Superconducting qubits are a leading platform for scalable quantum computing and quantum error correction. One feature of this platform is the ability to perform projective measurementsorders of magnitude more quickly than qubit decoherence times. Such measurements are enabled by the use of quantum-limited parametric amplifiers in conjunction with ferrite circulators – magnetic devices which provide isolation from noise and decoherence due to amplifier backaction. Because these non-reciprocal elements have limited performance and are not easily integrated on-chip, it has been a longstanding goal to replace them with a scalable alternative. Here, we demonstrate a solution to this problem by using a superconducting switch to control the coupling between a qubit and amplifier. Doing so, we measure a transmon qubit using a single, chip-scale device to provide both parametric amplification and isolation from the bulk of amplifier backaction. This measurement is also fast, high fidelity, and has 70% efficiency, comparable to the best that has been reported in any superconducting qubit measurement. As such, this work constitutes a high-quality platform for the scalable measurement of superconducting qubits.
The SSSV model is a simple classical model that achieves excellent correlation with published experimental data on the D-Wave machine’s behavior on random instances of its nativeproblem, thus raising questions about how „quantum“ the D-Wave machine is at large scales. In response, a recent preprint by Vinci et al. proposes a particular set of instances on which the D-Wave machine behaves differently from the SSSV model. In this short note, we explain how a simple modeling of systematic errors in the machine allows the SSSV model to reproduce the behavior reported in the experiments of Vinci et al.
Recently there has been intense interest in claims about the performance of the D-Wave machine. Scientifically the most interesting aspect was the claim in Boixo et al., based on extensiveexperiments, that the D-Wave machine exhibits large-scale quantum behavior. Their conclusion was based on the strong correlation of the input-output behavior of the D-Wave machine with a quantum model called simulated quantum annealing, in contrast to its poor correlation with two classical models: simulated annealing and classical spin dynamics. In this paper, we outline a simple new classical model, and show that on the same data it yields correlations with the D-Wave input-output behavior that are at least as good as those of simulated quantum annealing. Based on these results, we conclude that classical models for the D-Wave machine are not ruled out. Further analysis of the new model provides additional algorithmic insights into the nature of the problems being solved by the D-Wave machine.