Λ型三能级原子与两个谐振器的量子相位门

Quantum phase gate on a single superconducting Λ-type three-level and two superconducting resonators

量子纠缠的生成和操控在量子通信和量子信息处理中具有广泛的应用价值.通过构建单个Λ型三能级原子和两个超导谐振器之间相互耦合的模型,给出了实现控制Z门(Controlled-Z)的四种操作方案和实现交换门(Swap)的两种操作方案;同时对实现控制Z门的第一种操作方案进行了保真度的数值模拟仿真.结果表明:通过20.83 ns的运行时间,其保真度为96.67%,而衰减率、弛豫速率和移相比率的增加会降低系统的保真度,而耦合强度的增加会减少系统的运行时间,从而减小衰减参数的影响,提高系统的保真度.

Quantum phase gate is a necessary quantum component for quantum coding and quantum computing. Compared with the traditional gate circuit, quantum phase gate has the characteristics of unitarity and reversibility. Therefore,we construct a model of mutual coupling between a single Λ-type three-level atom and two superconducting resonators,which is connected by a capacitor. By separately controlling the disconnection time and connection time of the two superconducting resonators in the model as well as by controlling the magnetic flux of the superconducting quantum interference device（SQUID） to make a certain transition energy level of the Λ-type three-level atom equal the relevant resonance energy level, the interaction between the two levels can be achieved and the system can be manipulated.Afterwards, we propose four control schemes for implementing the controlled-Z gate through a three-step operation, and two operation schemes for implementing swap gate through a four-step operation. At the same time, the numerical simulations of fidelity are implemented for the first operation scheme for controlling the Z-gate. The results of fidelity discussion show that the fidelity of this scheme is 96.67% through the running time of 20.83 ns, thus it proves that this scheme is theoretically feasible. The increase in the three attenuation parameters, i.e., attenuation rate, relaxation rate,and phase shift ratio, will reduce the fidelity of the system, while the increase in coupling strength will cut down the time of system operation, thus reducing the influence of attenuation parameters and improving the system fidelity.In this paper we present a quantum phase gate scheme in which two superconducting resonators and a Λ-type three-level atom are coupled with two capacitors. Since the experimental setup is simplified, it is important to reduce the coherence between devices. In addition, the solution has no restriction on the strength of the classic pulse principally,through which the system operates faster and the fidelity of the phase gate is improved effectively.