Trapped-Ion Qubits
Trapped-ion qubits use individual charged atoms held in vacuum by electric fields, offering long coherence and all-to-all connectivity.
Atoms as qubits
A trapped-ion qubit is a single ionized atom, commonly ytterbium, calcium, barium, or beryllium, confined in ultra-high vacuum by oscillating and static electric fields. Because every ion of a species is identical, the qubits are naturally uniform, unlike fabricated solid-state qubits that vary chip to chip.
Qubit encodings
- Hyperfine qubits: two ground-state sublevels split by a few GHz, controlled with microwaves or Raman lasers, with coherence times up to seconds or longer
- Optical qubits: a ground state and a long-lived metastable state separated by an optical transition, controlled with a narrow-linewidth laser
- Zeeman qubits: sublevels split by a magnetic field
Strengths
Ions hold the records for single- and two-qubit gate fidelity, often exceeding 99.9 percent, and for coherence time. Because ions in a chain share collective motional modes, any pair can interact through those modes, giving full all-to-all connectivity within a chain, which reduces the routing overhead that limits fixed-lattice devices.
Limits
Gates are slow, microseconds to tens of microseconds, because they rely on moving the ions' shared motion. A single chain cannot grow indefinitely: more ions crowd the motional spectrum and slow gates. Scaling therefore turns to shuttling ions between zones (QCCD) or linking many small traps with photonic interconnects.
Trapped ions and superconducting circuits are the two most mature gate-model modalities, trading speed for fidelity and connectivity in opposite directions.