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arXiv:2604.16786

Engineering magnetically insensitive qubits in metastable electronic D-states of trapped ions

Ksenia Sosnova, Martin Lichtman, Allison Carter, Nora Crocker, Christopher Monroe

quant-phphysics.atom-phSubmitted Apr 18, 2026• Announced Apr 21, 2026• 0 citations
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Abstract

Ion trap quantum computers often store qubits on field-sensitive S_1/2 ground state Zeeman levels of the valence electron, such as in 40Ca+, 88Sr+, and 138Ba+ atomic systems. We experimentally synthesize magnetically insensitive qubit states in multiple metastable electronic D_3/2 Zeeman levels in such an atomic system. We demonstrate coherent operations within the D_3/2 manifold of 138Ba+, including coherent flopping between the synthesized qubit states, and our results agree with theory. Such an encoding may allow for more flexible use of atomic levels for photonic interfaces, and with a measured improvement in the qubit coherence time T2* by a factor of 3, this lays the foundation for further improvement for quantum computing and network applications.

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Trapped ions offer a controlled and versatile platform for the simulation of spin and spin-boson quantum systems. The Coulomb interaction between ions trapped in a harmonic potential can be used to mediate interactions between effective spins in the ion chain. Generally, coherent operations on ions are performed on ground-state qubits encoded in magnetically insensitive hyperfine sublevels. In this thesis, we extend this toolbox with the aim of simultaneously manipulating both ground-state and optical qubits, with the latter encoded in metastable optically excited states. The optical qubit provides another set of coherent and dissipative operations that can be performed on the ions, opening new avenues for quantum computing and simulation. In our experiment, we use 171Yb+ and 172Yb+ ions to encode ground-state and optical qubits. We discuss the laser setup needed to address the narrow quadrupole transition from 2S1/2 →2 D3/2 of 3.02 Hz linewidth, and we describe spectroscopy techniques to study this transition in both the isotopes. We describe how the low linewidth allows resolved sideband cooling on the shared motional modes in an ion chain, enabling sympathetic cooling with one ion, 172Yb+ , while simultaneously performing coherent operations on the 171Yb+ ion. Finally, we discuss the prospects of using this tool for light-shift entangling gates, showing the flexibility of this trapped-ion system for simulating coherent as well as dissipative quantum systems.

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We demonstrate a trapped-ion protocol in which a nearby, dedicated monitor qubit tracks magnetic field drifts in real time without interrupting data-qubit operations. Using two $^{40}\mathrm{Ca}^{+}$ ions and the optical-metastable-ground architecture, we encode the data qubit in the ground-state manifold and the monitor qubit in a metastable-state manifold to achieve spectral separation. The monitor qubit senses common magnetic fluctuations during data-qubit experiments, enabling feedforward corrections to the qubit-control drives. Under applied magnetic noise with a realistic spectrum ($1/{f}^{2}$), the protocol maintains coherence and, when compared with interleaved calibration, it extends usable data-qubit probe times by up to a factor of approximately $\sqrt{2}$ and doubles the experimental duty cycle. These results establish monitor qubits as a scalable tool for real-time recalibration in quantum information processors.

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Engineering large-scale quantum computers which simultaneously provide high-fidelity quantum operations, low memory errors, low crosstalk, and reasonable resource usage remains an outstanding challenge across quantum computing platforms. In trapped ions, progress has largely focused on alkaline-earth and ytterbium ions, whose simple electronic structures facilitate control over their internal state. Here we investigate singly-ionized yttrium ($^{89}\mathrm{Y}^+$), a two-valence-electron ion whose ground-state manifold hosts a nuclear-spin qubit and which also features a variety of low-lying metastable manifolds, for applications in quantum information processing. Because experimental data are limited, we perform high-resolution laser-induced fluorescence spectroscopy to measure the hyperfine structure of several low-lying levels, and carry out comprehensive electronic structure calculations to determine lifetimes, transition matrix elements, and hyperfine coefficients for manifolds addressable with visible, near-visible, or infrared wavelengths. Using these results, we analyze schemes for qubit storage, initialization, readout, leakage mitigation, and single- and two-qubit gates. These results position $^{89}\mathrm{Y}^+$ as a uniquely capable next-generation trapped-ion qubit, combining field-insensitive nuclear-spin or clock-qubit storage with spectrally isolated transitions for operations.

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A magnetic-field insensitive gate set for trapped-ion nuclear spin qubits

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We outline a complete set of operations for manipulating nuclear spin qubits of trapped-ions stored in high magnetic fields such as required for Penning trapping, where the nuclear and electron spins are largely decoupled due to the dominance of the external field Hamiltonian. The reduced nuclear magnetic moment results in insensitivity to external magnetic fields compared to the use of an electron spin, but also makes the nuclear spin hard to manipulate on fast timescales. To maintain speed, we propose a two-qubit phase gate technique which utilizes the electron spin, but for which the qubit remains protected from magnetic fields through retaining the nuclear encoding. This method works for magnetic-field insensitive qubits at lower fields as well as for both microwave and laser field gradients.

quant-pharXiv:2609.37318•Sep 30, 2026•0 citations
Ion trap quantum computers often store qubits on field-sensitive S_1/2 ground state Zeeman levels of the valence electron, such as in 40Ca+, 88Sr+, and 138Ba+ atomic systems. We experimentally synthesize magnetically insensitive qubit states in multiple metastable electronic D_3/2 Zeeman levels in such an atomic system. We demonstrate coherent operations within the D_3/2 manifold of 138Ba+, including coherent flopping between the synthesized qubit states, and our results agree with theory. Such an encoding may allow for more flexible use of atomic levels for photonic interfaces, and with a measured improvement in the qubit coherence time T2* by a factor of 3, this lays the foundation for further improvement for quantum computing and network applications.
Trapped ions offer a controlled and versatile platform for the simulation of spin and spin-boson quantum systems. The Coulomb interaction between ions trapped in a harmonic potential can be used to mediate interactions between effective spins in the ion chain. Generally, coherent operations on ions are performed on ground-state qubits encoded in magnetically insensitive hyperfine sublevels. In this thesis, we extend this toolbox with the aim of simultaneously manipulating both ground-state and optical qubits, with the latter encoded in metastable optically excited states. The optical qubit provides another set of coherent and dissipative operations that can be performed on the ions, opening new avenues for quantum computing and simulation. In our experiment, we use 171Yb+ and 172Yb+ ions to encode ground-state and optical qubits. We discuss the laser setup needed to address the narrow quadrupole transition from 2S1/2 →2 D3/2 of 3.02 Hz linewidth, and we describe spectroscopy techniques to study this transition in both the isotopes. We describe how the low linewidth allows resolved sideband cooling on the shared motional modes in an ion chain, enabling sympathetic cooling with one ion, 172Yb+ , while simultaneously performing coherent operations on the 171Yb+ ion. Finally, we discuss the prospects of using this tool for light-shift entangling gates, showing the flexibility of this trapped-ion system for simulating coherent as well as dissipative quantum systems.
We demonstrate a trapped-ion protocol in which a nearby, dedicated "monitor" qubit tracks magnetic-field drifts in real time without interrupting data-qubit operations. Using two 40Ca+^{40}\mathrm{Ca}^+40Ca+ ions and the optical--metastable--ground architecture, we encode the data qubit in the ground-state manifold and the monitor qubit in a metastable-state manifold to achieve spectral separation. The monitor qubit senses common magnetic fluctuations during data-qubit experiments, enabling feedforward corrections to the qubit-control drives. Under applied magnetic noise with a realistic spectrum (1/f21/f^{2}1/f2), the protocol maintains coherence and, when compared with interleaved calibration, it extends usable data-qubit probe times by up to a factor of ∼2{\sim}\sqrt{2}∼2​ and doubles the experimental duty cycle. These results establish monitor qubits as a scalable tool for real-time recalibration in quantum information processors.
We demonstrate a trapped-ion protocol in which a nearby, dedicated monitor qubit tracks magnetic field drifts in real time without interrupting data-qubit operations. Using two 40Ca+^{40}\mathrm{Ca}^{+}40Ca+ ions and the optical-metastable-ground architecture, we encode the data qubit in the ground-state manifold and the monitor qubit in a metastable-state manifold to achieve spectral separation. The monitor qubit senses common magnetic fluctuations during data-qubit experiments, enabling feedforward corrections to the qubit-control drives. Under applied magnetic noise with a realistic spectrum (1/f21/{f}^{2}1/f2), the protocol maintains coherence and, when compared with interleaved calibration, it extends usable data-qubit probe times by up to a factor of approximately 2\sqrt{2}2​ and doubles the experimental duty cycle. These results establish monitor qubits as a scalable tool for real-time recalibration in quantum information processors.
Engineering large-scale quantum computers which simultaneously provide high-fidelity quantum operations, low memory errors, low crosstalk, and reasonable resource usage remains an outstanding challenge across quantum computing platforms. In trapped ions, progress has largely focused on alkaline-earth and ytterbium ions, whose simple electronic structures facilitate control over their internal state. Here we investigate singly-ionized yttrium (89Y+^{89}\mathrm{Y}^+89Y+), a two-valence-electron ion whose ground-state manifold hosts a nuclear-spin qubit and which also features a variety of low-lying metastable manifolds, for applications in quantum information processing. Because experimental data are limited, we perform high-resolution laser-induced fluorescence spectroscopy to measure the hyperfine structure of several low-lying levels, and carry out comprehensive electronic structure calculations to determine lifetimes, transition matrix elements, and hyperfine coefficients for manifolds addressable with visible, near-visible, or infrared wavelengths. Using these results, we analyze schemes for qubit storage, initialization, readout, leakage mitigation, and single- and two-qubit gates. These results position 89Y+^{89}\mathrm{Y}^+89Y+ as a uniquely capable next-generation trapped-ion qubit, combining field-insensitive nuclear-spin or clock-qubit storage with spectrally isolated transitions for operations.
Over the past decade, the field of trapped ion quantum computing has emerged as one of the leaders in quantum information processing due the level of manipulation available and the long coherence times possible in the system. As this thesis will demonstrate, all of the necessary building blocks for a quantum computer have been exhibited in ion traps and small scale quantum algorithms have been implemented. In this trapped ion system, quantum bits consist of the first order magnetic field insensitive ground state hyperfine levels of 111^{111}111Cd+^++. The qubits are manipulated via resonant and off-resonant coherent laser interactions. We experimentally realize Grover's quantum search algorithm over a 4 element database with n=2 trapped 111^{111}111Cd+^++ ion qubits. One of the four states is marked, and with a single query it is recovered, on average, with 60% probability. This exceeds the performance of any possible classical search, which can only succeed with 50% probability following a single query. The algorithm consists of two Molmer-Sorensen entangling gates, that utilize bichromatic stimulated Raman transitions to create a spin dependent force, paired with several single-qubit rotations and near-perfect qubit measurements. The spectral arrangement of the Raman beams is tailored to suppress phase noise accumulation between gates. This suppression is critical for reliably performing consecutive operations during the algorithm. Additionally, this thesis discusses the possibility of combining trapped ions with trapped neutral atoms for the purpose studying ultra-cold charge exchange interactions. It may be possible to conceal quantum information, initially prepared in an ionic qubit, inside a pure nuclear spin qubit for the purpose of transportation and storage. As a first step, we present the laser-cooling and confinement of Cd atoms in a magneto-optical trap, and characterize the loading process from the background Cd vapor. The trapping laser drives the 1S0^{1}S_{0}1S0​~rightarrowrightarrowrightarrow~1P1^{1}P_{1}1P1​ transition at 229 nm in this two valence electron atom and also photoionizes atoms directly from the 1P1^{1}P_{1}1P1​ state. This photoionization dominates other loss mechanisms and allows a direct measurement of the photoionization cross section.
We outline a complete set of operations for manipulating nuclear spin qubits of trapped-ions stored in high magnetic fields such as required for Penning trapping, where the nuclear and electron spins are largely decoupled due to the dominance of the external field Hamiltonian. The reduced nuclear magnetic moment results in insensitivity to external magnetic fields compared to the use of an electron spin, but also makes the nuclear spin hard to manipulate on fast timescales. To maintain speed, we propose a two-qubit phase gate technique which utilizes the electron spin, but for which the qubit remains protected from magnetic fields through retaining the nuclear encoding. This method works for magnetic-field insensitive qubits at lower fields as well as for both microwave and laser field gradients.