Monitoring the energy of a cavity by observing the emission of a repeatedly excited qubit*

It is possible to count the number of photons in a microwave cavity by exploiting the dispersive interaction between a superconducting qubit and the cavity. In standard protocols, it requires to perform a series of gates on the qubit that are conditioned on the cavity occupation, followed by readout operations on the qubit using an ancillary readout resonator [1-3].

Recently, our group introduced a way to realize this counting by multiplexing the driving tones on the qubit and continuously extracting information about the number of photons. A previous experiment managed to show that the counting works on average on many realizations of the experiment and that the coherences between Fock states disappear as expected when the drive is turned on [4].

Here, we use a new device that is able to resolve the photon number in a single realization of the experiment. The cavity is a high-quality factor 3D superconducting cavity whose lifetime is above 200 µs. A carefully designed filter allows us to drive and measure the emitted field by the qubit at a rate that is orders of magnitude larger than the decay rate of the cavity. We use the proposed multiplexing scheme in [4], which is equivalent to repeatedly excite the qubit. The heterodyne detection of the fluorescence field that the qubit emits at all frequencies reveals the photon number in the cavity and produces a quantum backaction on the cavity state. The achieved measurement rate is measured to be more than one order of magnitude higher than the decay rate of the cavity, thus allowing us to track the number of photons in time.

[1] B. R. JOHNSON et al. : Quantum non-demolition detection of single microwave photons in a circuit. Nature Physics, 6(9):663–667, 2010.

[2] C. GUERLIN : Mesure quantique non destructive repetee de la luniere : etats de Fock et trajectoires quantiques. Thèse de doctorat, Universite Pierre et Marie Curie - Paris VI, 2007.

[3] R. DASSONNEVILLE et al. : Number-Resolved Photocounter for Propagating Microwave Mode. Physical Review Applied, 14(4):1, 2020.

[4] A. ESSIG et al. : Multiplexed photon number measurement. Physical Review X, 031045:1–33, 2020.