Ytterbium-171 Hyperfine Qubit

Figure

Description

The ytterbium-171 hyperfine qubit encodes quantum information in the two hyperfine clock states of the ${}^{171}{\text{Yb}}^{+}$ ion’s ${}^2{S}_{1/2}$ ground-state manifold: $|0⟩\equiv |F=0,m_F=0⟩$ and $|1⟩\equiv |F=1,m_F=0⟩$, separated by 12.642812 GHz. Because both states have $m_F=0$, the qubit is first-order insensitive to magnetic-field noise, and the transition frequency shifts only quadratically with $B$ near zero field.

The ${}^{171}{\text{Yb}}^{+}$ ion has nuclear spin $I=1/2$ and a simple $S_{1/2}$ ground-state hyperfine structure, which makes state preparation, microwave control, Raman control, and fluorescence readout comparatively clean. The main detection and cooling transition is ${}^2{S}_{1/2}\leftrightarrow {}^2{P}_{1/2}$ at 369.5 nm, with 935 nm repumping through the ${}^2{D}_{3/2}$ manifold. This combination of clock-state robustness, simple level structure, and high-quality optical control made ${}^{171}{\text{Yb}}^{+}$ the workhorse qubit species for commercial trapped-ion systems such as Quantinuum’s H-series processors.

Two-qubit entangling gates use shared motional modes of the ion chain, typically via Raman-driven geometric-phase or Mølmer-Sørensen-type interactions. In modern commercial systems, ${}^{171}{\text{Yb}}^{+}$ has also been combined with other ion species in dual-species architectures, where ytterbium provides sympathetic cooling or auxiliary functionality even when another species carries the data qubits.

Hamiltonian

A more complete ground-manifold Hamiltonian is

$H=A_{\mathrm{hfs}}\mathbf{I}\cdot \mathbf{J}+{\mu }_B\left(g_J{J}_z+g_I{I}_z\right)B,$

where $A_{\mathrm{hfs}}$ is the magnetic-dipole hyperfine constant, $\mathbf{I}$ is the nuclear spin, $\mathbf{J}$ is the electronic angular momentum, and $B$ is an external magnetic field.

For ${}^{171}{\text{Yb}}^{+}$, $I=J=1/2$, giving the ground-state hyperfine splitting

$\Delta E_{\mathrm{hfs}}=h\times 12.642812\mathrm{GHz}.$

The full field dependence is given by the Breit-Rabi formula. Near $B=0$, the clock transition $|F=0,m_F=0⟩\leftrightarrow |F=1,m_F=0⟩$ has no first-order Zeeman shift, so

${\omega }_{01}(B)\approx {\omega }_{01}(0)+\beta B^2,$

with $\beta /2\pi \approx 31kHz/m{\mathrm{T}}^2$ (equivalently about $310Hz/{\mathrm{G}}^2$). By contrast, the $|F=1,m_F=\pm 1⟩$ states acquire approximately linear Zeeman shifts at low field.

Motivation

Trapped-ion quantum computing needs qubits with long coherence, clean state preparation and measurement, and control compatible with high-fidelity microwave and laser operations. The ${}^{171}{\text{Yb}}^{+}$ clock-state qubit satisfies all three. It supports hour-scale coherence under dynamical decoupling, microsecond single-qubit gates, high-fidelity fluorescence readout, and reliable transport in QCCD architectures. Those properties are exactly why it became one of the dominant trapped-ion qubit implementations in both laboratory and commercial systems.

Experimental Status

First characterization, Olmschenk et al. (2007):

• Demonstrated manipulation and detection of a trapped ${}^{171}{\text{Yb}}^{+}$ hyperfine qubit.
• Established the canonical clock-state encoding and fluorescence-readout workflow.

Microwave control scaling, Shappert et al. (2013):

• Demonstrated spatially uniform microwave single-qubit control in a surface-electrode trap.
• Achieved $\pi$-rotations as fast as about 1 $\mu$s on the ${}^{171}{\text{Yb}}^{+}$ hyperfine transition.

Fast high-fidelity readout, Noek et al. (2013):

• Demonstrated 99.93% state-preparation-and-measurement fidelity for a ${}^{171}{\text{Yb}}^{+}$ hyperfine qubit.
• Used background-suppressed fluorescence detection on the 369.5 nm cycling transition.

Transport robustness, Kaufmann et al. (2018):

• Measured 99.9994% state fidelity per shuttling operation during repeated transport of a ${}^{171}{\text{Yb}}^{+}$ hyperfine qubit.
• Important evidence that QCCD transport can preserve hyperfine-qubit information with negligible added error.

Long coherence, Wang et al. (2021):

• Demonstrated a single-ion ${}^{171}{\text{Yb}}^{+}$ qubit with estimated coherence time exceeding one hour under dynamical decoupling.
• Showed how magnetic-noise suppression and sympathetic-cooling support can push physical clock-state coherence far beyond the raw $T_2^{\ast }$ limit.

Integrated-control fidelity, Hogle et al. (2023):

• Demonstrated ${}^{171}{\text{Yb}}^{+}$ single-qubit gate fidelities above 99.7% using scalable integrated photonic modulators.
• Relevant because it connects high-fidelity qubit control to an architecture compatible with large-scale optical fanout.

System-level deployment, Quantinuum H2 (2024):

• Quantinuum’s H2 processor used a fully connected ${}^{171}{\text{Yb}}^{+}$ QCCD register for logical-qubit and error-correction experiments.
• This includes the 4D surface-code demonstration on a commercial trapped-ion processor.

Ultra-long encoded coherence, Pi et al. (2026):

• Reported coherence beyond ten hours in a decoherence-free clock-qubit encoding built from a pair of ${}^{171}{\text{Yb}}^{+}$ ions.
• Shows that ytterbium clock qubits remain a leading platform for long-lived trapped-ion quantum memory.

Key Metrics

Metric	Value	Notes	Fidelity reference
$T_1$	Effectively infinite on circuit timescales	Ground-state hyperfine qubit	—
Hyperfine splitting	12.642812 GHz	Clock transition in ${}^2{S}_{1/2}$	Olmschenk et al. 2007
$T_2$ (single ion, DD)	>1 h	Best reported single-ion coherence under dynamical decoupling	Wang et al. 2021
$T_2$ (DFS pair)	>10 h	Pair-encoded decoherence-free clock qubit	Pi et al. 2026
1Q gate fidelity	>99.7%	Gate set tomography with integrated photonic modulators	Hogle et al. 2023
SPAM fidelity	99.93%	Background-suppressed fluorescence detection	Noek et al. 2013
1Q gate time	~1 $\mu$s	Microwave-driven $\pi$ rotations	Shappert et al. 2013
Transport fidelity	99.9994% per hop	280 $\mu$m shuttling operation in a microstructured trap	Kaufmann et al. 2018
System scale	56 qubits (H2, 2024)	Commercial all-to-all ${}^{171}{\text{Yb}}^{+}$ QCCD register	Berthusen et al. 2024

References

Original characterization

• S. Olmschenk et al., “Manipulation and detection of a trapped Yb⁺ hyperfine qubit,” Phys. Rev. A 76, 052314 (2007), arXiv:0708.0657

Control, transport, and readout

• C. M. Shappert et al., “Spatially uniform single-qubit gate operations with near-field microwaves and composite pulse compensation,” New J. Phys. 15, 083053 (2013), arXiv:1304.6636
• R. Noek et al., “High speed, high fidelity detection of an atomic hyperfine qubit,” Opt. Lett. 38, 4735 (2013)
• P. Kaufmann et al., “High-Fidelity Preservation of Quantum Information During Trapped-Ion Transport,” Phys. Rev. Lett. 120, 010501 (2018), arXiv:1704.02141

Coherence and integrated control

• P. Wang et al., “Single ion qubit with estimated coherence time exceeding one hour,” Nat. Commun. 12, 233 (2021)
• C. W. Hogle et al., “High-fidelity trapped-ion qubit operations with scalable photonic modulators,” npj Quantum Information (2023)
• J. Pi et al., “Beyond-Ten-Hour Coherence in a Decoherence-Free Trapped-Ion Clock Qubit,” arXiv:2603.19631 (2026)

System-level deployment

• N. Berthusen et al., “Experiments with the four-dimensional surface code on a quantum charge-coupled device quantum computer,” Phys. Rev. A 110, 062413 (2024), arXiv:2408.08865
• A. Ransford et al., “A 98-qubit trapped-ion quantum computer,” arXiv:2511.05465 (2025)

Linked Papers

• olmschenk-2007-yb171-qubit
• noek-2013-high-speed-detection
• wang-2021-single-ion-qubit

Related Entries

• trapped-ion-qubit — parent platform
• shuttling-ion-trap-qubit — QCCD transport architecture used in scalable ion processors
• cirac-zoller-gate — foundational trapped-ion gate proposal
• motional-mode-coupling-in-ion-traps — physics of the Coulomb-mediated bus
• coherence-time-hierarchy — context for coherence comparison