近日，美国哈佛大学的Kang-Kuen Ni&Annie J. Park及其研究团队取得一项新进展。经过不懈努力，他们实现了分子量子比特间的纠缠和iSWAP门。相关研究成果已于2024年11月13日在国际权威学术期刊《自然》上发表。

据悉，量子计算和模拟依赖于具有可控相互作用的长寿命量子比特。捕获的极性分子已被提出作为一种有前景的量子计算平台，它既能实现可扩展性和单粒子可寻址性，又能利用分子的固有复杂性和强耦合性。

最近在单个捕获分子的超精细-转动态的单量子态制备和相干性方面取得的进展，使它们有望成为有潜力的量子比特，分子间的偶极相互作用可以产生纠缠。然而，尚未实现用分子构建的通用两量子比特门。

本研究利用分子内在资源，使用单独捕获的X¹Σ⁺态的NaCs分子实现了一个两量子比特iSWAP门。通过让分子在1.9微米的距离上相互作用664微秒，研究人员在两个分子都存在的试验中生成一个最大纠缠的贝尔态，其保真度为94(3)%。

利用运动-转动耦合，研究人员测量了沿轴向囚禁方向最低几个运动态的残余激发，并发现它们是导致退相干的主要原因。最后，研究人员在基态转动能级内确定了两个不相互作用的超精细态，用于编码量子比特。通过在相互作用态和非相互作用态之间转移来切换相互作用，从而实现iSWAP门。研究人员通过测量其逻辑真值表来验证了门的性能。

附：英文原文

Title: Entanglement and iSWAP gate between molecular qubits

Author: Picard, Lewis R. B., Park, Annie J., Patenotte, Gabriel E., Gebretsadkan, Samuel, Wellnitz, David, Rey, Ana Maria, Ni, Kang-Kuen

Issue&Volume: 2024-11-13

Abstract: Quantum computation and simulation rely on long-lived qubits with controllable interactions. Trapped polar molecules have been proposed as a promising quantum computing platform, offering scalability and single-particle addressability while still leveraging inherent complexity and strong couplings of molecules. Recent progress in the single quantum state preparation and coherence of the hyperfine-rotational states of individually trapped molecules allows them to serve as promising qubits, with intermolecular dipolar interactions creating entanglement. However, universal two-qubit gates have not been demonstrated with molecules. Here we harness intrinsic molecular resources to implement a two-qubit iSWAP gate using individually trapped X¹Σ⁺ NaCs molecules. By allowing the molecules to interact for 664μs at a distance of 1.9μm, we create a maximally entangled Bell state with a fidelity of 94(3)% in trials in which both molecules are present. Using motion–rotation coupling, we measure residual excitation of the lowest few motional states along the axial trapping direction and find them to be the primary source of decoherence. Finally, we identify two non-interacting hyperfine states within the ground rotational level in which we encode a qubit. The interaction is toggled by transferring between interacting and non-interacting states to realize an iSWAP gate. We verify the gate performance by measuring its logical truth table.

DOI: 10.1038/s41586-024-08177-3

Source: https://www.nature.com/articles/s41586-024-08177-3