Trapped-Ion Qubit

Definition

Trapped-ion qubits use individual ions (electrically charged atoms) held in place by electromagnetic fields in a vacuum chamber, with their internal energy states used to represent quantum information. Lasers are typically used to manipulate and read out the ions' states.

In depth

A trapped-ion qubit is an individual charged atom, such as ytterbium, calcium, or barium, held in place by electric fields inside a vacuum chamber, with two of its internal energy levels serving as 0 and 1. Because every ion of a species is identical, the qubits are uniform in a way that fabricated devices are not, and the internal states are naturally isolated from the environment, so coherence times run from seconds to minutes. Lasers cool the ions, set their states, perform gates, and read them out by fluorescence.

Gates between ions use their shared motion. A laser pulse couples each ion's internal state to the collective vibration of the chain, which entangles ions. The Mølmer-Sørensen gate does this for any pair in a chain, which gives all-to-all connectivity within a trap. Two-qubit fidelities of 99.9 percent are routine on leading machines, and 99.99 percent has been reported in laboratory and company results. The costs are speed, with gates taking tens to hundreds of microseconds, and scale, since long chains become hard to control. Quantinuum's QCCD design moves ions between zones, and IonQ plans to link small traps with photons.

Ion machines are among the most accurate available. Quantinuum's H2 reached a quantum volume of more than 33 million in 2025, and Oxford groups reported single-qubit error rates near 10 to the minus 7. The main companies are IonQ, Quantinuum, and Alpine Quantum Technologies, plus Oxford Ionics, now part of IonQ. David Wineland shared the 2012 Nobel Prize in Physics for the techniques that made trapped-ion quantum control possible.

Technical Definition

Trapped-ion systems often achieve all-to-all qubit connectivity (any qubit can interact with any other) and notably long coherence times compared to superconducting alternatives, though gate operations are typically slower, on the order of microseconds rather than nanoseconds.

Visual Explanation: An Analogy

Think of trapped ions like marbles held in place by an invisible magnetic force field, arranged in a row — lasers act like precise tools that can be aimed at any individual marble (or pair of marbles) to perform an operation.

Key facts

Where the idea comes from

Cirac and Zoller proposed ion trap quantum computing in 1995, and the first CNOT was demonstrated by Monroe and Wineland's group at NIST that year. Commercial systems from IonQ and Honeywell, now Quantinuum, appeared in 2019 and 2020.

Real-World Use Cases

• Used by companies like IonQ and Quantinuum
• Particularly well suited to algorithms requiring high connectivity between qubits
• Often used in record-setting two-qubit gate fidelity demonstrations

How it differs from related ideas

• Ions are slower than superconducting qubits but more accurate and more connected.
• Compared with neutral atoms, ions are charged and interact strongly, which makes gates easy but limits chain length.
• Ion qubits are identical by nature, while superconducting qubits vary from device to device.

Frequently asked questions

Related Terms

• Qubit
• Superconducting Qubit
• Coherence Time
• QCCD Architecture
• Gate Fidelity

Keep exploring

• Algorithm: Quantum Error-Correcting Codes (Shor Code, Surface Code)
• Hardware: IonQ Forte (and Forte Enterprise)
• Hardware: Quantinuum H2
• Hardware: IonQ Tempo

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