Reconstruction of single-photon emission with a self-calibrated wQED device based on a transmon qubit

We present a detailed study of a microwave single-photon source utilizing a transmon qubit asymmetrically coupled to two transmission lines: a weakly coupled drive line and a strongly coupled emission line. Our design allows for the generation of single photons in the microwave regime, which are essential for quantum communication, computing, and sensing applications. In addition to single-photon generation, the same design functions as a power sensor and enables precise measurement of the system’s attenuation. This dual functionality allows us to calibrate the noise of our amplification chain. This enables a self-calibrated architecture in which the same device acts simultaneously as the photon source and as a reference standard, eliminating the need for external tones or calibrated inputs. With this calibration we reconstruct the density matrix of the emitted photon state, for which we achieve a fidelity of 61.4% and measure a second-order correlation function of g ( 2 ) ( 0 ) = 0.06 − 0.06 + 0.45 , indicating quantum nature and single-photon characteristics. These results confirm generation of single photons and demonstrate the potential of transmon qubits in advancing quantum technologies in the microwave domain.