Possibilities for physical implementation

• Implementations of quantum computers will be a difficult experimental challenge. Quantum computer equipment must satisfy a variety of constraints: (1) the qubits must interact very weakly with their environment to minimize decoherence and preserve their superpositions, (2) the qubits must interact very strongly with one another for the logic gates and information transfer to be effective, and (3) the initialization and readout of states must be efficient. Not many known physical systems can satisfy these requirements, although there are some possibilities.
• A collection of charged ions held in an electromagnetic trap is one possibility. Each atom stores a qubit of information in a pair of internal electron levels. Each atom's levels are protected from environmental influences. Scaling to larger numbers of qubits should be able to be done by adding more atoms to the collection. When appropriate laser radiation is applied to the atoms, only one of the two internal states fluoresces. This allows detection of the state of each qubit. The atoms are coupled by virtue of their mutual Coulomb repulsion. Experimental development of trapped ion quantum computation is at the level of single-ion and two-ion qubit systems. Extensions to larger numbers of trapped ions has been difficult, but there do not seem to be impossible theoretical limits to scaling.
• Another system which could be developed into a quantum computer is a single molecule, in which nuclear spins of individual atoms represent qubits. This is the basis of the NMR technique mentioned above. The spins can be manipulated, initialized, and measured. For example, the carbon and hydrogen nuclei in a chloroform molecule can be used to represent two qubits. Applying a radio-frequency pulse to the hydrogen nucleus addresses that qubit and causes it to rotate from a $\vert{0}\rangle$ state to a superposition $\frac{1}{\sqrt{2}}(\vert{0}\rangle + \vert{1}\rangle)$ state. Interactions through chemical bonds allow multiple-qubit logic to be performed. However, it is difficult to find molecules with more than 10 spins in them and with a large coupling constant between every pair of spins ...