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.
Dynamical Breaking of Charge Neutrality in Intrinsic Josephson Junctions: Common Origin for Microwave Resonant Absorptions and Multiple-branch Structures in the I-V Characterisitcs
Coherent control of a flux qubit by phase-shifted resonant microwave pulses
利用谐振微波成功地脉冲控制了磁通量子比特的量子态。我们在不引入失谐的情况下，通过施加一对相移π/2微波脉冲，观察到了Ramsey条纹。利用这种方法，量子比特态可以在旋转坐标系中在布洛赫球的x-y平面内的任意轴上旋转。我们从相干振荡中获得了量子比特信号，其角速度高达2 pi *11.4 Grad/s。结合拉比脉冲，这种方法使我们能够实现对单量子比特操作的完全控制。它还提供了数量级增加的可能性，在任意酉门操作的速度。
Measurement of the nonlinear Meissner effect in superconducting Nb films using a resonant microwave cavity: A probe of unconventional pairing symmetries
