Quantum limited measurement using superconducting circuits


     Superconducting circuits are demonstrated to be a very promising platform for constructing large scale quantum processor. Meanwhile, Josephson parametric amplifier (JPA) has become an important resource for superconducting quantum information experiments. We incorporated a flux-driven JPA as a preamplifier in our setup for the readout of superconducting qubits. This led to drastic improvement in the signal to noise ratio and enabled us to achieve single-shot readout and observation of quantum jumps. Same circuit of the flux-driven JPA can also work as a parametric phase-locked oscillator (PPLO) at the pump power above the oscillation threshold. Using a PPLO, we demonstrated a fast, high-fidelity, and non-destructive readout. The new readout scheme combines the advantages of several disparate schemes developed. Combining an artificial L-type three-level atom and PPLO, we demonstrated detection of a propagating microwave photon, which remains a challenging task due to its correspondingly small energy.