Integration of 4-8GHz Cryogenic Low Noise Mixer Using AC Josephson effect in Quantum Computation System

For RF signal processing, frequency conversion using diode mixer is one of the most essential techniques. In the case of various cryogenic RF systems (such as quantum computation system, MEMS, etc.), the thermal noise of the system is extremely low. The diode mixer based on Schottky effect has lower knee-voltage compared to other mixer, but it has a much higher voltage scale (~300mV) compared to the signals of the cryogenic RF system (~100nV). Therefore, despite the high purity of such RF systems, we usually mix signals in high temperature environments. Since the proposal for Josephson mixer in 1972, the method of using Josephson relation in superconductors has been extensively studied in mm-wavelength system of radio astronomy. We would like to apply the concept of Josephson mixer to our 4-8GHz single-photon-limit cryogenic system. First, we verified the manufacturability of Josephson mixer, which operates in our RF cryogenic system. Applying the Bessel-Fourier series expansion to the situation of mixing two RF signals, the amplitude of each mixed frequency components is reproduced as Bessel function of the first kind, and we could obtain realistic conditions of junction size and mixing signals. It is followed by the fabrication of Josephson mixer with suitable notch filters.