Catching Microwave Photons in a Superconducing Resonator with Tunable Coupling

When transferring a quantum state from a freely propagating mode to a resonator, reflections must be minimized to avoid energy loss. Performing this transfer with high fidelity requires tunable coupling. We experimentally studied a 50 Ohm transmission line with tunable coupling to a 6GHz superconducting coplanar waveguide resonator, which in turn is capacitively coupled to a phase qubit for calibration. We classically drove the resonator while measuring the reflected and captured signals using a HEMT amplifier. Following theory by Korotkov (PRB 84, 014510, 2011), we find that the photon capture efficiency is maximized with an exponentially increasing drive; further improvements come from varying pulse duration and dynamic coupling. With these techniques, we reduce reflections so that presently over 80\% of the pulse energy is captured by the resonator.