Dispersive readout, the standard method for measuring superconducting qubits, is limited by multiphoton qubit-resonator processes arising even at moderate drive powers. These processes degrade performance, causing dispersive readout to lag behind single- and two-qubit gates in both speed and fidelity. In this work, we propose a novel readout method, termed "junction readout". Junction readout leverages the nonperturbative cross-Kerr interaction resulting from coupling a qubit and a resonator via a Josephson junction. Furthermore, by adding a capacitive coupling in parallel to the junction, Purcell decay can be suppressed without the need for a Purcell filter. We also show that junction readout is more robust against deleterious multiphoton processes, and offers greater flexibility for resonator frequency allocation. Crucially, junction readout achieves superior performance compared to dispersive readout while maintaining similar hardware overhead. Numerical simulations show that junction readout can achieve fidelities exceeding $99.99\%$ in under $30$ ns, making it a promising alternative for superconducting qubit readout with current hardware.
开放量子系统可以经历耗散相变，并且它们的临界行为可以在传感应用中被利用。例如，它可以用于增强超导量子比特读出测量的保真度，这是创建可靠量子硬件的核心问题。最近引入的测量协议，称为"临界参数量子传感“，使用参数（双光子驱动）克尔谐振器的驱动耗散相变，以达到99.9\%的单量子位检测保真度[arXiv：2107.04503]。在这项工作中，我们通过使用基于机器学习的分类算法来改进以前的协议，以\textit{efficiently and rapidly}从这个关键的动态中提取信息，到目前为止，这一直被忽视，只关注静态属性。这些分类算法被施加到弱量子测量（零差检测）的电路QED实现耦合到超导量子比特的克尔谐振器的时间序列数据。这展示了机器学习方法如何在关键的开放量子系统中实现更快、更可靠的测量协议。
Ideal Quantum Nondemolition Readout of a Flux Qubit Without Purcell Limitations
在量子计算中，基于Rabi模型的大失谐色散耦合被广泛用于量子非破坏（QND）量子比特读出。然而，测量速度和保真度通常受到Purcell效应的显著限制，即：Purcell衰变，临界光子数和依赖于量子位的克尔非线性。为了避免这些影响，我们提出了如何实现一个理想的QND读出的梯度通量量子比特与可调间隙通过其非微扰色散耦合（NPDC）的频率可调测量谐振器。我们表明，这种基于NPDC的读出机制是免费的偶极场相互作用，量子比特QND测量不恶化腔内光子。量子位读出速度和保真度都可以\emph{avoid the Purcell limitations}。此外，NPDC可以方便地打开和关闭通过外部控制通量。我们将展示如何将此建议扩展到多量子位架构的联合量子位读出。
Mixed spin-boson coupling for qubit readout with suppressed residual shot-noise dephasing
QuTech and Kavli Institute of Nanoscience, Delft University of Technology（代尔夫特理工大学奎克研究所和卡弗里纳米科学研究所）；Kavli Institute of Nanoscience, Delft University of Technology（代尔夫特理工大学卡弗里纳米科学研究所）；School of Applied and Engineering Physics, Cornell University（康奈尔大学应用与工程物理学院）；Quantum Engineering Centre for Doctoral Training, H. H. Wills Physics Laboratory and Department of Electrical and Electronic Engineering, University of Bristol（布里斯托大学量子工程博士培训中心、H.H.威尔斯物理实验室及电气电子工程系）；QphoX
Two-photon coupling via Josephson element II: Interaction dressing, cross-Kerr coupling, and limits of low-energy bosonic model
Bogolyubov Institute for Theoretical Physics, National Academy of Sciences of Ukraine（乌克兰国家科学院博戈留博夫理论物理研究所）；Institute of Physics of the National Academy of Sciences（乌克兰国家科学院物理研究所）
Tunable nonlinear electromechanics at the zero-point motion scale
ICFO – Institut de Ciències Fotòniques, The Barcelona Institute of Science and Technology（ICFO光子科学研究所，巴塞罗那科技研究院）；Center for Hybrid Quantum Networks (Hy-Q), Niels Bohr Institute, University of Copenhagen（哥本哈根大学尼尔斯·玻尔学院混合量子网络中心）；Université de Bordeaux, CNRS, LOMA, UMR 5798（波尔多大学）；Chiral Nano AG（手性纳米股份公司）
A high-fidelity two-qubit gate for multimode superconducting P-mon qubits
Technical University of Munich（慕尼黑工业大学）；Walther-Meißner-Institut, Bayerische Akademie der Wissenschaften（瓦尔特·迈斯纳研究所，巴伐利亚科学院）；Saarland University（萨尔兰大学）
