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Acta Physica Sinica

Measurement of the first rotational excitation spectrum of ultracold ground-state 23Na40K molecules

CHEN Wei, GU Zhenyu, ZHOU Xuansheng, Peng Wang

Published Jan 1, 2026• 0 citations

Abstract

超冷极性分子具有长程电偶极相互作用与丰富的内部自由度，在多个前沿研究领域具有巨大的应用潜力。第一转动能态作为核心的旋转自由度，是实现分子量子态操控的首要选择，可以用来精确调控极性分子的长程相互作用强度，或者作为量子信息处理中的量子态。本文以超冷23Na40K分子为研究对象，开展了两个超精细基态对应的不同第一转动激发态能级结构的研究，并且实现了超冷分子在基态和第一转动激发态之间的相干操控。首先通过理论计算标定了第一转动态中的不同超精细能级以及基态到激发态跃迁频率随外磁场的变化，随后实验上通过测量基态分子损耗谱的方法确定了第一转动激发态的共振频率。结果表明，理论计算与实验数据呈现出高度一致性。进一步利用微波场驱动实现了分子在基态和第一转动激发态之间的拉比振荡。该研究为后续基于超冷极性分子第一转动态的研究提供了基础。

DOI: 10.7498/aps.75.20260232

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超冷极性分子具有长程电偶极相互作用与丰富的内部自由度，在多个前沿研究领域具有巨大的应用潜力。第一转动能态作为核心的旋转自由度，是实现分子量子态操控的首要选择，可以用来精确调控极性分子的长程相互作用强度，或者作为量子信息处理中的量子态。本文以超冷23Na40K分子为研究对象，开展了两个超精细基态对应的不同第一转动激发态能级结构的研究，并且实现了超冷分子在基态和第一转动激发态之间的相干操控。首先通过理论计算标定了第一转动态中的不同超精细能级以及基态到激发态跃迁频率随外磁场的变化，随后实验上通过测量基态分子损耗谱的方法确定了第一转动激发态的共振频率。结果表明，理论计算与实验数据呈现出高度一致性。进一步利用微波场驱动实现了分子在基态和第一转动激发态之间的拉比振荡。该研究为后续基于超冷极性分子第一转动态的研究提供了基础。

We present an intensive study of the coupling between different Feshbach states and the hyperfine levels of the excited states in the adiabatic creation of 23Na40K ground-state molecules. We use coupled-channel method to calculate the wave function of the Feshbach molecules, and give the short-range wave function of triplet component. The energies of the hyperfine excited states and the coupling strength between the Feshbach states and the hyperfine excited states are calculated. Our results can be used to prepare a specific hyperfine level of the rovibrational ground state to study the ultracold collisions involving molecules.

We report on the formation of ultracold weakly bound Feshbach molecules of 23Na40K, the first fermionic molecule that is chemically stable in its absolute ground state. The lifetime of the nearly degenerate molecular gas exceeds 100 ms in the vicinity of the Feshbach resonance. The measured dependence of the molecular binding energy on the magnetic field demonstrates the open-channel character of the molecules over a wide field range and implies significant singlet admixture. This will enable efficient transfer into the singlet vibrational ground state, resulting in a stable molecular Fermi gas with strong dipolar interactions.

原子与分子冷却技术的进步推动了超冷化学领域的快速发展,为深入理解化学反应本质以及实现对化学反应的调控提供了新的可能.对于包含分子的反应散射体系,已有相关理论将散射过程直观类比为光学干涉模型,用于解释体系损失速率在外场调控下的变化原因.与上述研究相关,本文采用一个解析的两体散射模型,研究了在超低温下多重反射路径之间的干涉对损失速率的影响.结果表明,当相邻反射路径之间的相位差为0时,产生相长干涉,分波损失速率取得极大值,且高于普适极限;当相位差为π时,产生相消干涉,分波损失速率取得极小值,且低于普适极限.干涉效应在损失参数y较小时显著增强.分波损失速率在极值处具有关于短程相移δs的对称性质.对于典型的反应散射体系23Na+23Na6Li,通过对其实验数据进行拟合,所得的损失参数y为一小量,与相关文献报道结果一致.此外,与已有工作相比,本研究明确了多重反射路径之间的干涉条件,为实验调控超冷化学反应提供了理论参考.

Ultracold diatomic molecules have achieved significant breakthroughs in recent years, enabling the exploration of quantum chemistry, precision measurements, and strongly correlated many-body physics. Extending ultracold molecular complexity to polyatomic molecules, such as triatomic and tetratomic molecules, has attracted considerable interest. However, the realization of ultracold polyatomic molecules remains technically challenging due to their complex energy-level structures. While only a few experiments have successfully demonstrated the formation of polyatomic molecules by magnetoassociation or electroassociation, here we present the first step toward producing tetratomic molecules through the development of a microwave association technique combined with microwave dressing. When the two lowest rotational states of the molecules are dressed by a microwave field, weakly bound tetramer states emerge in the entrance channel with free dark excited states

\ket{0}

and a dressed state

\ket{+}

. The spectroscopy of these weakly bound tetramers is probed by another microwave field that drives transitions from the populated dressed states

\ket{+}

. By precisely discriminating the complex hyperfine structure of the dark excited level

\ket{0}

from the dressed-state spectroscopy, the binding energy of the tetratomic molecules is measured and characterized. Our work contributes to the understanding of complex few-body physics within a system of microwave-dressed molecules and may open an avenue toward the creation and control of ultracold polyatomic molecules.