What are the quantum limits of a measurement and how to attain that limit in superconducting quantum information processing experiments.

Previous results: Novel, non-reciprocal amplifiers that enable measurement of superconducting quantum bits with minimal back action i.e. minimal interference to the qubit being measured (Ranzani et.al. “Wideband Isolation by Frequency Conversion in a Josephson-Junction Transmission Line”, Phys. Rev. Applied 8, 054035, (2017), F.Lecoq et.al. “Microwave Measurement beyond the Quantum Limit with a Nonreciprocal Amplifier”, Phys. Rev. Applied 044005 (2020)). The main challenge here is measurement efficiency. Typically between 10%-20% it is limited by quantum mechanics in most architectures to 50%. In our architecture, we were able to surpass 50% with a clear path to get close to 100%. These works, not only enable state detection of the qubit, but also feedback steering of the qubit state to a desired state (quantum feedback).