Quantum bits (qubits) based on individual trapped atomic ions are a promising technology for building a quantum computer. The elementary operations necessary to do so have been achieved with the required precision for some error-correction schemes. However, the essential two-qubit logic gate that is used to generate quantum entanglement has hitherto always been performed in an adiabatic regime (in which the gate is slow compared with the characteristic motional frequencies of the ions in the trap), resulting in logic speeds of the order of 10 kilohertz. There have been numerous proposals of methods for performing gates faster than this natural ‘speed limit’ of the trap. Here we implement one such method, which uses amplitude-shaped laser pulses to drive the motion of the ions along trajectories designed so that the gate operation is insensitive to the optical phase of the pulses. This enables fast (megahertz-rate) quantum logic that is robust to fluctuations in the optical phase, which would otherwise be an important source of experimental error. We demonstrate entanglement generation for gate times as short as 480 nanoseconds—less than a single oscillation period of an ion in the trap and eight orders of magnitude shorter than the memory coherence time measured in similar calcium-43 hyperfine qubits. The power of the method is most evident at intermediate timescales, at which it yields a gate error more than ten times lower than can be attained using conventional techniques; for example, we achieve a 1.6-microsecond-duration gate with a fidelity of 99.8 per cent. Faster and higher-fidelity gates are possible at the cost of greater laser intensity. The method requires only a single amplitude-shaped pulse and one pair of beams derived from a continuous-wave laser. It offers the prospect of combining the unrivalled coherence properties, operation fidelities and optical connectivity of trapped-ion qubits with the submicrosecond logic speeds that are usually associated with solid-state devices.

中文翻译：

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具有俘获离子量子位的快速量子逻辑门

基于单个俘获原子离子的量子位（qubits）是构建量子计算机的一项很有前途的技术。这样做所需的基本操作已经以某些纠错方案所需的精度实现。然而，用于产生量子纠缠的基本双量子位逻辑门迄今为止一直在绝热状态下执行（与陷阱中离子的特征运动频率相比，该门是慢的），从而导致逻辑速度10 kHz 左右。已经有许多方法提出了比陷阱的这种自然“速度限制”更快地执行门的方法。这里我们实现了一种这样的方法，它使用振幅形状的激光脉冲来驱动离子沿着设计的轨迹运动，从而使门操作对脉冲的光学相位不敏感。这实现了对光学相位波动具有鲁棒性的快速（兆赫兹速率）量子逻辑，否则这将成为实验误差的重要来源。我们证明了门时间短至 480 纳秒的纠缠生成——比陷阱中离子的单个振荡周期还短，比在类似的钙 43 超精细量子位中测量的记忆相干时间短 8 个数量级。该方法的威力在中间时间尺度上最为明显，它产生的门误差比使用传统技术所能达到的低十倍以上；例如，我们达到了 1。6 微秒持续时间的门，保真度为 99.8%。以更高的激光强度为代价，可以实现更快、更高保真度的门。该方法只需要一个振幅整形脉冲和一对来自连续波激光器的光束。它提供了将俘获离子量子位无与伦比的相干特性、操作保真度和光学连接性与通常与固态设备相关的亚微秒逻辑速度相结合的前景。