A power-meter based on superconducting qubits

Light-matter interaction is a fundamental process that has been widely investigated through atomic physics and quantum optics. Advancements in superconducting circuits have achieved strong coupling between transmission lines and transmon qubits, which are employed as artificial atoms. Because of this, microwave quantum optics experiments can now be performed, based on circuit quantum electrodynamics and superconducting qubits.

In this contribution, we present a device consisting of a superconducting transmon qubit strongly coupled to a transmission line that can be operated as a power sensor. This device has several applications in quantum technologies, such as the calibration of transmission line attenuation, microwave components, or other devices inside a dilution refrigerator. The fact that the transmon absorbs and emits only single photons ensures a non-perturbative measurement, but also can lead to other interesting applications. In fact, the qubit extracts a single-photon state from a coherent-state pulse, re-emitting a single photon in the backward direction, essentially extracting a non-classical field from a classical field. In this way, this device can be operated as a single-photon source. We show results toward quantifying the coherence of this device.