Trapped-Ion Qubit

A trapped-ion qubit encodes information in the internal electronic (or hyperfine) energy levels of a single charged atom, held in a vacuum chamber by oscillating electromagnetic fields and cooled with lasers until it is nearly motionless. Gates are applied with laser pulses rather than microwave wiring, and because every ion in the same trap is identical and can be linked to every other ion through their shared motional modes, trapped-ion systems get all-to-all qubit connectivity without the wiring problem that limits chip-based modalities. That connectivity, combined with gate fidelities that are currently among the highest of any modality (Quantinuum has reported two-qubit gate fidelities above 99.9%), comes at the cost of gate speed: laser-based gates take microseconds rather than the tens of nanoseconds typical of superconducting qubits. IonQ and Quantinuum are the two companies building commercial systems on this modality. Scaling trapped-ion systems past a few hundred qubits is now mostly an interconnect and trap-architecture problem rather than a gate-fidelity one.