Improved Superconducting Qubit State Readout by Path Interference

High fidelity single shot qubit state readout is essential for many quantum information processing protocols. In superconducting circuit, the usually determined by detecting dispersive frequency shift of a microwave cavity from either transmission or reflection. We demonstrate use constructive interference between transmitted and reflected signal to optimize readout, with which we find better resolved discrimination an improved fidelity. As simple convenient approach, our scheme can be combined other methods based on photon states further improve readout.