Tuning the Dispersive Coupling Rate and Suppressing the Photon Shot Noise in a Superconducting Qubit

We report on measurements of the dispersive coupling between a microwave cavity and a tunable coupling qubit (TCQ). By operating the TCQ in the straddling regime, we can tune $\chi$, the cavity pull of the qubit, from a few MHz to close to zero while maintaining a constant qubit frequency. The vanishing $\chi$ leads to the suppression of the photon shot noise, which is one of the sources of qubit decoherence. Readout of the qubit at small $\chi$ is achieved by transferring the qubit state to a higher energy level. Experimental results of tunable $\chi$ and its impact on qubit coherence will be presented.