A trapped ion qubit is a type of physical qubit, which consists of an atomic ion suspended, or trapped, in an electromagnetic field — an ion being an electrically charged atom, it is sensitive to electromagnetic fields. Trapped ions were the first physical qubits to be implemented, in 1995, just one year after Shor proposed the factoring algorithm. Trapped ion quantum computers are being developed by several companies, such as IonQ, Quantinuum, Quantum Factory, Alpine Quantum Technologies and Oxford Ionics.
Trapped ion quantum computer
Trapped ions quantum computer rely on ions confined in free space using electromagnetic fields. The ions are then slowed down and organized in a crystal like structure using laser cooling, a technique which is now a cornerstone of atomic physics. The principle of laser cooling is the following: when an atom (or molecule) is illuminated by a laser, it will absorb and re-emit a photon, which changes its momentum. Choosing the frequency of the laser properly allows to reduce the motion of the atom. We mention that the 1997 physics Nobel prize was awarded to Claude Cohen-Tannoudji, Steven Chu and William Daniel Phillips for the “development of methods to cool and trap atoms with laser light”.
To implement a qubit (a two-state system serving as a basic unit of information), one uses here the electronic states of the ion. Depending on the protocol, several types of trapped ion qubits can be implemented. There are two main categories. First, optical qubits, where the two states are the ground state and an excited state, generated by shining a laser at optical frequencies (THz) on the ion. Second, hyperfine qubits, where the two states are both ground states of different hyperfine levels. Hyperfine levels are very close in energy (MHz to GHz) and are generated by the interaction between the magnetic moments in the nucleus and the electrons. Hyperfine qubits have a longer coherence time than optical qubits, but are slower to operate, making gate long to apply.
The qubits are then manipulated using lasers, which allow to either create a coupling between the states of a qubit (for single qubit gates), or a coupling between the internal, electronic qubit states and its motion states (to create entanglement).
Frequently Asked Questions About Trapped Ion Qubits
1. What are the advantages of trapped ion qubits?
The main strength of trapped ion qubits is their high coherence time, which can be of a few minutes (as opposed to tens of milliseconds for superconducting qubits for instance). They also allow for high fidelity gates. Finally, state preparation and readout are easily implemented.
2. What are the limitations of trapped ion qubits?
The two main caveats of trapped ion qubits are their slow gate time, and the fact that trapped ions quantum computers do not scale well (it is difficult to maintain good control of the trapped ion qubits as their number increases).