An atom or molecule with a net electric charge due to loss or gain of electrons, used as qubits here.An ion is an atom or molecule that has gained or lost one or more electrons, giving it a net positive or negative electric charge. This charge makes ions responsive to electric and magnetic fields, which is why they are used as qubits in trapped-ion quantum computers. In such systems, ions are held in place by electromagnetic traps and manipulated with lasers to perform quantum operations.
The term 'ion' was coined by Michael Faraday in 1834, based on the Greek word for 'going', because ions move toward electrodes in an electric field. The use of trapped ions as qubits was proposed by Ignacio Cirac and Peter Zoller in 1995, and the first demonstrations followed shortly after.
When an atom loses an electron, it becomes a positively charged ion; when it gains one, it becomes negatively charged. The charge allows ions to be confined using electric fields in a Paul trap, where they form a crystal-like structure. Lasers are then used to excite the ions into different quantum states, and their internal energy levels serve as the 0 and 1 states of a qubit. The ions interact with each other through their Coulomb repulsion, enabling entangled operations.
Trapped ions are used in quantum computers for research and small-scale computation, as well as in mass spectrometry for chemical analysis and in ion thrusters for spacecraft propulsion.
The first trapped-ion quantum computer was built in 1995, and today's versions can maintain quantum coherence for over a minute, which is remarkably long for a quantum system.
