High fidelity continuous measurements of superconducting quantum bits

Recent progress in the development of quantum noise limited amplifiers has enabled high fidelity, continuous measurements of superconducting quantum bits (qubits). These systems are excellent test beds for investigating various ideas in quantum measurement. By controlling the measurement strength one can go from the “orthodox” picture of near instantaneous quantum measurement and collapse of the quantum state to a weak, continuous measurement regime where the collapse is more gradual. When such a weak measurement is implemented with an ideal detector, it is possible to continuously track the quantum system by analyzing the measurement record. This opens up the possibility of using the measurement record for quantum feedback and control. I will describe experiments using superconducting quantum bits and amplifiers to implement such a scheme. I will describe in detail the operation of the superconducting parametric amplifier which is crucial to the success of these experiments. I will also discuss the challenges involved in achieving an ideal, quantum limited measurement scheme and using the measurement signal to feedback on the quantum state of the qubit.