 "In superconducting quantum circuits, such as quantum bits, information is processed and transferred in the form of microwave quantum signals. Moreover, at the end of quantum information protocols, these signals have to be recorded by room temperature electronic devices. Since microwave quantum signals typically consist of very few photons, they must be amplified in order to achieve reasonable signal-to-noise ratios. Therefore, low-noise amplification of quantum signals is crucial. Modern low-noise microwave amplifiers are built upon superconducting Josephson parametric devices, such as a flux-driven Josephson Parametric Amplifier (JPA), which allows to reach the standard quantum limit of amplification and even go beyond it. The current JPA is formed by a superconducting quantum interference device (SQUID) combined with a superconducting coplanar waveguide resonator. The combined system acts as a tunable nonlinear microwave resonator, whose frequency can be varied in-situ via an external magnetic field. A mechanical analogue would be a pendulum of variable length, allowing one to tune its eigenfrequency. Tunability of the nonlinear microwave resonator can be exploited to parametrically pump the JPA via application of a strong microwave signal at twice the resonant frequency. This, in turn, can result in a strong parametric amplification of weak quantum signals incident at the JPA. The same parametric amplification mechanism can be exploited further for generation of genuine quantum signals in the form of squeezed vacuum states.
The students’ mission in this practical training is to experimentally study the parametric quantum-limited amplification phenomenon with the flux-driven superconducting JPA. This goal can be split in several parts: (i) analyze the magnetic field dependence of the JPA’s resonance frequency via microwave transmission measurements with a Vector Network Analyzer (VNA) and determine the JPA frequency modulation period in terms of the magnetic coil current, (ii) find a suitable working point for parametric amplification and record the corresponding resonance response, (iii) apply a microwave pump signal at an appropriate frequency in order to obtain and measure a substantial parametric amplification gain." www.wmi.badw.de/fileadmin/WMI/Lecturenotes/FOPRA/Manual_FOPRA__104_20211029.pdf
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