Trapped ion systems are pivotal for quantum information processing, simulation, and precision measurement. The manipulation of multi-species ion mixtures is not only crucial for advancing quantum technologies but also for fundamental research in plasma physics. However, existing theoretical models, often based on single-species or weakly coupled approximations, cannot accurately describe the complex collective dynamics of strongly coupled multi-species systems. This work aims to investigate these collective effects experimentally, specifically exploring cooperative confinement in multi-species rubidium ion clusters and establishing a highly selective ion manipulation method via parametric resonance.
An advanced ion-atom hybrid trap system was employed, integrating a rubidium magneto-optical trap (MOT) and a linear Paul trap. Laser-cooled ⁸⁷Rb atoms were continuously photoionized to generate multi-species ion clusters ⁸⁷Rb_N⁺ (N=1,2,...), whose mass-to-charge ratios form an arithmetic sequence. The ion dynamics were characterized by scanning the frequency (Ω_RF) and amplitude (U_RF) of the confining radio-frequency field and monitoring the corresponding time-of-flight mass spectra. The key parameter for analysis was the radial Mathieu stability parameter q_x.
The principal findings are as follows. First, we observed that ⁸⁷Rb⁺ ions remain stably trapped at q_x > 0.908, significantly surpassing the theoretical stability boundary (q_x < 0.908) for a single species. A lifetime analysis yielded a lower bound of 1.3 s for these atomic ions under such conditions, far exceeding the characteristic decay time of molecular ions (0.34 s). This extended stability is attributed to a cooperative confinement effect arising from the collective Coulomb interactions among ions whose secular frequencies form a subharmonic sequence (≈ 1:1/2:1/3: ...). Second, within this extended stable region, the total ion signal exhibited two sharp minima near q_x(⁸⁷Rb⁺) ≈ √2. Mass-resolved analysis confirmed the selective loss of ⁸⁷Rb⁺ atomic ions at these points. This phenomenon is identified as a parametric resonance directly excited by the trapping RF field (Ω_RF ≈ 2ω_r), with the dual-minimum structure presumably induced by trap nonlinearities and coupling to higher-order motional modes (e.g., 2ω_r±ω_z). Finally, the measured resonance linewidths (2.2-4.1 kHz in frequency, 2.5 V in amplitude) demonstrate the high selectivity of this resonant ejection method.
In conclusion, this work experimentally clarifies the dominant role of collective interactions in the dynamics of strongly coupled multi-species ion systems. The discovery of the cooperative confinement effect expands the effective operational parameter space of ion traps, while the direct excitation of parametric resonance by the trapping field provides a novel, high-selectivity pathway for targeted ion removal. These results advance the understanding of multi-component Coulomb systems and open new avenues for applications in quantum technology and laboratory plasma physics. Future work will entail molecular dynamics simulations to quantitatively model the cooperative confinement mechanism.