基于具备翻转量子态特征原子偶极相互作用的量子逻辑门研究

Large-scale quantum computing can outperform classical computing in various tasks. Although many quantum systems have been suggested as candidates for building a reliable quantum computer, each system has specific limitation to the realization of large-scale quantum circuits. Recently, intense efforts have been devoted to the study of quantum information processing via dipole blockade of highly excited neutral atoms, which is usually called Rydberg blockade. The last decade has witnessed remarkable experimental progress in the study of quantum computing with neutral atoms, including efficient and accurate loading of large-scale atomic arrays, realization of high-fidelity single-qubit rotation, and a steady improvement of the fidelity of entangling gates. The fidelity of a traditional entangling Rydberg gate, however, is limited to a value below the threshold of fault-tolerant quantum computing. To overcome this limitation, several methods were recently proposed, but it is an outstanding challenge to realize them experimentally because of certain prestigious technical conditions required therein. Although the Rydberg blockade effect has been focused on for decades, scientists have recently shown that the study of atom-based or photon-based (through Rydberg dark-state polariton) quantum information processing can also benefit from a type of dipole-dipole interaction which features flip-flop process in the orbital or spin degrees of freedom of Rydberg atoms. The study of quantum computing based on this latter process, which is distinct from a simple attractive or repulsive Rydberg interaction, is only on its infant stage. On the other hand, we have recently found that by exploration of detuned Rabi oscillation in the presence of Rydberg blockade, it is possible to exceedingly enhance the accuracy of the Rydberg quantum gates. Motivated by these challenges and new opportunities, we will study how to design accurate and experimentally feasible quantum logic gates based on Rydberg interactions of state-flip features and detuned Rabi oscillations.

大规模量子计算在多方面优越于经典计算。虽然许多系统都被认为可用于设计量子计算机，但是各个系统都有其局限性。近年来，基于中性原子及其里德堡态的量子计算研究取得多个实验突破，但是传统的基于里德堡阻塞的原子间或者单光子间逻辑门精度在理论上尚无法达到量子计算必需的精度。虽然有最新的理论方案问世，但是实现它们的条件是近期技术难以企及的，以致于寻求实验上简单易行的方案迫在眉睫。传统的里德堡逻辑门只是使用到了原子间的阻塞相互作用；最新理论和实验表明，翻转电子轨道宇称、自旋的里徳堡相互作用也可用于量子信息处理，但是尚未出现基于它们的、实验上极易实现的精确逻辑门。另有理论表明，精确使用由于偶极相互作用导致的广义失谐拉比振荡可以大大提高里德堡逻辑门的精度，但是还没有将它和具备翻转特征的偶极过程结合起来的方案。为此本项目拟基于具备翻转特征的偶极过程、使用失谐拉比振荡研究简单易行的原子间和单光子间的高精度逻辑门。

本项目以中性原子的偶极相互作用和失协拉比过程为出发点，研究精确度足够高的原子纠缠及其量子逻辑门。在本项目之前的研究中，有许多基于原子偶极阻塞相互作用的量子纠缠逻辑门。这些不同的方案似乎表明不太可能得到一个具备高的内禀保真度，既能避免原子在里德堡态上漂移、又高速的里德堡量子逻辑门。本项目提出了一类新型里德堡量子逻辑门方案。在这类方案中，控制比特和受控比特都只经历一个激光拉比振荡脉冲。这使得两个比特都不会在处于里德堡态时长时间的自由漂移，大大优越于传统的方案中需要单独对两个比特进行操控的工作模式。这种全新的逻辑门方案可望对基于中性原子里德堡态的量子计算有重要推动作用，至今不仅其扩展版本被哈佛和麻省理工的实验小组验证，而且被他们的量子计算专利所引用。同时，本项目提出了方案压制由原子热运动导致的基态-里德堡态跃迁退相干的新理论，并提出了基于原子偶极阻塞的一种被称为“跃迁减慢”的新量子调控方案实现高速量子逻辑门，为制备高精度原子逻辑门奠定了理论基础。此外，本项目还提出方案在三维原子阵列中激发一个原子到达里德堡态而不影响其它原子的方案和制备中性原子的多自由度纠缠方案。本项目执行期间共计在物理评论等期刊发表七篇论文。

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