1. In three lines / what to know first / where this sits in the series

• A trapped-ion qubit is a single atomic ion floated in vacuum by electric fields. 0 and 1 are two internal states of that atom (for example, hyperfine levels)[Ref. 1, 2]Sourced
• Ions of one species are all identical and can be controlled precisely with laser light or microwaves. High precision has been reported, such as an average two-qubit-gate infidelity of 7.9×10⁻⁴ (average over all zones of a 98-qubit machine). On the other hand, gates are slow (about 70 μs in one example)[Ref. 2, 1]Sourced
• Scaling up brings the difficulty of more electrodes, lasers and wiring. The QCCD approach that moves ions around, optics built into the chip, and control by electrical signals are advancing as answers[Ref. 3, 4, 5, 6]Sourced

2. Recap: a qubit is an "arrow"

In this series, a qubit's state is drawn as an arrow from the center of a sphere to its surface. Pointing at the north pole means "0," pointing at the south pole means "1," and pointing in between is a superposition. When the arrow is tilted, a measurement always gives 0 or 1, and the chance of each is set by the tilt. After the measurement the arrow is fixed at a pole, and the old tilt does not come back.

3. How is an ion floated? (the Paul trap)

(1) Why an "ion"?

An atom is electrically neutral, so an electric field cannot grab it. So one electron is stripped away to make a positively charged "ion." Because it carries charge, electric fields can push and pull it and hold it in mid-air. Ions used for qubits include barium (Ba⁺), ytterbium (Yb⁺) and calcium (Ca⁺)[Ref. 1, 2]Sourced.

(2) Static forces alone cannot hold it, so the field is shaken

Electric forces alone cannot confine an ion in all three directions. A "hill" always appears in some direction, and the ion rolls off (Earnshaw's theorem; commentary). So the direction of the electric field is flipped tens of millions of times per second (radio frequency, RF). On average the ion is pushed back to the center and floats between the electrodes. This is the Paul trap. It is often explained by placing a ball on a saddle-shaped surface and spinning the saddle so the ball no longer falls off (analogy; limits: the ball involves gravity and the ion electric force, and only the shape "valley and hill swap with direction" is shared).In traps for quantum computing the ion–electrode distance is 30 μm to 1 mm, the RF voltage is 10–1,000 V, and the frequency is 10–100 MHz. In ultra-high vacuum an ion can be held for hours, days and in some cases months[Ref. 1]Sourced.

(3) After floating, cool it with lasers

A floated ion is still shaking with heat. So the ion is cooled with laser light (laser cooling), using the process in which the ion absorbs and re-emits light to slow its motion. The limit of the first stage of cooling (the Doppler limit) is a few hundred μK (as explained in a technical blog by IonQ)[Ref. 7]Sourced. The important point is that even though the ion is cooled, the chip (the trap) does not need to be at a cryogenic temperature. Compared with the roughly 20 mK refrigerator of superconducting qubits, a trapped-ion trap can run at room temperature (Ballance et al. used a room-temperature trap)[Ref. 8]Sourced. Even so, a trap is sometimes cooled to about 4 K to reduce noise from the electrode surface (Chapter 9).

4. The qubit is "two states inside an atom"

An electron inside an atom can sit only on "steps" of fixed energy (energy levels). A trapped-ion qubit takes two convenient steps as 0 and 1. The commonly used pair is two steps of very close energy (hyperfine levels, separated by about a GHz), moved between with microwaves or two laser beams (a Raman transition)[Ref. 1]Sourced. A pair of a ground level and a level with a lifetime of seconds (an optical qubit) is also used[Ref. 1]Sourced.

Here is a big difference from superconducting qubits. A superconducting qubit is a circuit that people designed, and each one comes out slightly different. An ion is an atom made by nature, and all ions of the same species and isotope are identical. Every ion responds to the same microwave or laser frequency and has the same coherence time. Less tuning and high reproducibility underlie the precision of trapped ions[Ref. 1]Sourced.

Some state-holding times are very long even compared with other approaches. A 171Yb⁺ "clock-state" qubit (a state insensitive to magnetic field) holds on the order of seconds, and 600 seconds was reached with dynamical decoupling (using pulses to cancel disturbances)[Ref. 1]Sourced. In Gaebler et al.'s 9Be⁺ the memory coherence time exceeded 1 second, and Wang et al. reported a value estimated to exceed one hour for 171Yb⁺ (the paper's title says "estimated")[Ref. 9, 10]Sourced. To suppress magnetic-field fluctuations, mu-metal magnetic shielding and permanent magnets gave 300 ms (2.1 s with dynamical decoupling) in one case[Ref. 1]Sourced. These are values from different laboratories measuring different things in different ways; they are not to be set beside values of other kinds of qubits to rank them.

5. Control: one-qubit and two-qubit gates

(1) One-qubit control: turning the arrow

Microwaves matched to the gap between 0 and 1 (or a frequency equal to the difference between two laser beams) turn the arrow. The pulse's length and strength set the angle. To do this to a chosen ion, the laser is focused on one ion, or the electrical signals are varied from ion to ion. Gaebler et al. lowered the one-qubit-gate error to 3.8×10⁻⁵ in 9Be⁺, and Ballance et al. showed one-qubit-gate fidelity of 99.9934(3)% in 43Ca⁺[Ref. 9, 8]Sourced. The Helios paper reports an average one-qubit-gate infidelity of 2.5×10⁻⁵[Ref. 2]Sourced.

(2) Two-qubit control: using the "shared motion" as a go-between

Ions carry the same charge and repel one another, so inside the trap's valley they sway together like beads joined by springs. A two-qubit gate connects two ions' states using this shared motion as a go-between[Ref. 1]Sourced. The idea was proposed by Cirac and Zoller in 1995[Ref. 11], and Sørensen and Mølmer showed in 1999 a form that works even with leftover thermal motion (the Mølmer–Sørensen gate)[Ref. 12]Sourced.

Examples of precision (definitions and conditions differ): Ballance et al. studied two-qubit-gate fidelity of 99.9(1)% in a room-temperature trap with 43Ca⁺ over gate times of 3.8 μs to 520 μs, showing that going faster lowers fidelity (a speed–precision trade-off)[Ref. 8]Sourced. Gaebler et al. estimated the two-qubit-gate error in 9Be⁺ as 8(4)×10⁻⁴[Ref. 9]Sourced. The paper on Quantinuum's Helios (98 qubits) reports an average two-qubit-gate infidelity of 7.9(2)×10⁻⁴ (average over all zones) and an average state-preparation-and-measurement (SPAM) infidelity of 4.8(6)×10⁻⁴ (as stated in the preprint; after peer review it was published in Nature on 17 June 2026)[Ref. 2]Sourced.

6. Readout: bright or dark

Readout in a trapped-ion machine is easy to picture. When a readout laser is applied, only ions in one state (for example |1⟩) absorb the light and keep emitting fluorescence (re-emitted light). Ions in the other state (|0⟩) do not glow and stay dark. Counting the photons (particles of light) that come out with a camera or photodetector tells you bright (1) or dark (0)[Ref. 1]Sourced. High-fidelity state preparation and readout have been done in under 1 ms[Ref. 1]Sourced.

Very little light can be collected at once. A 2019 review summarized the overall efficiency of collecting an ion's fluorescence at roughly 2–4%. Superconducting-nanowire single-photon detectors (which work only at cryogenic temperatures) have reported efficiencies of about 0.8 for ultraviolet photons[Ref. 1]Sourced. Even so, a few photons are enough to decide, so readout can be done in under 1 ms. Helios's SPAM (preparation and measurement errors together) averages 4.8×10⁻⁴[Ref. 2]Sourced. Other approaches amplify a faint resonator signal (superconducting), take a picture (neutral atoms) or count photons (photonic), whereas ions are distinctive in that you look directly at "bright or dark."

7. The key points on a common "card," and a side-by-side with the other three approaches

Here is everything so far as a "card" that answers the series' five shared questions (what it is made of, what 0 and 1 are, how it is controlled, how it is read, strengths and weaknesses). The other three articles use the same card.

Reading the four approaches in the same format

The arrangement is commentary; the numbers and facts in each cell come from the references (checked 16 October 2026). The numbers are different quantities under different conditions, so please do not rank the approaches by them.

8. Scaling up: the QCCD that moves ions, and wiring

Lining ions up in a single row to add qubits runs into a limit: the more ions in one row, the slower two-qubit gates generally become (the ions are farther apart and couple more weakly to the shared motion)[Ref. 1]Sourced. So the idea arose of moving ions a few at a time. Kielpinski et al. proposed it in 2002 as the "quantum charge-coupled device (QCCD)"[Ref. 3], and Pino et al. demonstrated it in 2021 with a cooled trap chip combining all the necessary ingredients[Ref. 4]Sourced. Only the ions to be gated are carried to a logic zone, unused ions rest in a memory zone, and junctions route ions to where they need to go.

Helios: moving 98 ions

Quantinuum's Helios is a 98-qubit QCCD machine. It uses hyperfine levels of barium (137Ba⁺) as qubits, and a four-way "X" junction connects a rotatable ion storage ring to two logic regions. The logic region has eight zones and handles up to 16 qubits at a time. Ions heated by moving are re-cooled by cooling ytterbium (171Yb⁺) ions trapped alongside them with laser light at 369 nm, which draws off the heat (sympathetic cooling)[Ref. 2]Sourced. The two-qubit gate itself takes about 70 μs, and the paper notes an additional cooling of about 300 μs after the move[Ref. 2]Sourced.

To keep wiring from growing too much as machines scale, a design that "broadcasts" the same voltage to many electrodes is used (Helios paper). Independent control signals per qubit fell from 9.9 in H1 and 4.8 in H2 to 2.8 in Helios[Ref. 2]Sourced. On another path, a team at Oxford Ionics (now part of IonQ) showed how to control ions with electrical signals — current-carrying wiring on the chip — instead of lasers. In a seven-zone trap that can control up to 10 qubits, they reported one-qubit-gate fidelity of 99.99916(7)% and creation of maximally entangled two-qubit states at 99.97(1)%[Ref. 6]Sourced. In October 2025 an IonQ team announced on arXiv that electrically controlled two-qubit gates, without cooling the ions to the ground state, had an estimated error of 8.4(7)×10⁻⁵ (preprint)[Ref. 34]Sourced. IonQ announced this on its blog as a "99.99%" two-qubit-gate fidelity and said the same technology will form the basis of 256-qubit systems scheduled for demonstration in 2026 (the company's claim; completion of a 256-qubit machine could not be confirmed here)[Ref. 7]Not yet confirmed.

9. Materials, optics and vacuum

An ion merely floats 30 μm to 1 mm away from the electrodes, so the state of the electrode "surface" affects the ion directly. The issues relevant to materials engineers are summarized below, by the numbers in the figure.

(1) Electrodes and substrate: built with semiconductor techniques

The mainstream trap today is the surface trap, in which metal electrodes are patterned on a substrate. Substrates include sapphire, quartz and silicon; they can be made on wafers up to 12 inches, and tiling small chips has been proposed[Ref. 1]Sourced. Compared with older traps assembled from machined metal parts, the advantage is that fine electrode shapes can be made reproducibly in volume (commentary).

(2) Surface noise of the electrodes: the most materials-like challenge

There is a little electric-field noise on the electrode surface. Ions are very sensitive to it and heat up by themselves (anomalous heating). Brownnutt et al. reviewed electric-field-noise measurements in ion traps and summarized that there are several causes and much is poorly understood[Ref. 35]Sourced. According to the review by Bruzewicz et al., cooling the electrodes from room temperature to about 4 K reduces anomalous heating by about two orders of magnitude regardless of material. Even at room temperature, laser irradiation, plasma treatment and ion milling (scraping the surface with ions) reduce the electric-field noise, and surface contamination and morphology are the suspected causes. For ion–electrode distances of 40–80 μm, such treatments are said to make heating low enough for fault-tolerant codes with high thresholds[Ref. 1]Sourced.

(3) Optics: building it into the chip

Ions need laser light at several wavelengths from ultraviolet to infrared (Helios uses 369 nm for cooling ytterbium)[Ref. 2]Sourced. Beams used to be delivered with optical parts set up in the air, which is sensitive to vibration and misalignment and cannot be assembled at large scale. So research advanced on building light paths (waveguides) into the chip, and gratings that direct light to the ion in mid-air. Mehta et al. connected optical fibers directly to a cooled trap and showed high-fidelity gates on several ions[Ref. 5]Sourced, and Niffenegger et al. delivered all the wavelengths from violet to infrared needed for ionization, cooling, control and readout of strontium ions (Sr⁺) through on-chip waveguides (CMOS-compatible fabrication)[Ref. 36]Sourced. The materials and manufacturing of optical parts are covered in our explainers on silicon nitride waveguides and silicon photonics.

10. Where things stand (checked 16 October 2026) and plans

(1) Quantinuum Helios (a peer-reviewed paper by a company research team)

Helios is a QCCD machine using 98 barium ions as qubits, with all qubits fully connected (any two can be gated directly). Averaged over all zones it reports infidelities of 2.5(1)×10⁻⁵ for one-qubit gates, 7.9(2)×10⁻⁴ for two-qubit gates and 4.8(6)×10⁻⁴ for state preparation and measurement. The authors state that random-circuit sampling shows the machine operating beyond the reach of classical simulation (the authors' claim)[Ref. 2]Sourced. The paper was published in Nature on 17 June 2026[Ref. 2]Sourced. We could not confirm a reproduction of the same performance by another laboratory.

(2) The earlier generation and the move to electrical control

H2, reported by Moses et al. in 2023 (a race-track trap, initially 32 qubits), had an average state-preparation-and-measurement infidelity of 1.6(1)×10⁻³, one-qubit-gate infidelity of 2.5(3)×10⁻⁵ and two-qubit-gate infidelity of 1.84(5)×10⁻³, and measured a quantum volume of QV = 2¹⁶[Ref. 37]Sourced. As seen in Chapter 8, the Oxford Ionics and IonQ team reported control by electrical signals and two-qubit gates without cooling to the ground state (a preprint and a company blog)[Ref. 6, 34, 7]Sourced.

(3) Plans (not yet confirmed)

In its official blog, IonQ mentions a roadmap toward millions of qubits by 2030 and 256-qubit systems scheduled for demonstration in 2026 (a company plan)[Ref. 7]Not yet confirmed. As of the check date we could not confirm an official announcement that a 256-qubit machine was completed, nor the progress of other companies' plans. For any planned number, check whether it counts physical or logical qubits and under what error-rate conditions.

11. Limits, what we could not confirm, and a summary

(1) Full texts of standards

We did not read the full text of ISO/IEC 4879:2024, which is paid. For IEEE P7130 and P7131 we could not retrieve the IEEE pages and confirmed only the existence of the projects through StandICT.eu.

(2) Company claims

IonQ's 99.99% is a company blog claim based on an "estimated" value in an arXiv paper. The Helios result is in a peer-reviewed paper, but we have not confirmed reproduction by other laboratories.

(3) Date of the review

The review by Bruzewicz et al. is from 2019, and the "best values" and "fastest gates" it cites may have been surpassed since. We supplement it with 2023–2026 papers where possible.

(4) Approaches not covered

We do not cover Penning traps, molecular ions, the detailed operation of optical qubits, or networking (photon links between ions).

(5) Progress on plans

Progress on plans such as IonQ's 256-qubit machine is stated only as far as official reports of achievement could be confirmed.

12. How to read evaluations and standards

A trapped-ion machine's performance is read through gate error rates (one- and two-qubit), state-preparation and measurement errors, state-holding time and the quality of the whole machine. High precision is its hallmark, but each number changes with "which pair, averaged over what, measured how." For standards, vocabulary has come first, while standards for how to measure performance are still at the project stage (checked 16 October 2026).

The arrangement of the table is Our calculation (organization). Because the standards' full texts are paid, we give only titles and status (checked 16 October 2026).

Cautions in reading (for materials engineers)

• "How many qubits" alone does not tell performance. In ions, lengthening a row slows gates, and the time to move ions in a QCCD is added[Ref. 1, 2]
• Check average versus best, one pair versus all pairs. Helios's 7.9×10⁻⁴ is an average over all zones; Ballance et al.'s 99.9% is a conditional value for one pair[Ref. 2, 8]
• A company's announcement and third-party verification are different. IonQ's 99.99% is the company's claim based on an "estimated" value; we have not confirmed a reproduction by another laboratory[Ref. 34, 7]
• Check whether a standard is a recommendation or a requirement, and whether it is legally binding. ISO/IEC 4879 is a vocabulary standard and does not guarantee performance[Ref. 40]. Plans are Not yet confirmed

13. Glossary

14. References (primary sources)

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15. Claim-to-source audit

Last updated: 16 October 2026. Sources are limited to primary ones (peer-reviewed papers, arXiv papers, ISO's official page, and a company blog treated as the company's claim). The standards' full texts are paid and were not read. The readout numerical example is our own assumed calculation, not the performance of any real machine. We could not confirm reproductions by other laboratories, the IEEE official pages, or completion of IonQ's 256-qubit machine, so we do not state them. All figures are explanatory concept diagrams or drawings that include our own calculations; Figures 1 and 12 are ImageGen-generated conceptual illustrations.