Generation of squeezed states of microwave radiation in a superconducting resonant circuit

High-quality superconducting oscillators have been successfully used for quantum control and readout devices in conjunction with superconducting qubits. Also, it is well known that squeezed states can improve the accuracy of measurements to subquantum, or at least subthermal, levels. Here we show theoretically how to produce squeezed states of microwave radiation in a superconducting oscillator with tunable parameters. The circuit impedance, and thus the resonance frequency, can be changed by controlling the state of an RF SQUID inductively coupled to the oscillator. By repeatedly shifting the resonance frequency between any two values, it is possible to produce squeezed and subthermal states of the electromagnetic field in the (0.1--10) GHz range, even when the relative frequency change is small. We propose experimental protocols for the verification of squeezed state generation, and for their use to improve the readout fidelity when such oscillators serve as quantum transducers.