Transmon-based single-photon generator

Superconducting circuits enable strong coupling between artificial atoms (transmon qubits) and transmission lines, making them an interesting platform for studying light-matter interactions in the microwave frequency range. We are developing a device consisting of a superconducting transmon qubit that is strongly coupled to a transmission line that we plan to use in several quantum technology applications and for calibration purposes. This device can be used as an absolute power-meter and it can calibrate and characterize transmission line attenuation, microwave components, or other devices inside a dilution refrigerator. Since the transmon can be approximated as a two-level system, it absorbs and emits only single microwave photons, making it an on demand microwave single-photon source. To demonstrate the quantum nature of the emission state we reconstructed its Wigner function and measured the second-order correlation function.

To perform the second order correlation function we use a 90-degree hybrid coupler that enables single photons to be emitted at two outputs with equal probability. The design of the source and the use of Josephson parametric amplifiers (JPAs) enables us to improve the efficiency of these millikelvin microwave measurements.