Ye Yufeng, Peng Kaidong, Naghiloo Mahdi, Cunningham Gregory, O'Brien Kevin P
Department of Electrical Engineering and Computer Science, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA.
Research Laboratory of Electronics, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA.
Phys Rev Lett. 2021 Jul 30;127(5):050502. doi: 10.1103/PhysRevLett.127.050502.
Strong nonlinear coupling of superconducting qubits and/or photons is a critical building block for quantum information processing. Because of the perturbative nature of the Josephson nonlinearity, linear coupling is often used in the dispersive regime to approximate nonlinear coupling. However, this dispersive coupling is weak and the underlying linear coupling mixes the local modes, which, for example, distributes unwanted self-Kerr nonlinearity to photon modes. Here, we use the quarton to yield purely nonlinear coupling between two linearly decoupled transmon qubits. The quarton's zero ϕ^{2} potential enables an ultrastrong gigahertz-level cross-Kerr coupling, which is an order of magnitude stronger compared to existing schemes, and the quarton's positive ϕ^{4} potential can cancel the negative self-Kerr nonlinearity of qubits to linearize them into resonators. This ultrastrong cross-Kerr coupling between bare modes of qubit-qubit, qubit-photon, and even photon-photon is ideal for applications such as single microwave photon detection, ultrafast two-qubit gates, and readout.
超导量子比特和/或光子的强非线性耦合是量子信息处理的关键组成部分。由于约瑟夫森非线性的微扰性质，线性耦合常用于色散 regime 以近似非线性耦合。然而，这种色散耦合较弱，并且潜在的线性耦合会混合局部模式，例如，将不需要的自克尔非线性分布到光子模式。在这里，我们使用四元子在两个线性解耦的跨导量子比特之间产生纯非线性耦合。四元子的零ϕ²势能够实现超强大的千兆赫兹级交叉克尔耦合，与现有方案相比强一个数量级，并且四元子的正ϕ⁴势可以抵消量子比特的负自克尔非线性，将它们线性化为谐振器。这种量子比特 - 量子比特、量子比特 - 光子甚至光子 - 光子裸模式之间的超强大交叉克尔耦合非常适合单微波光子检测、超快双量子比特门和读出等应用。
