Quantum measurement in superconducting qubits*

High-fidelity, quantum non-demolition qubit measurement is a vital prerequisite for robust, large-scale quantum machines. In superconducting quantum circuits, the typical information carriers for qubit readout are coherent states of light, which must be amplified before they can be efficiently recorded in room-temperature electronics. Typically, these amplifiers consist of one, or two, microwave modes linked by a parametrically driven coupling. Such amplifiers regularly approach the quantum limit for amplification, allowing us to closely track qubits’ states. However, conventional parametric amplifiers lack almost every other desirable property, including high saturation power, large bandwidth, and directional operation. I'll discuss our recent efforts to address these shortcomings by suppressing unwanted terms in the device's Hamiltonian and combining multiple, simultaneous parametric drives between a pair of microwave modes. In a single device by varying the parametric drives, we can produce desired behaviors including transmission-only phase-sensitive amplification, input match, and gain-independent bandwidth. We have used observation of quantum jumps and weak measurements of superconducting qubits to benchmark the device's performance. I will discuss the prospects for adding the final desired property, directionality, via further parametric couplings to a third microwave mode. Another route to higher measurement fidelity is to replace coherent states with squeezed light as the information carrier. I will present data from a recent experiment which uses an interferometric scheme for qubit readout with two-mode squeezed light, achieving a voltage signal-to-noise ratio improvement of ~25 % versus coherent state readout. I will also discuss the prospects for using two-mode squeezed light to remotely entangle distant qubits.