Fast quantum logic gates with trapped-ion qubits
Fast quantum logic gates with trapped-ion qubits
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DOI：
10.1038/nature25737
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发表时间：
2018-03-01
期刊：
影响因子：
64.8
通讯作者：
Lucas, D. M.
中科院分区：
文献类型：
--
作者：
Schafer, V. M.;Ballance, C. J.;Lucas, D. M.
基于单个捕获原子离子的量子比特是一种很有前途的构建量子计算机的技术(1)。这样做所需的基本运算已经达到了某些纠错方案(2-4)所需的精度。然而，到目前为止，用于产生量子纠缠的基本的两量子比特逻辑门一直在绝热状态下执行(在绝热状态下，与陷阱中离子的特征运动频率(3-7)相比，门的速度较慢)，导致逻辑速度约为10千赫兹。已经有许多建议的方法，以执行门更快的自然‘速度限制’的陷阱(8-12)。在这里，我们实现了一种这样的方法(11)，它使用幅度形状的激光脉冲来驱动离子沿设计的轨迹运动，从而使门操作对脉冲的光学相位不敏感。这使得快速(兆赫速率)量子逻辑对光学相位的波动具有健壮性，否则光学相位波动将成为实验误差的重要来源。我们证明了纠缠产生的栅极时间短到480纳秒--比陷阱中离子的单个振荡周期还短，比类似的钙-43超精细量子比特的记忆相干时间短8个数量级。该方法的能力在中间时间尺度上最为明显，在该时间尺度下，它产生的门误差比使用传统技术可以获得的门误差低十多倍；例如，我们实现了保真度为99.8%的1.6微秒持续时间的门。更快和更高保真的门是可能的，但代价是更大的激光强度。该方法只需要一个幅值整形脉冲和一对来自连续波激光的光束。它提供了将囚禁离子量子比特的无与伦比的相干特性(2，13)、操作保真度(2-4)和光学连接性(14)与通常与固态设备(15，16)相关的亚微秒逻辑速度相结合的前景。
Quantum bits (qubits) based on individual trapped atomic ions are a promising technology for building a quantum computer(1). The elementary operations necessary to do so have been achieved with the required precision for some error-correction schemes(2-4). 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(3-7)), 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(8-12). Here we implement one such method(11), 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(2,13), operation fidelities(2-4) and optical connectivity(14) of trapped-ion qubits with the submicrosecond logic speeds that are usually associated with solid-state devices(15,16).