Researchers at Fudan University have demonstrated a new method for rapidly loading barium ion qubits from a naturally abundant source, bypassing the need for artificially created isotopes. The work details a three-step photoionization scheme to selectively load either ^(137)Ba^+ or ^(135)Ba^+ ions, critical for scaling up quantum computing systems. Spectral-overlap analysis revealed “relayed selectivity” where the process suppresses unwanted isotope excitation, with no contaminant ions detected during the experiment. This strategy, the researchers report, can be extended to achieve high-purity loading of other ion species for quantum information processing and simulation.

Relayed Selectivity Enables High-Purity Barium Ion Loading

A three-step photoionization scheme developed at Fudan University achieves high-purity loading of barium ions without requiring pre-enriched isotopes, a significant simplification for scaling trapped ion quantum computing. The process begins with excitation at 791 nm, followed by resonant transfer at 554 nm, and concludes with photoionization at 532 nm, establishing a method for selectively populating ion traps with either ^(137) $B a^{+}$ or ^(135) $B a^{+}$ ions. This approach circumvents the need for costly and time-consuming isotope separation typically required in trapped ion experiments.

Researchers report no contaminant ions were detected throughout the entire experiment. Calculations detailed in the supporting information show that the relayed selectivity functions via specific excitation pathways, with strengths satisfying $s u m_{F^(′′)} t_{A, F^′F^(′′)}$ = 1 for excitation from $F^{′}$ = 1/2 to $F^{′′}$ = 1/2 and 3/2 in both ^(135)Ba and ^(137)Ba.

The resonant drive during the second excitation step yields a prominent loading rate for the selected isotope, further enhancing the efficiency of the process. For ^(135)Ba sourced from naturally abundant barium, calculated relayed selectivity maps demonstrate the effectiveness of the scheme showing $ε_{1}^{A_t}$ and $ε_{2}^{A_t}$ on logarithmic color scales. The team, affiliated at Fudan University published their findings with Zichun Zhou as first author.

The ability to load ions from a naturally abundant source represents a practical advantage for building larger quantum systems, reducing both the complexity and expense associated with isotope preparation. This advancement addresses a critical need for scalable quantum technologies.

791 nm Excitation Drives Isotope Selection

The 791 nanometer excitation wavelength offers a distinct advantage in isolating barium isotopes due to its well-resolved isotope-shifted hyperfine structure, surpassing simpler two-photon ionization methods limited by spectral overlap and efficiency. This precision stems from the excitation of the 6s² ¹S₀ → 6s6p ³P₁⁰ transition, a key element in a newly demonstrated three-step photoionization scheme. The process moves beyond cost-effective but less selective single-laser approaches by incorporating a resonant transfer at 554 nm to the 6s6d ³D₁ state before final ionization at 532 nm.

This multi-step approach builds on earlier work by refining the excitation sequence and detailing the resulting isotope purity. Calculations reveal that the selectivity for a target isotope, denoted as $η_{A_t} (ν)$ , is determined by the ratio of its response to the sum of responses from all isotopes.

For the 791 nm excitation, the equation $s_{A, F^′}$ = ( $2 F^{′}$ + 1)/[3( $2 I_{A}$ + 1)], where $I_{A}$ represents the nuclear spin, defines the strength of excitation for each isotope and hyperfine component. The researchers focused on selecting the component with the highest line-center selectivity for each transition and target isotope, specifically ¹³⁷Ba and ¹³⁵Ba. Defining the unwanted-isotope excitation fraction after one or two steps as $ε_{k}^{A_t}$ = (sum_(A ≠ A_t)R_A^(k))/sum_AR_A^(k), where k = 1,2, the team demonstrated a significant reduction in contamination.

For ¹³⁷Ba, the value $ε_{1}^{A_t}$ recovers the main-text $ε_{k}$ , indicating a strong correlation between the calculated and experimental results. The 554 nm laser addresses the calculated $F^{′}$ = 1/2 → $F^{′′}$ = 1/2 resonance, further enhancing the purity of the loaded ions. Averaging over magnetic sublevels, the peak-normalized Voigt profile V, with Lorentzian FWHM $Γ_{L}$ and Gaussian FWHM $Γ_{G}$ = Δ $v_{e f f}^{F W H M} / λ$ , contributes to the overall resolution of the isotope shifts.

554 nm Resonant Transfer Enhances Barium Loading Rate

The technique detailed achieves an exceptionally high loading purity, with an unwanted-isotope excitation fraction of approximately 6.8 x 10^-7, a result stemming from the carefully orchestrated two-step resonant excitation process. This level of purity surpasses previous attempts using ultraviolet lasers, which also suffer from the drawback of inducing unwanted charging on the ion trap itself. By eliminating the need for ultraviolet light, this new method avoids that issue while simultaneously enhancing isotope selectivity. Spectral-overlap analysis confirmed this mechanism, revealing how the second excitation step further refines the isotope purity.

The demonstrated method is not limited to the specific wavelengths used in this experiment, offering a pathway to adapt the technique for loading other ion species with high purity. This adaptability is important for scaling up quantum systems, as it removes the reliance on pre-enriched, artificially created isotopes, which are both expensive and time-consuming to produce.

The team’s work builds on earlier schemes that used 791 nm excitation followed by a second resonant transition, but importantly details the second-step transition and the resulting isotope selectivity for the first time. This detailed analysis provides a foundation for optimizing the process and extending it to other ion species. The ability to rapidly and selectively load ions from naturally abundant sources represents a step toward realizing practical, large-scale quantum computers and simulators.

532 nm Raman Beam Facilitates UV-Free Ionization

A 51-ion Coulomb crystal of ^(137) $B a^{+}$ was loaded in 10 seconds from a natural barium source, despite the isotope constituting only 11.23% of the element’s abundance, demonstrating a significant increase in loading efficiency. This achievement relies on a three-step photoionization process using a 532 nm Raman beam for the final ionization stage, bypassing the need for ultraviolet light typically employed in such procedures.

The experimental setup intersects 791 nm, 554 nm and 532 nm beams within the ion trap region, alongside 493 nm Doppler-cooling and 650 nm repumping light tuned to the selected isotope, either ^(137) $B a^{+}$ or ^(135) $B a^{+}$ . The process incorporates a “protection” beam, a 493 nm cooling beam detuned by -210 MHz, applied concurrently with the photoionization beams. This far-red-detuned beam provides additional cooling for newly loaded ions, improving robustness during capture and crystallization at the trap.

Researchers used a thermal-beam excitation model to fit the observed profile, incorporating factors like slit acceptance, residual Doppler shifts and the spatial profile of laser intensity. This scheme addresses isotope discrimination through two complementary spectral features: a larger first-step separation from competing isotopes and an additional narrowband isotope filter provided by the resonant 554 nm step. These approaches enhance final-step ionization rather than resolve the overlap between closely spaced 413 nm excitation resonances.

^(137)Ba^+ and ^(135)Ba^+ as Promising Qubit Candidates

The demonstrated photoionization scheme circumvents a significant obstacle to scaling trapped ion quantum computing: the reliance on artificially enriched isotopes. Researchers at Fudan University achieved rapid loading of both ^(137) $B a^{+}$ and ^(135) $B a^{+}$ directly from naturally abundant barium, a feat previously hindered by the need for complex isotope separation processes. The 554 nm transition effectively suppresses excitation of unwanted barium isotopes. Spectral-overlap analysis revealed this phenomenon, demonstrating that the second excitation step enhances discrimination beyond what would be expected from the initial atomic selection alone.

This suppression is critical, as the absence of contaminant ions throughout the experiment confirms the high purity of the loaded qubit candidates. The team reports no detection of other species during the entire process, a result indicative of the scheme’s effectiveness and precision.

The ability to bypass isotope enrichment has substantial implications for the cost and scalability of trapped ion quantum technologies. ^(133) $B a^{+}$ is often favored for its hyperfine qubit properties, but its radioactivity and reliance on nuclear synthesis make it impractical for large-scale systems. In contrast, ^(137) $B a^{+}$ and ^(135) $B a^{+}$ are stable isotopes, but previously required more complex preparation. “Selective and rapid loading of a specific isotope lays the critical foundation for such purpose particularly when scaling up the system size,” the researchers state, highlighting the importance of this advancement for building larger, more robust quantum processors.

The work also suggests that this multi-step excitation strategy, using relayed selectivity, can be adapted for high-purity loading of other ion species, broadening its potential impact on the field. The team’s success builds on existing techniques, but introduces a novel approach to isotope discrimination.

Various atomic ion species have been demonstrated as good candidates for qubits, and barium isotopes, including ^(137) $B a^{+}$ , ^(135) $B a^{+}$ and ^(133) $B a^{+}$ , show great promise due to their rich level structures and optical transitions. This new method offers a pathway to harness the benefits of stable barium isotopes without the limitations of traditional isotope preparation methods, potentially accelerating the development of scalable quantum information processing and quantum simulation platforms.

Spectral Analysis Confirms Isotope and Hyperfine Shifts

Spectral analysis of barium isotopes confirms the effectiveness of a new loading technique, revealing precise isotope and hyperfine shifts critical to suppressing unwanted excitation. These calculations were essential for optimizing the multi-step photoionization scheme and maximizing the purity of the final ion sample. The technique relies on the second excitation step actively suppressing the presence of undesired isotopes. The team’s work extends beyond simply identifying these shifts. It demonstrates how these values can be used to refine the excitation process and achieve high-purity loading.

The team used finite-field extensions of a pCI workflow to calculate mass-shift coefficients, enabling accurate predictions of spectral positions for different barium isotopes. The analysis also accounted for hyperfine splitting, particularly for isotopes ^(135) and ^(137)Ba, using established formulas to calculate the intervals between different hyperfine levels. The ability to predict and control these spectral characteristics is paramount for scaling up trapped ion quantum technologies. By accurately modeling isotope and hyperfine shifts, researchers can fine-tune the excitation parameters to selectively load specific barium isotopes.

Three-Step Photoionization Scheme for Neutral Barium Atoms

The demonstrated scheme bypasses the need for pre-enriched barium isotopes by using a three-step photoionization process, beginning with selection of neutral barium atoms on a narrow 791 nanometer transition. The final stage, ionization, employs an existing 532 nanometer Raman beam, but it is the interplay of the first two steps that achieves high-purity loading. The team’s method, detailed in the paper, achieves “extremely high-purity loading” through this relayed selectivity, a phenomenon stemming from the initial two excitation steps.

A linear dependence between the loading rate and the intensity of the 532 nanometer ionization beam was observed within the parameters of the experiment, suggesting that further increases in laser power could yield even faster loading times. They note that the scheme is a significant advantage for simplifying experimental setups and reducing potential sources of error. In summary, they have demonstrated a three-step, ultraviolet-free scheme for isotope-selective loading from natural-abundance barium.

👉 More information
🗞 Rapid high-purity loading of barium ion via relayed selectivity from naturally abundant source
✍️ Zichun Zhou, Xuhui Yao, Jiaze Li, Yuncheng Xie, Haozhe Shi, Yanxin Liu, Weibin Chu and Lei Feng (Department of Physics, Fudan University, Shanghai 200433, China)
🧠 ArXiv: https://arxiv.org/abs/2611.11990

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