trapped-ion

An ion is an atom carrying an electric charge, which makes it something an electromagnetic field can grip. In a trapped-ion machine, individual atomic ions are confined in an electromagnetic trap inside a vacuum chamber, then cooled and addressed with laser beams. Each ion is one qubit, and the 0 and the 1 are two of the ion's own internal electronic energy states.

In practice the trap is divided into several zones. Static voltages applied to segmented electrodes move the ions between those zones, individually or in groups, and precisely timed laser pulses drive information between energy levels, whether to run a logic operation or to read the answer out. Some pairs of energy states are picked because they are insensitive to magnetic fields: drifting magnetic fields were the main source of error in NIST's own multi-zone experiments, and states chosen this way blunt that.

The appeal is that all ions of a given species and isotope are fundamentally identical, so the qubits do not vary from one to the next the way manufactured circuits do, and each ion has the same coherence time. That improves reproducibility and cuts the calibration a machine needs. The difficulty is scale. Reviews of the field are direct about it: the open task is to increase the number of trapped ions while mitigating decoherence and control errors.