To control and measure the state of a quantum system, it must necessarily be coupled to external degrees of freedom. This inevitably leads to spontaneous emission via the Purcell effect, photon-induced dephasing from measurement backaction, and errors caused by unwanted interactions with nearby quantum systems. To tackle this fundamental challenge, we make use of the design flexibility of superconducting quantum circuits to form a multimode element—an artificial molecule—with symmetry-protected modes. The proposed circuit consists of three superconducting islands coupled to a central island via Josephson junctions. It exhibits two essential nonlinear modes, one of which is flux insensitive and used as the protected qubit mode. The second mode is flux tunable and serves via a cross-Kerr-type coupling as a mediator to control the dispersive coupling of the qubit mode to the readout resonator. We demonstrate the Purcell protection of the qubit mode by measuring relaxation times that are independent of the mediated dispersive coupling. We show that the coherence of the qubit is not limited by photon-induced dephasing when detuning the mediator mode from the readout resonator and thereby reducing the dispersive coupling. The resulting highly protected qubit, which we refer to as P-mon, with tunable interactions may serve as a basic building block of a scalable quantum processor architecture, in which qubit decoherence is strongly suppressed. Published by the American Physical Society 2024

中文翻译：

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非线性量子比特模式与其环境的高效解耦


为了控制和测量量子系统的状态，它必须与外部自由度耦合。这不可避免地导致通过 Purcell 效应自发发射、光子诱导的测量反作用消相，以及与附近量子系统不必要的相互作用引起的误差。为了应对这一基本挑战，我们利用超导量子电路的设计灵活性来形成具有对称保护模式的多模元件——人工分子。拟议的电路由三个超导岛组成，通过约瑟夫森结点与一个中央岛耦合。它表现出两种基本的非线性模式，其中一种对磁通量不敏感，用作受保护的量子比特模式。第二种模式是磁通量可调的，通过交叉 Kerr 型耦合作为介质来控制量子比特模式与读出谐振器的色散耦合。我们通过测量独立于介导的色散耦合的弛豫时间来证明量子比特模式的 Purcell 保护。我们表明，当从读出谐振器中失谐介质模式时，量子比特的相干性不受光子诱导的消相的限制，从而减少了色散耦合。由此产生的高度受保护的量子比特（我们称之为 P-mon）具有可调交互，可以作为可扩展量子处理器架构的基本构建块，其中量子比特退相干受到强烈抑制。美国物理学会 2024 年出版