Noise Characterization and Optimization in a System for the Measurement of the Coherence Time of Superconducting Qubits

We present the development of a project for the reliable measurement of the dephasing times of superconducting qubits. Here we focus specifically on the relevance of the phase noise affecting the system and on the approach we follow to evaluate it and to reduce its influence as much as possible. We focus, in particular, on the phase noise due to the the frequency synthesizer of the up- and down-converter, consisting of a microwave phase locked loop oscillator. We also compare the performance of our frequency synthesizer with that of a commercial evaluation board using the same integrated circuit, and discuss the role of the loop filter in determining the phase noise level in the region close to the carrier frequency.