanced radiometry that senses the excitation population p in the receiver cavity (Fig. 1c) via QEC. b The measured P_g as a function of the initial phase φ₀. The purple dots and green triangles correspond to experiments with p = 0 and 0.037, respectively. The experiment is performed with the probe state \(|{\psi }_{1,3}\rangle =(|1\rangle +|3\rangle )/\sqrt{2}\) and τ_int = 0.1T₁ for the QEC repetition number M = 1, 5, 10 (from left to right). c The measured P_g as a function of pt_int/T₁ for the probe states \(|{\psi }_{1,3}\rangle\), \(|{\psi }_{1,5}\rangle\), and \(|{\psi }_{1,7}\rangle\) with a single round of QEC (M = 1 and t_int = Mτ_int = τ_int). The fitted oscillation periods are proportional to n − m. d–f Sensitivity of measuring p (σ_p) of the radiometry for the probe states \(|{\psi }_{1,3}\rangle\), \(|{\psi }_{1,5}\rangle\), and \(|{\psi }_{1,7}\rangle\), respectively. A sensitivity enhancement of 5.3 dB over TLS (the encoding with the two lowest Fock states) is obtained. QEC+QJT (Fig. 3): the deduced sensitivity from the results with the QEC+QJT strategy in Fig. 3. The error bars are obtained through error propagation of the fit parameter uncertainties.