Trapped-Ion Qubits
A trapped-ion qubit is a single charged atom held in vacuum by electromagnetic fields, with gates driven by lasers and two-qubit interactions mediated by the ions' shared motion. Any pair in a chain can interact directly and demonstrated fidelities are the best of any platform, but gates run thousands of times slower than superconducting ones.
What is a trapped ion?
An ion is an atom that has lost (or gained) an electron and therefore carries a net electric charge. Charge is the handle: electric fields push on charged particles, so a carefully shaped, rapidly oscillating electromagnetic field can hold an ion at a fixed point in space. Do this inside an ultra-high-vacuum chamber and the ion hovers, touching nothing, isolated from almost everything that could disturb it.
The qubit is stored in two internal energy states of the ion — two configurations of its electrons and nucleus — chosen because they are extraordinarily stable. Stored quantum information survives for seconds or longer, compared with tens to hundreds of microseconds for a superconducting qubit.
One more contrast with fabricated qubits: every ion of the same isotope is literally identical, guaranteed by physics rather than by manufacturing tolerance. There is no unit-to-unit variation to calibrate away, though the lasers and electronics controlling the ions still need careful, continuous calibration of their own.
You’ve read the opening of chapter 4 — 6 more sections follow, with worked examples and circuits you can run on the page. A free account unlocks every chapter of every course (paid plans aren’t live yet — early readers get everything free).